Accidents at junctions on one-way urban roads · junctions on one-way roads; for example, such a...

Accidents at junctions on one-way urbanroads

Prepared for Road Safety Division, Department for Transport,

Local Government and the Regions

I Summersgill and J V Kennedy (TRL Limited),R D Hall, A Hickford and S R Barnard (University of Southampton)

TRL Report TRL510

First Published 2001ISSN 0968-4107Copyright TRL Limited 2001.

This report has been produced by TRL Limited, under/as partof a contract placed by the Department for Transport, LocalGovernment and the Regions. Any views expressed in it arenot necessarily those of the Department.

TRL is committed to optimising energy efficiency, reducingwaste and promoting recycling and re-use. In support of theseenvironmental goals, this report has been printed on recycledpaper, comprising 100% post-consumer waste, manufacturedusing a TCF (totally chlorine free) process.

CONTENTS

Page

Executive Summary 1

1 Introduction 3

1.1 Background 3

1.2 Objectives 3

2 Reconnaissance survey and site selection 3

2.1 Reconnaissance survey 3

2.2 Site selection 4

3 Main survey 4

3.1 Time-dependent data 4

3.2 Geometric data 5

3.3 Signal control data 5

3.4 Accident data 5

3.5 Data processing 5

4 Junction characteristics 5

4.1 Geographical spread 5

4.2 Geometric characteristics 5

4.3 Vehicle and pedestrian flows 6

5 Accident tabulations 6

5.1 Introduction 6

5.2 Accident numbers, frequency, severity and rates 6

5.3 Accidents by type 7

5.3.1 Junction type and operational form 7

5.3.2 Severity and casualty involvement 8

5.3.3 Road user involvement rates 8

5.4 Comparison with accident studies of other junction types 8

5.4.1 Other junction accident studies 8

5.4.2 Comparison of accident frequency, severity and rate 9

5.4.3 Comparison of vehicle involvement rate 9

6 Statistical analysis 10

7 Whole junction accident models 10

7.1 Introduction 10

7.2 Form of the models 10

7.2.1 Vehicle and pedestrian flow functions 10

iii

iv

Page

7.2.2 Factors 11

7.2.3 Flow functions and junction features 11

7.3 Total accidents (all operational forms combined) 11

7.4 Total accident models by operational form 12

8 Accident-flow models by accident group 12

8.1 Introduction 12


9 Accident-flow-geometry models by accident group 14

9.1 Introduction 14


9.3 Variables and factors 14

9.4 Outline of key results 14

10 Application of the models 15

10.1 Total accident-flow models 15

10.2 Accident-flow models by accident group 15

10.3 Full accident-flow models by accident group 15

10.4 Software 16

11 Summary and conclusions 16

12 Acknowledgements 17

13 References 17

Appendix A: Operational forms 18

Appendix B: Vehicle and pedestrian flow functions 22

Appendix C: Whole junction models by junction type 25

Appendix D: Whole junction models by junction type andoperational form 27

Abstract 30

Related publications 30

1

Executive Summary

ii Accident frequency for each accident group was relatedto various functions of the vehicle and pedestrian flowsand to key characteristics (accident-flow models);

iii The best accident-flow models developed in (ii) wereextended to include the full range of measuredgeometric and signal control characteristics (accident-flow-geometry models).

Whole junction models were developed separately foreach of the main operational forms included in the study;however accident-flow and accident-flow-geometrymodels were only developed for each junction type sincethere were too few sites to develop full models forindividual operational forms. This report contains the best-fitting whole junction models. Accident-flow and accident-flow-geometry models for the different accident groups arenumerous and complex and are reported elsewhere.

Some of the key findings of the study are listed below.

i The mean pedestrian flow at the sample of junctions,generally located in town or city centres, was very highand over one third of accidents involved a pedestrian.

ii Comparison of the data for this study of junctions withone or more one-way arms with that from correspondingstudies of junctions with all two-way arms showed thefollowing. At 3-arm junctions, both the mean accidentfrequency and the mean accident rate were similar.However at 4-arm junctions, both accident frequency andrate were substantially lower if there was a one-way arm.The mean severity of accidents at both the 3-arm and 4-arm junctions was lower if there was a one-way arm.

iii Accident involvement rates (number of involvementsper 100 million vehicles of the class) were much higherfor pedal cycles and motorcycles than for cars and lightgoods vehicles. Comparing with the data from theprevious samples of junctions with all two-way arms,the relative rate for motorcycles was lower at junctionswith a one-way arm, while relative rates for othervehicle types were similar whether or not the junctionshad a one-way arm.

iv Whether the arm of association or an adjacent oropposite arm was one-way or two-way had an effect onaccident risk. In general, more conflict points wereassociated with higher risk.

v A high percentage of the sample of junctions (especially3-arm) had islands which impinge on more than onearm and which were intended to channel the flow. At 3-arm priority sites, kerbed central islands were associatedwith fewer merging and diverging accidents. Howeverat signal sites, these islands were associated with anincrease in pedestrian accidents.

vi Four-arm signal sites with a separate signal stagebetween the conflicting right angle movements wereassociated with lower risk of right angle accidents.

vii Variables describing the amount of queueing affected anumber of accident groups at priority junctions, with theeffect that longer queues were generally associated withincreased accident risk.

One-way roads are commonly used in towns and cities tohelp relieve congestion, but little is known about hownumbers of accidents and their distribution differ fromthose on two-way roads. In particular, the only means ofmodelling accidents at junctions on one-way roads hashitherto been to use models developed for junctions ontwo-way roads and set some of the flows to zero. This isunlikely to be a good approximation where behaviour maybe very different, for example, where there are 6 entrylanes on one arm. Geometry may also be different atjunctions on one-way roads; for example, such a junctionmay have an island to channel traffic.

The report describes a study of personal injury accidentsat junctions with one or more one-way arms on urban30 miles/hr single carriageway roads, undertaken on behalfof the Road Safety Division (RSD) of the Department forTransport, Local Government and the Regions (DTLR). Theaim was to determine how these accidents are related tovehicle and pedestrian flows and to the features and layoutof the junction. The study is one of a series investigatingaccidents at different junction and link types, mostly on30 miles/hr single-carriageway roads with two-way traffic.

The study comprised 3- and 4-arm priority junctions and3- and 4-arm signal junctions. For each type of junction,several different ‘operational forms’ were included. Theterm ‘operational form’ refers to the arrangement of one-way and two-way arms at the junctions. A wide variety ofoperational forms of junctions with at least one one-wayarm exist, and it was not possible to study all of thesewithin the resources of this study. The approach adoptedwas to identify the more common operational forms forbusy junctions, and to base the study on these.

A national sample of 433 junctions was identified(196 3-arm priority, 76 4-arm priority, 69 3-arm signal and92 4-arm signal junctions). The sample was chosen to have agood geographical spread, with a substantial component inLondon, and was stratified by vehicle and pedestrian flows.An in-depth data collection exercise was carried out at thejunctions, comprising a 12-hour vehicle and pedestrian surveyand comprehensive geometric and signal control data.

Records of all reported personal injury accidentsoccurring at the junctions were obtained for the period from1987 to 1994 inclusive. Each of the 3,622 accidents wasallocated to an arm of the junction and to a detailed accidenttype code, determined by the nature of the accident and themovements of the vehicles and pedestrians involved.Accidents were grouped into similar types for subsequentanalysis. Tabulations are presented showing accidentfrequencies, severities and rates by junction type.

The technique of generalised linear modelling was usedto develop accident predictive relations from the data.Models were developed in three stages:

i Total accident frequency at the junctions was related tovarious functions of the vehicle and pedestrian flowsand to key characteristics (whole junction models);

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viii For all junction types, there was no evidence that thepresence of pedestrian guard railing was associatedwith fewer pedestrian accidents. This suggeststherefore that there may be scope for improvement inthe design of these facilities to achieve their objectives.

ix In the final models, measures of the speed of trafficwere not significant at the 5 per cent level for 3-armpriority junctions. At the other junction types, a highermean speed or higher speed variability were associatedwith increases in some accident types. The modelscontain other variables that are correlated with speed,for example, visibility and the presence of a majorjunction within 200m; it is possible that these othervariables affect accidents through a modifying effecton speed. This study was not intended nor designed toinvestigate speed mechanisms and relationships indepth, and only coarse measures of speed wereincluded. The results do not preclude there being otherengineering measures not included in the study (trafficcalming measures, for example), which could reduceaccidents by reducing approach speeds. Speedmeasurements were made only for freely movingvehicles on the approach to the junction.

x Other key results included the following. Vehicleproportion variables increased risk for various accidenttypes, for example single vehicle accidents at 3-armjunctions with higher proportions of buses andmotorcycles. Junctions in Greater London, particularlysignal junctions, had increased risk of some accidenttypes. These results will be more important forprediction than design purposes, since they do notconcern layout variables.

The models are intended to be used mainly to identifypotential design improvements and to provide accidentestimates for the economic appraisal of roadimprovements. In conjunction with traffic assignmentmodels, they can be used: to predict the effect on accidentsof traffic management schemes; to identify casualty-reducing strategies; and to optimise safety/mobility for allroad users. The whole junction models have beenincorporated into the computer program SafeNET, whichcan be used to estimate the frequency of injury accidentsfor a road network (junctions of different types and roadsections) operating under different traffic/safetymanagement schemes.

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1 Introduction

1.1 Background

One-way roads are commonly used in towns and cities tohelp relieve congestion, but little is known about hownumbers of accidents and their distribution differ fromthose of two-way roads. In particular, the only means ofmodelling accidents at junctions with one-way arms hashitherto been to use models developed for junctions withall two-way arms and set some of the flows to zero. This isunlikely to be a good approximation where behaviour maybe very different, for example, where there are 6 entrylanes on one arm. Geometry may also be different atjunctions on one-way roads; for example, such a junctionmay have an island to channel traffic.

The report describes a study of personal injury accidentsat junctions with one or more one-way arms on urban30 miles/hr single carriageway roads, undertaken on behalfof the Road Safety Division (RSD) of the Department forTransport, Local Government and the Regions (DTLR).The aim was to determine how these accidents are relatedto vehicle and pedestrian flows and to the features andlayout of the junction.

The study is one of a series investigating accidents atdifferent junction and link types, mostly on single-carriageway roads with two-way traffic. The reportspreviously published are for: 4-arm roundabouts on ruraland urban, single and dual carriageway roads (Maycockand Hall, 1984); 3-arm priority junctions on rural roads(Pickering et al., 1986), 4-arm signal junctions on30 miles/hr urban roads (Hall, 1986); one-way and two-way links (between junctions) on 30 and 40 miles/hr urbanroads (Summersgill and Layfield, 1996); 3-arm priorityjunctions on 30 and 40 miles/hr urban roads (Summersgillet al., 1996); 4-arm priority junctions on 30 and 40 miles/hr urban roads (Layfield et al., 1996); 3-arm signaljunctions on 30 miles/hr urban roads (Taylor et al., 1996);and 3-arm and 4-arm mini-roundabouts on 30 miles/hrurban roads (Kennedy et al., 1998).

The study comprised 433 junctions (196 3-arm priority,76 4-arm priority, 69 3-arm signal and 92 4-arm signaljunctions). For each type of junction, several different‘operational forms’ were included. The term ‘operationalform’ refers to the arrangement of one-way and two-wayarms at the junctions. A wide variety of operational formsof junctions with at least one one-way arm exist, and it wasnot possible to study all of these within the resources ofthis study. The approach adopted was to identify the morecommon operational forms for busy junctions, and to basethe study on these.

The study was divided into a number of phases:

Phase I design and execution of a reconnaissance surveyand selection of a sample of junctions for themain survey;

Phase II design of the main survey, data collection andverification, and accident coding and tabulation;

Phase III development of accident predictive models.

The contents of the report are as follows. Section 2describes the reconnaissance survey and the selection of asample of junctions for the main survey. Section 3 givesdetails of the main survey. Section 4 presents some of thekey site characteristics, whilst Section 5 gives accidenttabulations. The methodology of the regression analysis isdescribed in Section 6. Section 7 presents the accidentpredictive relations for the whole junction with flows andmain features only. Sections 8 and 9 describe briefly thedevelopment of accident-flow and accident-flow-geometrymodels for each group of accidents. These models arenumerous and complex; full details are provided inSummersgill et al. (2001). Section 10 sets out the expectedapplications of the models and Section 11 provides a briefsummary and conclusions.

1.2 Objectives

The main objectives of the project were:

i to produce accident tabulations for the junctions whichwill give insights into the main accident problems;

ii to develop accident predictive relations between accidentfrequency and the explanatory variables such as vehicleand pedestrian flows, junction type and operational form,features, layout and signal control variables.

The accident predictive relations are intended to beused: to identify potential improvements in junction designand signal control strategies; to provide accident estimatesfor the economic appraisal of road improvements; and, inconjunction with traffic assignment models and similarrelations for other junction types, to identify casualty-reducing traffic management schemes and to optimisesafety/mobility for all road users.

2 Reconnaissance survey and site selection

2.1 Reconnaissance survey

A reconnaissance survey was undertaken to obtain arepresentative sample of 3- and 4-arm priority and signaljunctions on 30 mile/hr single carriageway roads, with atleast one arm that was a one-way road. A widegeographical spread was required, with a substantialcomponent in London so that effects similar to thoseoccurring in other studies (e.g. Summersgill et al., 1996)could be investigated. These showed that the accident ratein Greater London was about 50 per cent greater than thatelsewhere when all explanatory variables used in the studywere taken into account.

The junctions had to satisfy the following specificconditions, in part to be consistent with other accidentstudies within the series:

� not to be part of a housing or an industrial estate;

� not to be part of signalised or non-signalisedroundabouts, though gyratories were acceptable;

� not to have a bus lane, a tram lane, or a contraflow cyclelane;

� to be lit at night;

� not to be closed for part of the day (or night).

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The sample of signal junctions additionally was to:

� have full time signals;

� include junctions with any number of prohibited turns;

� have Mellor high intensity signals.

About 900 junctions were visited in the reconnaissancesurvey and data recorded at 793 that were potentiallysuitable for the study. The basic data recorded included thetype and operational form of the junction and whether thearms were one-way inbound, one-way outbound or two-way. In addition, the presence of features such aspedestrian crossings and traffic islands was noted, alongwith road markings, parking regulations, land use andbasic junction dimensions.

Vehicle and pedestrian flows were estimated from shortperiod counts. The vehicle flows were the two-way ‘major’and ‘minor’ annual average daily totals (AADTs). Thepedestrian flow was the total flow crossing the centre ofthe junction and all arms within 20m of the junction.

2.2 Site selection

The key criteria used to select the final sample of junctionswere as follows:

i A minimum of about 20 to 25 junctions of eachoperational form was considered desirable, so that onlythe more common forms were included in the finalsample.

ii The sample of 4-arm junctions was to includecrossroads, left-right and right-left staggered junctions.

iii Approximately 25 per cent of the sample should be inGreater London and there should be a good distributionacross the former Department of Transport (DOT) regionsand in Wales and Scotland. In each DOT region, thejunctions were to be located in at least 5 different towns orurban areas, and ideally across several different counties.

iv Junctions were stratified according to the estimates oftotal ‘major’ road inflow, total ‘minor’ road inflow andpedestrian flow. The sample of each of the four junctiontypes (3- and 4-arm priority and signal junctions) wasstratified by the estimated vehicle and pedestrian flows.

v A wide distribution of geometric, signal control andother junction features, for example, frontage land use,parking, pedestrian crossing facilities and traffic islands.

vi Sufficient priority junctions with zebra or pelicancrossings to establish their effect.

vii Junctions were to have been stable over the period from1st January 1987 to 30th June 1994 with no significantchanges in junction layout, control or in vehicle andpedestrian flows.

The selected sample comprised 433 junctions. A numberof junctions that were stable over a slightly reduced periodor with one or more changes in layout or to the signallingarrangements were accepted. Each of these was coded astwo, or in some cases three, separate ‘sites’, giving a totalof 490. Table 1 shows the number of junctions, sites andoperational forms of each type in the final sample.

The operational forms for each junction type are shownin Appendix A, in Figures A1 to A4 for 3-arm priority, 4-arm priority, 3-arm signal and 4-arm signal junctionsrespectively. It should be noted that the numbering is notconsecutive, for example, no junctions in the final samplehad operational form 5 and this is therefore omitted fromFigure A1. The diagrams also show the number ofjunctions of each type in the sample. Arms are numberedclockwise: the major arms are 1 and 2 for 3-arm junctionsand 1 and 3 for 4-arm junctions.

3 Main survey

The following data were collected at each of the 433selected junctions.

3.1 Time-dependent data

Time-dependent data were recorded continuously from0700 to 1900 on one weekday between February 1994 andSeptember 1995 inclusive (excluding December, January,July and August) avoiding market days, school and bankholidays, as outlined below:

i Vehicle flow. Full classified turning counts on each arm(cars, taxis and light goods vehicles (LGVs); heavygoods vehicles (HGVs); buses and coaches;motorcycles; and pedal cycles).

ii Pedestrian counts within 20m. For each crossingmovement on each arm within 20m of the junction.Pedestrians on a zebra or pelican crossing were includedif the edge of the crossing nearest the junction waswithin 20m.

iii Parking activity and occupancy within 20m. Thenumber of parking acts (vehicles entering or leaving aparking space) on each side of each arm up to 20m fromthe junction was recorded every hour. Kerb occupancywas recorded at the start of each hour.

iv Vehicle queueing on each arm with an inbound flow.At priority junctions, the number of vehicles queueingwithin 50m was recorded at 10 minute intervals; thisgenerally applied to the minor arm (or arms) of thejunction. At signal junctions, the number of vehicleswithin 50m at the start of the red period was counted;this procedure also gave a count of the number ofsignal cycles at the junction. From thesemeasurements, the mean, standard deviation, 85thpercentile and coefficient of variation (ratio of

Table 1 Sample characteristics

No. ofNo. of No. of operational

Junction type sites junctions forms

3-arm priority 205 196 94-arm priority 81 76 73-arm signal 84 69 54-arm signal 120 92 7

Total 490 433 28

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standard deviation to the mean) of the numbers ofvehicles queueing were calculated.

In addition, data were recorded as follows duringeach of four 15-minute periods (morning/afternoon;peak/off peak):

i Vehicle speed. The speeds of up to 25 freely movinglight vehicles were measured with a radar speed meteron each arm with an inbound flow; at signal junctions,only vehicles approaching a green signal were included.From these measurements, the mean, standarddeviation, 85th percentile and coefficient of variation ofsample speeds were determined.

ii Pedestrian flow proportions. The sex and estimated age(under 15; between 15 and 60; over 60) of a sample ofpedestrians were recorded.

iii Driver proportions. The sex and estimated age (under25; between 25 and 60; over 60) of a sample of driversof light vehicles were recorded.

iv Parking occupancy within 100m. The number ofvehicles parked on each side of each arm was counted atthe end of each of the four 15-minute periods.

3.2 Geometric data

A comprehensive record was made during a site visit ofthe geometry of the junction centre and of each arm. Theseincluded, for example, numbers of lanes, carriagewaywidths, positions and lengths of islands, signing and roadmarkings, road layout, features such as pedestriancrossings, guard rails, bus stops, gradients, parking andloading regulations, sight distances and land use.

In addition, large scale (1:500) plans were obtained fromthe Local Highway Authorities and from the OrdnanceSurvey. These were checked against photographs and thesurvey data obtained on site. In general the location of theboundary kerbs was correct, but the positions of thesmaller islands in particular were less reliably marked. Thedata obtained from plans included angles, minimum radiiof curvature, the area of the core of the junction and,where applicable, the stagger length.

3.3 Signal control data

Data on the form of signal control (SCOOT, UTC, VA andothers) and details of the staging/phasing, green times andinter-greens were obtained from the Local HighwayAuthorities. The number of signal cycles was recorded inconjunction with the queueing data (Section 3.1). Thenumber and types of signal head were recorded on site atthe same time as the geometric survey.

3.4 Accident data

For most junctions, the accident period covered the 7.5 yearperiod from 1 January 1987 to 30 June 1994. However forsome junctions, changes to the geometry or signal control,vehicle or pedestrian flows, meant that the study period hadto be restricted. In addition, for 40 junctions, the studyperiod was extended to 31 December 1994.

Records of personal injury accidents were obtained fromLocal Highway Authorities, who also supplied plainlanguage descriptions, and cross-referenced to TRL’s copyof the STATS 19 database. An accident type code and anarm of association were assigned for each relevant accident.

The accidents were those occurring within 20m of thejunctions.

3.5 Data processing

The 12-hour vehicle counts were converted to annualaverage daily totals (AADTs) for the year in which theywere observed. Scaling factors dependent on the roadclass, vehicle type and day or month of the observationperiod were obtained from DTLR. The AADTs wereadjusted to allow for changes in flow level and trafficcomposition over the accident period. The pedestriancounts were not scaled.

A comprehensive programme of verification wasundertaken to ensure the robustness of the data.

4 Junction characteristics

4.1 Geographical spread

The sample contained a good geographical spread of sites(Table 2) with just over 30 per cent in Greater London.

Table 2 Number of sites by type and by region

Junction type

3-arm 4-arm 3-arm 4-armpriority priority signal signal

Greater London 59 17 32 47South East 28 12 2 3South West 32 11 6 9Eastern 16 5 13 10East Midlands 8 2 4 8West Midlands 13 7 3 1Yorks & Humberside 3 0 1 3North West 20 12 12 11Northern 5 3 4 10Wales 10 3 0 5Scotland 11 9 7 13

Total 205 81 84 120

4.2 Geometric characteristics

Some of the key geometric characteristics of the sampleare listed below; further details are given in Summersgillet al. (2001).

Direction of flow and number of entry lanesThe number of arms that were inbound or outbound only isshown in Table 3, together with the distribution of entrylanes on inbound or two-way arms.

Junction islands

Movements within junctions with one-way arms are oftencontrolled by kerbed or painted islands, as well as by signs

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and markings. The numbers of sites with one or more kerbedislands are listed in Table 3. Islands intended to channel flowtypically impinge on more than one arm, whereas islandssolely within one arm are more likely to be intended aspedestrian refuges. At the 3-arm sites, about three-quarters ofthe islands were considered to channel flow, whereas at 4-armsites, the split between islands acting as refuges and thoseconsidered to channel flow was approximately even.

Land-use

About 75 per cent of the sites had at least one shopadjacent to them, reflecting their town centre location, andabout 35 per cent had a public house.

Pedestrian crossings and guard railing

About 16 per cent of arms at 3-arm priority sites and 10per cent at 4-arm sites had pedestrian crossings within 20m(Table 3). Guard railing on the entry corner was mostcommon at the signal sites, being present on about 30 percent of arms at these sites compared with about 9 per centof arms at priority sites.

Side roads and accesses

A small number of arms had side roads, public accesses orprivate drives on the entry corner (Table 3).

4.3 Vehicle and pedestrian flows

Tables 4a–4c present the range of total vehicle inflows andpedestrian crossing flows for the sample by site type.Pedestrian flows were separately recorded crossing an armor crossing to or from an island. The ‘off-kerb’ total(Table 4b) was the number of pedestrian movements fromthe outer boundary of the junction. The total count (Table 4c)

includes all pedestrian movements: between kerbs; fromthe kerb to an island; and between islands.

The mean vehicle flow was higher for the signal than forthe priority sites, but similar for the 3-arm and 4-arm sites.Average pedestrian flows were much lower at priority thanat signal sites.

5 Accident tabulations

5.1 Introduction

A total of 3,622 personal injury accidents (PIAs) havebeen identified as occurring within 20m of the junctionsduring the study period.

A number of key tabulations of the accidents arepresented in the following sub-sections; more extensivetabulations, which include data broken down byoperational form, are given in Summersgill et al. (2001).

The tabulations use two measures of accident occurrence:

i Average accident frequency: the average number ofinjury accidents per site per year over the study period;

ii Average accident rate: the average number of injuryaccidents per 100 million vehicles entering the site overthe study period.

Road user involvement uses average involvement ratesdefined as follows:

i Average vehicle involvement rate: the average number ofvehicles of the particular class involved in injury accidentsper 100 million vehicles of that class entering the site;

ii Average pedestrian involvement rate: the averagenumber of pedestrians involved in injury accidents per100 million pedestrians crossing the arms of the site.The pedestrian flows used in calculating the pedestrianinvolvement rates are the 12-hour counts (from 0700 to1900) times the number of site days with no attemptbeing made to account for seasonal variation or flowoutside these hours. The pedestrian involvement ratesare not, therefore, directly comparable to the vehicleinvolvement rates.

It is important to recognise that since the sample ofjunctions was stratified by region (over-representing theLondon sites) and by traffic and pedestrian flows, thetabulations reflect the sample characteristics and notnecessarily the characteristics of the population of thesetypes of junctions as a whole.

5.2 Accident numbers, frequency, severity and rates

Table 5 shows the numbers of injury accidents occurringduring the study period, the mean accident frequency andthe mean accident severity (percentage of accidentsinvolving either fatal or serious casualties).

As expected, 4-arm sites had a higher mean frequencythan 3-arm sites, and signal sites had a higher meanfrequency than priority sites. There was little differencebetween the severity (percentage of accidents that were fatalor serious) for the priority and the signal sites (possiblybecause both types occur in the same parts of town), but the4-arm sites had a higher severity than the 3-arm sites.

Table 3 Key site characteristics

3-arm 4-arm 3-arm 4-armNumber priority priority signal signal

Sites 205 81 84 120

Arms 615 324 252 480Outbound only arms 185 98 67 142Inbound only arms 216 97 95 148

Arms1 with one entry lane 280 167 37 115two entry lanes 123 44 83 138three entry lanes 22 13 47 56four entry lanes 5 2 12 27five entry lanes 0 0 5 2six entry lane 0 0 1 0

Islands 130 29 83 144Islands which channel flow 104 13 57 73

Arms with pedestrian crossing within 20m 97 32 1 0Arms with guard railing on entry corner 63 20 109 113

Arms with a side road on entry corner 17 0 4 0Arms with a public access on entry corner 34 2 19 9Arms with a private drive on entry corner 61 11 15 15

1 inbound or two-way

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Table 6 shows the average vehicle inflow (AADT) andthe average 12-hour pedestrian crossing counts totalledacross all arms and accident rates for each site type. Itshould be noted that the average flows of vehicles andpedestrians in Table 6 are weighted by the number of studyyears at each site, while the mean values shown inTables 4a–4c give each site equal weight. Accident rateswere higher for the signal than for the priority sites, andfor the 4-arm than for the 3-arm sites.

5.3 Accidents by type

The 3,622 injury accidents were classified according to themanoeuvres of the vehicles and the movements of thepedestrians involved. The numbers of accidents are givenby detailed type in Summersgill et al. (2001).

An accident was defined as a vehicle accident if theprimary cause or the first impact did not involve a pedestrian.Thus some vehicle accidents involved pedestrian casualties, ifa vehicle in the primary impact went on to injure a pedestrian.

In the tables that follow, the detailed accident types havebeen amalgamated into six groups: single vehicle; rearshunts and side collisions; right angle; right turns; ‘other’vehicle; and pedestrian accidents. This was intended tohelp to determine the most suitable grouping of theaccident types for model development, although thisgrouping was subsequently altered (see Section 8).

5.3.1 Junction type and operational formTable 7 shows the percentage of accidents by junction typeand accident group. Pedestrian accidents formed thelargest group. Right angle accidents occur when vehiclesgoing ahead on adjacent roads collide. This cannot happenat 3-arm junctions, but formed the largest vehicle accidentgroup at 4-arm junctions. Proportionately, the 3-arm siteshad more single vehicle accidents and more rear shuntsand side collisions than 4-arm sites.

Percentages of accidents in the various groups were verysimilar at 3-arm priority and 3-arm signal sites, apart from‘other’ vehicle accidents. The main difference between 4-arm priority and 4-arm signal sites was that the latter had alower percentage of right angle accidents (although theseaccidents were not eliminated) and a higher percentage ofrear shunts and side collisions.

Table 4a Total vehicle flow by site type

3-arm 4-arm 3-arm 4-armAADT priority priority signal signal 3-arm 4-arm Priority Signal Total

Minimum 1606 4101 8974 4296 1606 4101 1606 4296 1606Mean 18538 13668 28787 24812 21517 20321 17159 26449 21026Maximum 50841 32110 76944 63404 76944 63404 50841 76944 76944

Table 4b Off-kerb pedestrian flow by site type

Pedestrians 3-arm 4-arm 3-arm 4-armin 12 hours priority priority signal signal 3-arm 4-arm Priority Signal Total


Table 4c Total pedestrian flow by site type

Pedestrians 3-arm 4-arm 3-arm 4-armin 12 hours priority priority signal signal 3-arm 4-arm Priority Signal Total


Table 6 Accident rate

AverageAverage 12hr Accident

No. of 24hr pedes rateNo of Site acci vehicle -trian per 108

sites years -dents inflow flow vehicles

3-arm priority 205 1410.6 1004 18,403 2,414 10.64-arm priority 81 565.8 525 13,528 4,821 18.83-arm signal 84 440.7 755 27,178 7,331 17.34-arm signal 120 647.7 1338 24,003 8,283 23.6

Table 5 Accidents, accident frequency and accidentseverity

Acci AcciNo. of accidents -dent -dent

No of Site frequ seversites years Fatal Serious Slight Total -ency -ity %

3-arm priority 205 1410.6 6 138 860 1004 0.71 14.34-arm priority 81 565.8 2 90 433 525 0.93 17.53-arm signal 84 440.7 4 102 649 755 1.71 14.04-arm signal 120 647.7 16 225 1097 1338 2.07 18.0

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5.3.2 Severity and casualty involvementTable 8 shows accident severity and casualty involvementby accident group.

5.4 Comparison with accident studies of other junctiontypes

5.4.1 Other junction accident studiesA comparison of the values obtained in the differentjunction accident studies on single carriageway roads, foraccident frequency, rate and severity is presented inTable 11. Road user involvement rates from the studies arecompared in Table 12. The comparisons have beenrestricted to 30 miles/hr subsets of the junctions whereseparate values are available. The results for roundaboutsand mini-roundabouts with two-way arms are included forthe sake of completeness, although there are no results forjunctions of these types with one-way arms.

The study of 3-arm priority junctions with two-wayarms (Summersgill et al., 1996) on 30 and 40 miles/hrurban roads included two sets of T-junctions. The first wasa fully stratified set of 300 junctions. The second was of680 junctions, for which the main road traffic flow wasstratified but the minor road flow and the pedestrian flowwere not. For convenience, both samples have beencombined in Table 11 to give 790 junctions on 30 miles/hrroads at which 2,277 injury accidents occurred during thestudy period. Each junction formed only one site.

The study of 3-arm signal junctions with two-way arms(Taylor et al., 1996) had a stratified sample of 221junctions (238 sites), all with 30 miles/hr speed limits.

The study of 4-arm priority junctions with two-wayarms (Layfield et al., 1996) included 300 junctions ofwhich 233 had a 30 miles/hr speed limit. The sample wasstratified and each junction formed only one site. Bothcrossroads and staggered junctions were included.

The study of 4-arm signal junctions with two-way arms(Hall, 1986) had 177 junctions, all with 30 miles/hr speedlimits, and at which 1,772 injury accidents occurred duringthe study period. The sample was stratified and eachjunction formed only one site.

The study of 4-arm roundabouts included 36 junctionson urban 30-40 miles/hr roads (Maycock and Hall, 1994).The study of mini-roundabouts with two-way arms(Kennedy et al., 1998) included a stratified sample of 3- and4-arm junctions, all with a 30 miles/hr speed limit. Therewere 200 3-arm junctions (206 sites) and 100 4-armjunctions (105 sites).

It should be noted that the study periods (shown inTable 11) differed considerably.

Table 10 Accident involvement rates by vehicle type/pedestrians and junction type

Vehicle type

Car,Pedal Motor taxi, Pedescycle cycle LGV1 HGV2 PSV -trian

3-arm priority 89.9 91.2 14.0 8.2 39.4 29.84-arm priority 98.5 127.5 26.5 16.9 66.1 18.43-arm signal 114.6 80.2 19.8 14.0 96.5 27.54-arm signal 156.4 105.3 34.1 22.5 89.1 24.1

1 LGV Light goods vehicle2 HGV Heavy goods vehicle

Shunts and side collisions were the least severe. Almost80 per cent of the fatal and about half of the serious injuryaccidents involved a pedestrian.

The average number of casualties per injury accidentwas consistently low for pedestrian accidents and high forright angle accidents (Table 9).

5.3.3 Road user involvement ratesTable 10 shows the accident involvement rate of differentvehicle types (per 100 million vehicles of that type) and ofpedestrians (per 100 million pedestrians) for the fourjunction types.

Two-wheelers were disproportionately involved inaccidents at all junction types, with pedal cycles and motorcycles experiencing several times the involvement rate ofcars. Public service vehicles (PSVs) also had higherinvolvement rates than cars. PSVs were particularlyinvolved in single vehicle accidents, in which a passengeris injured when boarding, alighting or travelling on a bus.

Table 7 Percentage of accidents by accident group andby junction type

Accident group

Single Rear Right Right Other Pedes No ofvehicle shunts angle turn vehicle -trian Total accs

No of accidents 385 704 437 362 443 1291 36223-arm priority 12.0 23.9 0.0 9.4 19.6 35.2 100.0 10044-arm priority 6.1 9.7 29.3 10.9 11.4 32.6 100.0 5253-arm signal 16.0 25.8 0.0 7.3 9.5 41.3 100.0 7554-arm signal 8.4 16.3 21.2 11.7 8.5 34.0 100.0 1338All sites 10.6 19.4 12.1 10.0 12.2 35.6 100.0 3622

Table 8 Severity of accidents (% fatal or serious) byaccident group and by junction type

Accident group

Single Rear Right Right Other Total Pedesvehicle shunts angle turn vehicle vehicle -trian Total

3-arm priority 14.2 5.0 – 13.8 16.2 11.4 19.8 14.34-arm priority 28.1 5.9 15.6 12.3 8.3 13.6 25.7 17.53-arm signal 11.6 4.1 – 12.7 13.9 8.8 21.5 14.04-arm signal 15.2 10.6 21.2 13.5 7.9 14.7 24.4 18.0

Table 9 Casualty involvement (casualties per accident)by accident group and by junction type

Accident group

Single Rear Right Right Other Total Pedesvehicle shunts angle turn vehicle vehicle -trian Total

3-arm priority 1.22 1.22 – 1.17 1.15 1.19 1.08 1.154-arm priority 1.06 1.14 1.36 1.21 1.18 1.25 1.05 1.183-arm signal 1.08 1.19 – 1.16 1.24 1.16 1.06 1.124-arm signal 1.13 1.24 1.70 1.33 1.22 1.39 1.05 1.27

9

5.4.2 Comparison of accident frequency, severity and rateIn general, the 3-arm sites with one-way arms had similaraccident frequencies and rates but much lower severities thanthe equivalent junctions with all two-way arms (Table 11).They had higher mean vehicle flows and much highermean pedestrian flows.

The 4-arm sites with one-way arms had much loweraccident frequencies and rates as well as lower severitiesthan the equivalent junctions with all two-way arms. Theyhad similar mean vehicle flows but again had much highermean pedestrian flows.

5.4.3 Comparison of vehicle involvement rateTable 12 compares the road user involvement rates(number of involvements per 100 million vehicles of theclass) by junction type. Involvement rates for all junctiontypes were much higher for pedal cycles and motorcyclesthan for cars and light goods vehicles. The relative rate formotorcycles was lower at junctions with a one-way arm,while relative rates for other vehicle types were similarwhether or not the junctions had a one-way arm.

Table 11 Comparison of accident frequency, accident rate and severity for different junction types

Accident Average AccidentSpeed frequency 24 hour Average rate Severity

limit Number Number of (accs per vehicle 12 hour (accs per (% fatal Years(miles/hr) of sites accidents site year) flow ped flow 108 vehicles) + serious) of study

3-arm sitesPriority – two-way 30 790 2277 0.58 13,100 1,390 12.1 22 1983-1988

– one-way 30 205 1004 0.71 18,403 2,414 10.6 14 1987-1994

Signals – two-way 30 238 2262 1.67 25,730 1,950 17.8 18 1985-1991– one-way 30 84 755 1.71 27,178 7,331 17.3 14 1987-1994

Mini-roundabouts 30 206 1198 0.92 19,974 968 12.5 12 1986-1992

4-arm sitesPriority – two-way 30 233 2440 1.77 15,188 2,056 31.9 22 1984-1989

– one-way 30 81 525 0.93 13,528 4,821 18.8 18 1987-1994

Signals – two-way 30 177 1772 2.65 21,180 3,260 34.4 20 1979-1982– one-way 30 120 1338 2.07 24,003 8,283 23.6 18 1987-1994

Small island roundabouts 30-40 25 497 4.38 32,330 1,2361 37.1 18 1974-1979Conventional roundabouts 30-40 11 146 2.36 30,470 1,3921 21.2 27 1974-1979

Mini-roundabouts 30 105 902 1.35 16,258 1,442 22.8 14 1986-1992

116 hour pedestrian count

Table 12 Comparison of vehicle involvement rates for different junction types

Speed limit Number Pedal Motor(miles/hr) of sites cycle cycle Car, LGV HGV PSV

3-arm sitesPriority - two-way 30 790 95 (6) 123 (8) 16 (1) 9 (0.6) 41 (3)

- one-way 30 205 90 (6) 91 (7) 14 (1) 8 (0.6) 39 (3)

Signals - two-way 30 238 137 (6) 148 (6) 24 (1) 16 (0.7) 72 (3)- one-way 30 84 115 (6) 80 (4) 20 (1) 14 (0.7) 97 (5)

Mini-roundabouts 30 206 163 (9) 140 (8) 18 (1) 12 (0.7) 69 (4)

4-arm sitesPriority - two-way 30 233 177 (4) 358 (8) 46 (1) 20 (0.4) 79 (2)

- one-way 30 81 99 (4) 128 (5) 27 (1) 17 (0.6) 66 (2)

Signals - two-way 30 177 175 (4) 240 (5) 48 (1) 32 (0.7) 126 (3)- one-way 30 120 156 (5) 105 (3) 34 (1) 23 (0.7) 89 (3)

Small island roundabouts 30-40 25 785 (15) 663 (12) 54 (1) 43 (0.8) 62 (1)Conventional roundabouts 30-40 11 291 (13) 267 (12) 22 (1) 18 (0.8) 45 (2)

Mini-roundabouts 30 105 249 (7) 267 (8) 35 (1) 26 (0.7) 84 (2)

( ) The figures in brackets are the relative involvement ratios (Car, LGV = 1)

10

6 Statistical analysis

The objective of the analysis was to relate the accidentfrequency at the junctions to vehicle and pedestrian flow,and to junction characteristics. The statistical method usedwas a form of multiple regression analysis and is the sameas that employed in the other accident studies (Maher andSummersgill, 1996).

The set of ‘explanatory’ or ‘independent’ variables ofthe regression are functions of the vehicle and pedestrianflows, and the site, geometric and other characteristics ofthe junctions. Since, however, the numbers of accidents ina given period do not follow a normal distribution and, inparticular, do not have a constant variance, the‘generalised linear modelling’ technique, available forexample in the computer program GENSTAT (Alvey et al.,1977), has been used instead of classical least squaresregression. This allows the dependent variable to be drawnfrom one of a family of distributions.

The regression modelling was undertaken in three mainstages:

i relating total accident frequency at the junctions (andvehicle-only and pedestrian accident frequencies) tovarious functions of the traffic and pedestrian flows andkey site features (whole junction models);

ii relating accident frequency on each arm of the junctionfor each of the main accident groups to various functionsof the traffic flow and site factors (accident-flow models);

iii extending the best accident-flow models of (ii) using thefull range of geometric variables and factors observed(accident-flow-geometry models).

This report includes the whole junction (stage (i))models only. The full set of models is given inSummersgill et al. (2001).

7 Whole junction accident models

7.1 Introduction

The first stage of the modelling was to relate the totalaccident frequency at the junctions to various functions ofthe vehicle and pedestrian flows. The basic unit of analysiswas the whole junction. The models predict accidentfrequency at the whole junction.

The model with the best-fitting flow function wasthen extended to include some basic site classificationfactors, where these were statistically significant at the5 per cent level.

The procedure was carried out separately for vehicle-only accidents and for pedestrian accidents, as well as forall accidents together.

7.2 Form of the models

7.2.1 Vehicle and pedestrian flow functionsThe basic form of the model relating accident frequency toflow that has been found appropriate in the previousjunction studies is:

A = k Qα (7.1)

where A is the accident frequency (accidentsper year),

Q is the flow function, an algebraiccombination of the vehicle (andsometimes the pedestrian) movements,

k and α are parameters to be estimated by theregression.

Before fitting, the model was transformed to linear formby taking logarithms.

Often a product of two flows was tried as the flowfunction, giving the alternative form of model:

A = k Qaα Q

bß (7.2)

where A is the accident frequency,

Qa and Q

bare separate flow functions,

k, α and ß are parameters to be estimated.

This model simply allows the exponents of the two partsof the flow product to take separate values rather thanbeing constrained to one value as in model (7.1).

In model (7.2), Qa will generally represent a function of

the vehicle flows, for example QT (total vehicle inflow inthousands of vehicles per 24-hour period). Often Q

b is a

function of the pedestrian flows, for example PT (the totaljunction pedestrian flow across all the arms and thejunction centre, summed over both directions of crossing,in thousands of pedestrian per 12-hour period). The modelform is then:

A = k Qaα PTß (7.3)

This model has the property that it predicts zeroaccidents for zero pedestrian flow and is appropriate fortotal pedestrian accidents.

Vehicle-only accidents do not involve pedestrians in theprimary collisions, and pedestrian flow appears in themodels for two main reasons. Firstly, pedestrian activity islikely to increase the complexity of the driving task andhence increase accident risk. Secondly, pedestrian flowvariables may simply be acting as proxies for causalvariables with which they are associated and which have notbeen tested at this level of the modelling. In either case,vehicle-only accidents are not likely to be eliminated if thepedestrian flow is zero. Total accidents, which include bothvehicle-only accidents and pedestrian accidents, have asimilar property. For these accident categories, analternative form of model was tested which has the form:

A. = k Qaα exp{b PTß} (7.4)

The parameter b is determined by the regression but ßmust be obtained by trial and error until a good fit to thedata is obtained.

In all the models (7.1) to (7.4), in order that thedependent variable may be regarded as having a Poissonerror distribution, both sides of the model are multiplied by

11

the number of years (YR) for which each junction isstudied, so that for example (7.2) becomes:

A. YR = YR. k Qaα Q

bß (7.5)

where (A.YR) is the number of accidents at the junctionduring the several years of the period studied. This alsoapplies to the models described in the following sections(7.2.2, 8.2 and 9.2).

7.2.2 FactorsIn order to test the effect on accidents of the main featuresof the junctions, the data were grouped into two mutuallyexclusive subsets (junctions ‘without’ and ‘with’ thefeature). This grouping was done by defining a factor foreach feature which has a level value of 1 for junctions‘without’ the feature and 2 for those ‘with’.

The effect of including a two-level factor is to add one‘dummy’ variable (taking only the values 0 or 1) to themodel to represent the feature:

A = k Qaα exp{γ D} (7.6)

A = k Qaα Q

bß exp{γ D} (7.7)

A = k Qaα PTß exp{γ D} (7.8)

A = k Qaα exp{b PTß + γ

D} (7.9)

where D is the dummy variable and γ is the coefficient

estimated by the regression for the second level of the factor.

7.2.3 Flow functions and junction featuresThe basic vehicle and pedestrian flows are illustrated inFigure 1a for 3-arm and Figure 1b for 4-arm sites. The

flow functions appearing in the models in this report aregiven in Appendix B for 3- and 4-arm junctions separately.The vehicle flows are average 24-hour flows (inthousands) over the study period, whilst the pedestrianflows are 12-hour flows (in thousands) for the day onwhich the counts were taken.

Factors used in the models are also listed in Appendix B.

7.3 Total accidents (all operational forms combined)

Tables C1 to C3 in Appendix C present the whole junctionmodels developed for total, vehicle-only and pedestrianaccidents respectively, for each junction type.

Models that include only vehicle flows are included fortotal accidents in Table C1. These models did notgenerally provide the best fit; they are presented here fortotal accidents at each junction type because they arepublished for many types of junction and they can providea useful broad estimate of risk at one-way sites.Alternatives are also given so that the ‘total inflow’ and the‘encounter product flow’ models are always included.

Where pedestrian flow was significant, this model ispresented in Tables C1 and C2. In addition, the best-fittingmodel including vehicle flow, pedestrian flow and factorswhere significant is presented.

For pedestrian accidents (Table C3), all the modelspresented include both vehicle and pedestrian flows (withfactors, where significant).

As an example, consider total accidents at 3-arm prioritysites. The best-fitting vehicle-flow function was the totalinflow at the junction QT:

A = 0.0580 QT0.865

P31 P21

P13 P12P23

P32

Q1

Q2 Q4

Q5 Q6 Q3

Arm 1 Arm 2

Arm 3

Figure 1a Vehicle flows (Qn) and pedestrian flows (P

nn) at 3-arm sites

12

The function of pedestrian flow used was the ‘off-kerb’flow PK, giving the best-fitting flow-only model as:

A = 0.0110 QT1.075 exp{0.874.PK0.3}

In addition the factor LON, indicating whether the sitewas in London, was significant, giving the model:

A = 0.0210 QT0.875 exp{0.706.PK0.3 + 0.364 LON}

The effect of the factor LON is to increase accidents by1.4 at 3-arm priority sites in London.

7.4 Total accident models by operational form

Separate whole junction models were also developed foreach operational form of junction; the results are displayedin Appendix D for total, vehicle-only and pedestrianaccidents. A wide variety of appropriate flow functionswere tested, but only a single model is included here foreach operational form. The flow functions were not alwayssignificant even where there were a substantial number ofsites with the same operational form. It was not generallythe case that the flow function was the same as that for themodel for all operational forms combined; where it was,the exponents were not necessarily the same.

8 Accident-flow models by accident group

8.1 Introduction

In the second stage of modelling of accidents at junctionson one-way roads, the accidents have been divided intogroups as shown in Table 13. It should be noted that theseaccident groups are not in general the same as those inSection 5. Since each junction type includes a variety ofcombinations of one-way inbound, one-way outbound andtwo-way arms, it was considered more appropriate todevelop arm-based models by accident group and the newgroups were selected to be appropriate for arm-basedmodels. The relationship between the original and revisedaccident groups is indicated in Summersgill et al. (2001).

The group of single vehicle accidents remainedunchanged. These accidents are non-pedestrian accidentsin which the vehicle is approaching the junction on the armof association, is in the junction centre or is exiting thejunction on another arm having originally entered on thearm of association.

For two-vehicle accidents, the main groups were:

� Same direction on entry accidents (same entry, same exit).

� Diverging accidents (same entry, different exit).

� Crossing accidents (different entry, different exit).

� Merging accidents (different entry, same exit).

Arm 2

Arm 4

Arm 1 Arm 3

P41

P21

P31

P14

P34

P43

P13P23

P42

P12

P32

P24

Q1

Q9

Q7

Q8

Q12

Q11

Q10

Q6Q4

Q5

Q3

Q2

Figure 1b Vehicle flows (Qn) and pedestrian flows (P

nn) at 4-arm sites

13

At 4-arm sites, the crossing accidents were furtherdivided into right angle crossing accidents and ‘other’crossing accidents.

‘Other’ vehicle accidents included all the remainingtwo-vehicle accidents (other left turn accidents, head-onaccidents, U-turns, exiting accidents, parked and parkingaccidents and accidents at private drives and side roads).

There were four pedestrian accident groups. The twomain groups involved pedestrians crossing the road andbeing hit by vehicles entering and exiting the junctionrespectively. Since there were substantial numbers ofaccidents involving pedestrians being hit while crossingthe centre of the junction (except at 4-arm priority sites), aseparate group was formed for these. At 4-arm sites,however, the large mixture of vehicle movements andpedestrian movements between junction central islandsmade it impossible to group accidents under a particulararm of association in any meaningful way, so all suchaccidents at the junction have been grouped together.Remaining pedestrian accidents were grouped into ‘other’pedestrian accidents.

At each of the four junction types, there were a numberof accidents involving vehicles contravening the one-waysystem, or at 4-arm signals only, making a bannedmanoeuvre. These accidents were excluded from the mainaccident groups and no models were developed for them.

Apart from the ‘contravention’ accident group(accidents involving a vehicle contravening the one-wayrule), all except two groups had at least 20 accidents,which was considered the absolute minimum number for aviable accident model. For these two groups (divergingaccidents at 3-arm signal sites - 12 accidents - andpedestrian across centre accidents at 4-arm priority sites –5 accidents), a viable model was not feasible, but onbalance the grouping was the best that could be achieved.It was not feasible to develop models for individualoperational forms for each accident group.

The choice of arm-based models is in contrast to thewhole junction analysis adopted for T-junctions bySummersgill et al. (1996) and the major/minor analysis

for 4-arm priority junctions by Layfield et al. (1996),but is the same as that used for mini-roundabouts(Kennedy et al., 1998), for 4-arm roundabouts(Maycock and Hall, 1984) and for signal junctions(Hall, 1986; Taylor et al., 1996).

The accident frequency for each accident group wasrelated to various functions of the vehicle and pedestrianflows and to the main features. For nearly all of theanalysis, each arm of a junction and the accidentsassociated with it were considered as individual units. Thenumber of arm units analysed for each accident groupvaried according to the accident type. For instance,diverging accidents can only occur on those arms wheretwo diverging flows are present.


The vehicle and pedestrian flows used in the modeldevelopment were those illustrated in Figures 1a and 1band appropriate sums and products, for example thevehicle flow entering the junction on the arm ofassociation. Although a range of vehicle and pedestrianflow functions was tested, the most logical forms generallyproduced the best fit to the data.

The form of the models varied according to the accidentgroup. For vehicle-only accident groups involving onlyone stream of vehicles, a model of the form of equation(7.1) was used, whereas for groups representing collisionsbetween vehicles in different streams, the model was of theform of equation (7.2). For pedestrian accidents, models ofthe form of equation (7.3) were used. Non-collision flowswere also tested in all groups and where necessaryincluded, using a model of the form of equation (7.8). Theflow terms were usually a single stream, but in some caseswere combinations of different flows.

The factors that appeared in the models are the same asthose for the whole junction models (Table B3 inAppendix B).

Table 13 Numbers of accidents by group and site type

3-arm priority 4-arm priority 3-arm signal 4-arm signal

Number of Number of Number of Number ofGroup Description accidents % accidents % accidents % accidents %

1 Single vehicle on entry 120 12.0 31 5.9 119 15.8 112 8.42 Same direction on entry 172 17.1 36 6.9 176 23.3 196 14.63 Diverging 53 5.3 37 7.0 12 1.6 65 4.94 Right angle crossing – – 149 28.4 – – 271 20.35 Other crossing 34 3.4 21 4.0 28 3.7 94 7.06 Merging 79 7.9 28 5.3 24 3.2 30 2.27 Other vehicle 178 17.7 40 7.6 79 10.5 86 6.48 Pedestrian, vehicle entering 121 12.1 66 12.6 111 14.7 151 11.39 Pedestrian, vehicle exiting 99 9.9 66 12.6 75 9.9 175 13.110 Pedestrian across centre or to/from central island 64 6.4 5 1.0 76 10.1 40 3.011 Other pedestrian 68 6.8 31 5.9 48 6.4 86 6.4

Accidents contravening one-way 16 1.6 15 2.9 7 0.9 32 2.4

Total 1004 100 525 100 755 100 1338 100

14

9 Accident-flow-geometry models byaccident group

9.1 Introduction

In the third stage of modelling, the accident-flow modelsdeveloped in Section 8 for each junction type and eachmain accident group were extended by the inclusion ofgeometric variables and other site characteristics.


The forms of models used to examine the effects of theadditional geometric and other site characteristics (which areof either a discrete or continuous nature) are shown below:

A = k Qaα exp { Σ γ

ij D

ij + Σ ε

i G

i } (9.1)

A = k Qaα Q

bß exp { Σ γ

ij D

ij + Σ ε

i G

i } (9.2)

A = k Qaα exp { bQ

bß + Σ γ

ij D

ij + Σ ε

i G

i } (9.3)

where A is the accident frequency (peryear per site);

Qa, Q

bare functions of the vehicleand pedestrian flows;

Dij

(j = 2, n) are dummy variables(taking only the values 0 and 1)representing the 2nd to nthlevel of each discrete factor;

Gi

are continuous variables e.g.flow proportions, geometricand site variables, signalcontrol variables;

k, α, ß, b, γij, and ε

iare parameters to be estimated.

9.3 Variables and factors

The vehicle and pedestrian flow functions from thepreferred accident-flow models developed in Section 8 wereused as the basis of the accident-flow-geometry models.

The wide range of geometric and other featuresmeasured at each junction, as outlined in Section 3, andothers derived from these were used at this stage of themodelling. These were either continuous variables ordiscrete variables (factors).

Starting from the accident-flow model, a forwardselection procedure was used to build up the modelsequentially. A number of criteria were applied to decidewhich terms to include:

i statistical significance of the variable at the five per centlevel;

ii the plausibility of the effect. It was desirable that theeffect of a variable or factor was comprehensible interms of simple logic, common sense and trafficengineering judgement;

iii the stability of the model. If variables or factors areassociated with each other, then introducing one willtend to strongly affect the model parameters for theother. Since causal rather than associative variables are

sought, such instability was carefully investigated andoften resulted in the selection of one variable inpreference to another, or the rejection of a variableapparently introducing such instability.

9.4 Outline of key results

The following paragraphs give the effect of some of themain explanatory variables and factors. Full details aregiven in Summersgill et al. (2001).

One-way and two-way armsWhether the arm of association or an adjacent or oppositearm was one-way or two-way had an effect on accidentrisk. More conflict points were associated with higheraccident risk.

Vehicle flow proportionsThe proportions of certain vehicle types in the flow wereassociated with increased risk for various accident groups,for example: single vehicle accidents at junctions withhigher proportions of buses (buses and motorcycles for3-arm junctions, buses and two-wheelers for 4-arm signaljunctions).

Number and width of lanesAt 3-arm priority sites, wider entries were associated withincreased risk of ‘same direction on entry’ accidents.

At 4-arm sites, a wider minimum exit width or more exitlanes on the opposite arm was associated with fewer rightangle crossing accidents. At 4-arm priority sites, rightangle crossing accidents increased with the number of leftor ahead lanes.

IslandsAt 3-arm priority sites, kerbed central islands wereassociated with fewer merging and diverging accidents.However at signal sites, these islands were associated withincreased pedestrian accidents.

Speed variablesNone of the speed variables were included in the models for3-arm priority junctions. At 4-arm priority junctions, a highermean speed on the opposite arm was associated with anincrease in merging accidents. The coefficient of variation ofthe mean off-peak speed was associated with an increase insingle vehicle accidents at 3-arm signal junctions. At 4-armsignal junctions, the mean off-peak speed was associated withan increase in ‘other’ crossing accidents. In addition themodels contain other variables that are correlated with speed,for example visibility and the presence of a major junctionwithin 200m. Longer visibility was associated with anincrease in accident risk, whereas the presence of a majorjunction within 200m reduced accident risk.

Queueing variablesQueueing variables affected a number of accident groupsat priority junctions, with the effect that longer queueswere generally associated with increased accidents.

15

Angles

There were fewer right angle crossing accidents at 4-armpriority junctions and fewer diverging accidents at 4-armsignal junctions at sites where the angle between one pairof opposite arms differed from 180 degrees.

Bus stops, bus bays and parking bays

The presence of a bus stop, bus bay or parking bay close tothe junction was associated with increased accident risk fora number of accident groups.

Pedestrian crossings and guard railing

At priority junctions, the presence of a pedestrian crossingfacility on the arm within 20m of the junction wasassociated with an increase in ‘same direction on entry’accidents (for example rear shunts) and in some pedestrianaccidents.

The presence of guard railing on the entry corner or oneither side of the arm was associated with increasedaccident risk for a number of accident groups, particularlypedestrian accidents.

Signal staging

At 4-arm signal sites, the presence of another stage either:

between the ‘ahead’ from the previous arm and the‘ahead’ from the current arm in the usual sequence, or

between the ‘ahead’ from the current arm and the‘ahead’ from the previous arm,

was associated with fewer right angle crossing accidents.At sites where the principal right turn conflict

movements never occurred together, accident risk wasreduced for ‘other’ crossing accidents.

London factorSites in Greater London were associated with increasedrisk for single vehicle accidents at all except 4-arm prioritysites and also for ‘same direction on entry’ accidents atsignal sites and ‘pedestrian accidents with vehiclesentering the junction’ at 4-arm signal sites.

10 Application of the models

Accident predictive models have been developed at threelevels: whole junction models (Section 7); accident-flowmodels by accident group (Section 8); and full accident-flow-geometry models by accident group (Section 9). Theapplications for which each of these sets of models aresuitable are discussed below.

10.1 Total accident-flow models

These models treat the whole junction as a unit. They caneither predict the total number of accidents overall orpredict the vehicle-only and pedestrian accidentsseparately. The models are built from functions of theindividual vehicle streams, functions of individualpedestrian crossing flows and a limited number of factors

representing the main characteristics of the junction. Thisrestricts their application in two ways.

Firstly, the flow functions that are used at this aggregatelevel of modelling may not properly represent the differentinteractions of vehicle and pedestrian flows for accidents ofdifferent types, and the effect on safety of particular vehicleclasses is ignored. Secondly these models are likely toinclude associative as well as causal factors. Hence they arenot reliable indicators of the features that would be suitablefor attention in accident remedial treatment.

In general, these models can be expected to providereasonably good predictions of total accident numbersfrom a limited knowledge of the characteristics of thejunction. For urban traffic management assessment theyare likely to be of most use outside the immediate areawhere remedial measures are to be applied - for example,on untreated diversion routes. They could also be useful inthe economic appraisal of road schemes where broaddecisions need to be taken before the detailed design ofindividual intersections is considered.

10.2 Accident-flow models by accident group

These models are arm-based and offer the capability topredict separately for the different accident groups defined.They are built from individual vehicle and pedestrianflows relevant to the type of accident in question, andinclude factors. They serve largely as an intermediate stagein the development of the full accident-flow-geometrymodels but nevertheless have potential for application intheir own right. They were developed for each junctiontype but not for the individual operational forms.

These models also have the limitation that they arelikely to include some associative as well as causal factorsand are therefore not reliable indicators of the features thatdirectly affect accident risk. However, they have theadvantage that they do reflect the different flowinteractions associated with different accident types.

The applications for these models are likely to be similarto those for the total accident-flow models but the resultingaccident predictions seem likely to be somewhat better.

10.3 Full accident-flow models by accident group

The development of these full models was the keyobjective of this project. These models are again arm-based and predict separately for the different accidentgroups. At this stage, all plausible, measurable, physicalvariables and factors that might affect accident risk havebeen tested as far as is practicable in the models. It istherefore likely that the resulting relationships arecausative (but that is not to say that the mechanisms arefully understood). The models indicate the physicalvariables and factors that have an effect on accident riskand which might be considered in accident remedialtreatment. They were developed for each junction type, butnot for individual operational forms.

The models take full account of the interactions betweenvehicle and pedestrian flows for each accident type, and ofvehicle class. Their only disadvantage is the amount ofdetailed information required to use them.

16

The main application of these models is likely to be inurban road design and traffic management appraisal. Inthis context, they are the only forms of model availableallowing proper evaluation of accidents at junctions withone-way arms subject to remedial treatment or other directtraffic engineering measures.

10.4 Software

The computer program SafeNET (TRL, 1999) can be usedto estimate the frequency of injury accidents for a roadnetwork (junctions of different types and road sections).The total number of accidents predicted in the network isobtained as a sum of the estimates for each junction orroad section. SafeNET can therefore be used as an aid intraffic/safety management studies for an area, either as astand-alone program or linked to an assignment program togive a complete assessment of traffic flow, delays andqueueing as well as safety.

SafeNET incorporates whole junction accident-flowmodels (Section 7) for junctions on one-way roads,including total accident-flow models by operational form.

11 Summary and conclusions

A full-scale study of accidents at 3- and 4-arm priority and3- and 4-arm signal junctions on one-way roads subject toa 30 miles/hr speed limit has been carried out. The studywas based on a national stratified sample of 205 3-arm, 814-arm priority and 84 3-arm, 120 4-arm signal sites at 433junctions, and on 3,622 personal injury accidents whichoccurred at these junctions during the eight-year period1987 to 1994.

Full details of the vehicle and pedestrian flows, and ofall other physical variables and factors that might affectaccident risk at this type of junction, were measured.

Accident tabulations have been prepared that give usefulinsights into the characteristics of the accidents. However,since they relate to a stratified sample of junctions, they donot necessarily reflect the distribution of characteristics ofthe overall accident population for this junction type.

Accident predictive models have been developed. Themodels presented in this report are junction-based totalaccident models for each junction type and for individualoperational forms. In addition, arm-based accident-flowand accident-flow-geometry models have been developedfor individual accident groups at each junction type andare reported in Summersgill et al. (2001). The modelsrelate annual accident frequency to functions of traffic andpedestrian flows and to characteristics of the junctionlayout and signal control variables.

Some of the key findings of the study are listed below.

i The mean pedestrian flow at the sample of junctions,generally located in town or city centres, was very highand over one third of accidents involved a pedestrian.

ii Comparison of the data for this study of junctions withone or more one-way arms with that fromcorresponding studies of junctions with all two-wayarms showed the following. At 3-arm junctions, both

the mean accident frequency and the mean accidentrate were similar. However at 4-arm junctions, bothaccident frequency and rate were substantially lower ifthere was a one-way arm. The mean severity ofaccidents at both the 3-arm and 4-arm junctions waslower if there was a one-way arm.

iii Accident involvement rates (number of involvementsper 100 million vehicles of the class) were muchhigher for pedal cycles and motorcycles than for carsand light goods vehicles. Comparing with the datafrom the previous samples of junctions with all two-way arms, the relative rate for motorcycles was lowerat junctions with a one-way arm, while relative ratesfor other vehicle types were similar whether or not thejunctions had a one-way arm.

iv Whether the arm of association or an adjacent oropposite arm was one-way or two-way had an effecton accident risk. In general, more conflict points wereassociated with higher risk.

v A high percentage of the sample of junctions(especially 3-arm) had islands which impinge on morethan one arm and which were intended to channel theflow. At 3-arm priority sites, kerbed central islandswere associated with fewer merging and divergingaccidents. However at signal sites, these islands wereassociated with an increase in pedestrian accidents.

vi Four-arm signal sites with a separate signal stagebetween the conflicting right angle movements wereassociated with lower risk of right angle accidents.

vii Variables describing the amount of queueing affecteda number of accident groups at priority junctions, withthe effect that longer queues were generally associatedwith increased accident risk.

viii For all junction types, there was no evidence that thepresence of pedestrian guard railing was associated withfewer pedestrian accidents. This suggests therefore thatthere may be scope for improvement in the design ofthese facilities to achieve their objectives.

ix In the final models, measures of the speed of trafficwere not significant at the 5 per cent level for 3-armpriority junctions. At the other junction types, a highermean speed or higher speed variability were associatedwith increases in some accident types. The modelscontain other variables that are correlated with speed,for example, visibility and the presence of a majorjunction within 200m; it is possible that these othervariables affect accidents through a modifying effecton speed. This study was not intended nor designed toinvestigate speed mechanisms and relationships indepth, and only coarse measures of speed wereincluded. The results do not preclude there being otherengineering measures not included in the study (trafficcalming measures, for example), which could reduceaccidents by reducing approach speeds. Speedmeasurements were made only for freely movingvehicles on the approach to the junction.

x Other key results included the following. Vehicleproportion variables increased risk for various

17

accident types, for example single vehicle accidents at3-arm junctions with higher proportions of buses andmotorcycles. Junctions in Greater London, particularlysignal junctions, had increased risk of some accidenttypes. These results will be more important forprediction than design purposes, since they do notconcern layout variables.

The models are intended for a range of applications:

i to identify potential design improvements - that is, toenable engineers to ‘design out’, as far as possible,features found to result in increased accidents and to‘design in’ those features found to result in reducedaccidents;

ii to provide accident estimates for the economicappraisal of road improvements - that is, to quantifythe safety benefits of improvements to junctions withone-way arms;

iii in conjunction with traffic assignment models, topredict the effect on accidents of traffic managementschemes, to identify casualty-reducing strategies, andto optimise safety/mobility for all road users.Alternative traffic and safety management schemeswhich involve re-distribution of traffic, and the re-design of junctions to accommodate changed trafficand pedestrian flows safely may be assessed.

Whole junction accident-flow models of junctions withone-way arms from the current study have beenincorporated into the computer program SafeNET, whichcan be used to estimate the frequency of injury accidentsfor a road network (junctions of different types and roadsections) operating under different traffic/safetymanagement schemes.

12 Acknowledgements

The work described in this Report forms part of theresearch programme of the Road Safety Division of theDepartment for Transport, Local Government and theRegions. The study was conducted by means of an extra-mural contract with the University of Southampton’sTransportation Research Group. TRL was responsible forthe overall planning of the study and its initial phases,management and reporting and has contributed extensivetechnical input to all stages of the project.

The authors are also grateful to: the Local HighwayAuthorities for supplying the relevant accident informationand details of any changes to the sites during the studyperiod; members of STC Division (DTLR) for the supplyof traffic flow scaling factors; and TRL colleagues for thesupply of STATS 19 accident data and for valuablediscussions during the course of the work.

13 References

Alvey N G et al. (1977). GENSTAT: A General StatisticalProgram. Harpenden: Rothamsted Experimental Station.

Baker R J and Nelder J A (1978). Generalised LinearInteractive Modelling - the GLIM system. Harpenden:Rothamsted Experimental Station.

Hall R D (1986). Accidents at four-arm singlecarriageway urban traffic signals. Contractor ReportCR65. Crowthorne: TRL Limited.

Kennedy J V, Hall R D and Barnard S R (1998).Accidents at urban mini-roundabouts. TRL Report TRL281.Crowthorne: TRL Limited.

Layfield R E, Summersgill I, Hall R D and Chatterjee K(1996). Accidents at urban priority crossroads andstaggered junctions. TRL Report TRL185. Crowthorne:TRL Limited.

Maher M J and Summersgill I (1996). A comprehensivemethodology for the fitting of predictive models. AccidentAnalysis and Prevention, Vol 28, No. 3. pp. 281-296.

Maycock G and Hall R D (1984). Accidents at four-armroundabouts. Laboratory Report LR1120. Crowthorne:TRL Limited.

Maycock G and Maher M J (1988). Generalised linearmodels in the analysis of road accidents - somemethodological issues. International Symposium on TrafficSafety Theory and Research Methods. Institute for RoadSafety Research (SWOV). Netherlands: Leidschendam.

Pickering D, Hall R D and Grimmer M (1986).Accidents at rural T-junctions. Research Report RR65.Crowthorne: TRL Limited.

Summersgill I, Kennedy J V and Baynes (1996).Accidents at three-arm priority junctions on urban singlecarriageway roads. TRL Report TRL184. Crowthorne:TRL Limited.

Summersgill I and Layfield R E (1996). Non-junctionaccidents on urban single carriageway roads. TRLReport TRL183. Crowthorne: TRL Limited.

Summersgill I, Kennedy J V, Hall R D , Hickford A andBarnard S R (2001). Accidents at junctions on one-wayurban roads. Papers and Articles PA3769/01. Crowthorne:TRL Limited.

Taylor M C, Hall R D and Chatterjee K (1996).Accidents at 3-arm traffic signals on urban singlecarriageway roads. TRL Report TRL135. Crowthorne:TRL Limited.

TRL (1999). SafeNET User Guide. Application GuideAG34. Crowthorne: TRL Limited.

18

Appendix A: Operational forms

Figure A1 Operational forms at 3-arm priority junctions

(Numbers in brackets are the numbers

of junctions in the study with this form)

1.

3.

7.

9.

2.

4.

6.

8.

10.

1 2

3

1 2

3

1 2

3

1 2

3

1 2

3

1 2

3

1 2

3

1 2

3

1 2

3

(14) (19)

(25) (16)

(22) (20)

(20)(23)

(46)

19

Figure A2 Operational forms at 4-arm priority junctions



11.

1 3

4

2

(6)

13.

1 3

4

2

(14)

12.

1 3

4

2

(9)

14.

1 3

4

2

(14)

16.

1 3

4

2

(13)

18.

1 3

4

2

(13)

19.

1 3

4

2

(12)

20

Figure A3 Operational forms at 3-arm signal junctions



20.

1 2

3

(11)

21.

1 2

3

(21)

23.

1 2

3

(19)

22.

1 2

3

(16)

24.

1 2

3

(17)

21

25.

1 3

4

2

(23)

27.

1 3

4

2

(18)

26.

1 3

4

2

(14)

28.

1 3

4

2

(19)

30.

1 3

4

2

(13)

29.

1 3

4

2

(18)

31.

1 3

4

2

(15)

Figure A4 Operational forms at 4-arm signal junctions



22

Table B1 Vehicle and pedestrian flow functions for whole junction models at 3-arm sites, with range and mean values

(Values without brackets are for priority sites, values with brackets are for signal sites)Mean

Flow function (refer to Figure 1A, main text) Min (thousands) Max

QT Q1+Q2+Q3+Q4+Q5+Q6 Total inflow 1.61 18.54 50.84[8.97 28.79 76.94]

QD Q1.Q2+Q3.Q4+Q5.Q6 Diverging flow products 0.0 29.18 277.26[0.0 82.43 742.94]

QM Q1.Q6+Q2.Q3+Q4.Q5 Merging flow products 0.05 36.30 368.81[10.4 113.6 689.1]

QR Q2.Q4+Q6.Q2+Q4.Q6 ‘Right turn’ crossing products 0.0 1.48 30.051[0.0 10.54 173.22]

QN QR+QD+QM Encounter flow products 0.56 66.95 646.08[20.2 206.6 1432.1]

QNC 2.QR+QM 0.35 39.25 368.82[12.7 134.7 689.1]

PK P13+P31+P23+P32+P12+P21+ Off-kerb pedestrian flow1 0.066 2.418 23.717flow from entrycorners to island. [0.008 7.839 29.100]

PT P13+P31+P23+P32+P12+P21+ Total junction pedestrian flow2 0.083 2.865 23.717flow between islands and to and [0.02 11.04 50.68]from islands and entry corners.

PK0n PK0.n

PT0n PT0.n

QPR QT.PK Total vehicle and ped 0.38 38.96 234.14off-kerb flow product [0.6 215.5 1230.1]

QPU QT.PT Total vehicle and ped 0.38 49.78 465.61flow product [1.2 307.9 1290.5]

QPV Sum of products of vehicle inflow 0.052 7.697 90.270and ped flow on all arms [0.00 43.28 421.84]

QPW Sum of products of two-way vehicle 0.09 13.36 90.72flows and ped flows on all arms [0.00 60.11 439.66]

1 Off-kerb pedestrian flow PK includes the two-way flow across each arm and the flow from each entry corner to the central junction island (if any).2 Total pedestrian flow PT includes the two-way flow across each arm and the flow from each entry corner to and from the central junction island (if

any) on each arm.

Appendix B: Vehicle and pedestrian flow functions

23

Table B2 Vehicle and pedestrian flow functions for whole junction models at 4-arm sites, with range and mean values


Flow function (refer to Figure 1b, main text) Min (thousands) Max

QT Q1+Q2+Q3+Q4+Q5+Q6+ Total inflow 4.10 13.67 32.11Q7+Q8+Q9+Q10+Q11+Q12 [4.30 24.81 63.40]

QD Q1.Q2+Q2.Q3+Q3.Q1+ Diverging flow products 0.11 14.60 110.43Q4.Q5+Q5.Q6+Q6.Q4+ [1.68 54.30 332.21]Q7.Q8+Q8.Q9+Q9.Q7+Q10.Q11+Q11.Q12+Q12.Q10

QM Q1.Q11+Q1.Q9+Q11.Q9+ Merging flow products 0.61 11.45 77.97Q4.Q2+Q4.Q12+Q2.Q12+ [1.80 63.14 600.98]Q7.Q5+Q7.Q3+Q5.Q3+Q10.Q8+Q10.Q6+Q8.Q6

QTA Q1+Q2+Q3+Q7+Q8+Q9 Entering flow arms 1 and 3 2.73 11.41 31.61[0.15 12.16 44.07]

QTI Q4+Q5+Q6+Q10+Q11+Q12 Entering flow arms 2 and 4 0.236 2.258 9.018[0.21 12.65 46.73]

QXA Q2.Q5+Q5.Q8+Q8.Q11+ Sum of right angle 0.03 6.324 53.855Q11.Q2 crossing products [0.0 50.34 302.90]

QXB Q2.Q6+Q5.Q9+Q8.Q12+ Sum of products of 0.0 1.729 16.133Q11.Q3+Q2.Q9+Q5.Q12+ ahead with next or [0.0 15.20 230.61]Q8.Q3+Q11.Q6 opposite right flows

QXR Q3.Q6+Q6.Q9+ Adjacent right turn flow 0.0 0.2014 4.025Q9.Q12+Q12.Q3 products [0.0 1.174 15.325]

QXRR Q3.Q9+Q6.Q12 Opposite right turn 0.0 0.1627 7.279flow products [0.0 2.713 114.03]

QN QD+(QXA+QXB+QXR)+ Encounter flow products 3.60 34.63 162.252.QXRR+QM [4.4 189.6 1008.4]

QAI QTA.QTI Cross product 2.57 22.12 84.22(‘major’ * ‘minor’) [1.7 142.4 779.3]

QNL (QXA+QXB+QXR)+ 0.182 8.58 56.3772.QXRR [0.0 72.14 595.99]

QML Q1.Q11+Q1.Q9+Q4.Q2+ Left turn merging flow 0.058 7.432 39.169Q4.Q12+Q7.Q5+Q7.Q3+ products [0.0 40.56 504.31]Q10.Q8+Q10.Q6

QNR (QXA+QXB+QXR)+ 0.44 12.13 64.89QXRR+0.5*QML [1.29 89.70 537.64]

QTH Q2+Q5+Q8+Q11 Sum of ahead flows 2.59 10.50 31.46[1.66 15.05 43.15]

PK Off-kerb pedestrian flow1 0.158 4.631 43.659[0.206 8.737 39.951]

PT Total junction 0.158 4.746 43.659pedestrian flow2 [0.38 10.19 50.86]

PK0n PK0.n

PT0n PT0.n

QPR QT.PK Total vehicle and ped. 1.15 70.37 912.30off-kerb flow product [5.2 207.1 1382.6]

QPU QT.PT Total vehicle and ped. 1.15 72.78 912.30flow product [5.2 250.6 1671.5]

Continued ....

24

Table B2 (Continued) Vehicle and pedestrian flow functions for whole junction models at 4-arm sites, with range andmean values


Flow function (refer to Fig 1b, main text) Min (thousands) Max

QPV QE1.(P41+P14)+ Sum of products of vehicle 0.22 10.71 85.37QE2.(P12+P21)+ inflow and pedestrian [0.20 40.51 249.88]QE3.(P32+P23)+ flow on all armsQE4.(P34+P43)

QPW (QE1+QX1).(P41+P14)+ Sum of products of two- 0.55 20.91 132.92(QE2+QX2).(P12+P21)+ way vehicle and [0.34 75.41 502.81](QE3+QX3).(P32+P23)+ pedestrian flow on all arms(QE4+QX4).(P34+P43)

QPQ Sum of products of 0.55 22.84 147.81vehicle and all ped flows [1.52 94.04 504.67]encountered (all arms)

1 Off-kerb pedestrian flow PK includes the two-way flow across each arm and the centre of the junction when no central islands are present. When acentral island or one or more splitter islands are present, the flows across the junction centre are replaced where appropriate by the flows from theentry corners to the island.

2 Total pedestrian flow PT is the same as PK when no central or splitter islands are present. When a central island or one or more splitter islands ispresent, the flows across the junction centre are replaced where appropriate by the two-way flows between the entry corners and islands and theflows between islands.

Table B3 Junction factors

Number of sites

3-arm 4-arm 3-arm 4-armpriority priority signal signal

LON London site1= not London 146 64 52 732= London 59 17 32 47

ISL Junction central island(3-arm sites only)1 = absent 112 – 32 –2 = present 93 – 52 –

25

Table C2 Total vehicle-only accident-flow models by junction type (all operational forms combined)

Number of Exponent ofJunction type accidents Description Constant (k) Vehicle flow1 vehicle flow (α) Pedestrian term and factors1

3-arm priority sites 651 Vehicle flow 0.0183 QT 1.104Without factors 0.00863 QT 1.254 exp{0.183 PK07}With factors 0.0182 QT 0.970 exp{0.116 PK07 + 0.504 LON}

4-arm priority sites 354 Vehicle flow 0.193 QNR 0.508Without factors '' '' ''With factors '' '' ''

3-arm signal sites 443 Vehicle flow 0.0295 QNC 0.749Without factors 0.0164 QNC 0.791 exp{0.128 PT05}With factors 0.0258 QNC 0.604 exp{0.093 PT05 + 0.584 LON +

0.390 ISL}

4-arm signal sites 883 Vehicle flow 0.0769 QTH 1.076Without factors '' '' ''With factors 0.0863 QTH 0.934 exp{0.596 LON}

1Defined in Appendix B

Table C1 Total accident-flow models by junction type (all operational forms combined)

Number of Exponent ofJunction type accidents Description Constant (k) Vehicle flow1 vehicle flow (α) Pedestrian term and factors1

3-arm priority sites 1004 Vehicle flow 0.0580 QT 0.865Alternative 0.224 QN 0.301Without factors 0.0110 QT 1.075 exp{0.874.PK03}With factors 0.0210 QT 0.875 exp{0.706 PK03 + 0.364 LON}

4-arm priority sites 525 Vehicle flow 0.270 QAI 0.430Alternative 0.196 QN 0.458Alternative 0.119 QT 0.793Without factors 0.0468 QAI 0.402 exp{1.622.PK01}With factors '' '' '' ''

3-arm signal sites 755 Vehicle flow 0.237 QNC 0.432Alternative 0.234 QN 0.402Alternative 0.105 QT 0.848Without factors 0.120 QNC 0.465 exp{0.099 PT07}With factors 0.186 QNC 0.348 exp{0.092 PT07 + 0.320 LON}

4-arm signal sites 1338 Vehicle flow 0.257 QTH 0.794Alternative 0.179 QN 0.489Alternative 0.0872 QT 0.996Without factors 0.0491 QTH 0.833 exp{1.274 PT01}With factors '' '' '' ''


Appendix C: Whole junction models by junction type

26

Table C3 Total pedestrian accident-flow models by junction type (all operational forms combined)

Number of Vehicle Exponent of Pedestrian1 Exponent ofJunction type accidents Description Constant (k) flow1 vehicle flow (α) flow pedestrian flow (b)

3-arm priority sites 353 Flow 0.0272 QT 0.614 PT 0.562With factors '' '' '' '' ''

4-arm priority sites 171 Flow 0.0272 QT 0.738 PK 0.383With factors '' '' '' '' ''

3-arm signal sites 312 Flow 0.0530 QPU 0.484With factors '' '' ''

4-arm signal sites 455 Flow 0.0440 QT 0.604 PT 0.430With factors '' '' '' '' ''


27

Table D1 Total accident models by junction type and operational form

Number of Vehicle Exponent of PedestrianOperational form accidents Constant (k) flow1 vehicle flow (α) flow term1

3-arm priority sites1 63 0.181 QN 0.3412 97 0.0371 QN 0.8343 92 0.172 QPR 0.3624 144 0.145 QM 0.4556 298 0.000449 QT 1.230 exp{3.60 PK01}7 60 0.0269 QD 0.8268 35 0.0395 QT 0.9719 96 0.000143 QT 1.223 exp{4.53.PK01}10 119 0.0000246 QT 1.727 exp{5.22 PT01}

4-arm priority sites11 29 0.64712 72 0.0270 QN 0.272 exp{2.29 PK01}13 85 0.0384 QN 0.85014 87 0.142 QN 0.57516 110 0.167 QN 0.57118 67 0.0172 QT 1.280 exp{0.057 PT}19 75 0.0298 QT 0.450 exp{1.97 PT02}

3-arm signal sites20 93 0.0205 QN 0.708 exp{0.072 PT}21 153 0.0343 QM 0.710 exp{0.083 PK}22 156 0.00125 QN 0.388 exp{4.38 PT01}23 127 0.0871 QN 0.57924 226 0.0239 QT 0.331 exp{2.87 PT01}

4-arm signal sites25 208 0.180 QTH 0.91226 167 0.200 QTH 0.88527 196 0.00190 QT 0.983 exp{3.12 PT01}28 160 0.0000690 QT 1.566 exp{4.12 PK01}29 276 0.0746 QN 0.71830 167 0.719 QTH 0.47031 164 0.0238 QN 0.781 exp{0.052 PT}


Appendix D: Whole junction models by junction type and operational form

28

Table D2 Vehicle-only accident models by junction type and operational form

Number of Vehicle Exponent of PedestrianOperational form accidents Constant (k) flow1 vehicle flow (α) flow term1

3-arm priority sites1 39 0.0197 QN 0.7892 65 0.00592 QN 1.2253 57 0.187 QT 0.2694 97 0.0406 QM 0.6636 194 0.00393 QT 1.368 exp{0.429 PT04}7 35 0.00477 QD 1.1698 25 0.0388 QT 0.8379 62 0.00181 QT 1.527 exp{0.718.PK04}10 77 0.000639 QT 2.006 exp{0.802 PT04}

4-arm priority sites11 15 0.33512 53 0.168 QN 0.45513 48 0.0214 QN 0.85214 54 0.194 QNL 0.72916 86 0.301 QNL 0.50918 44 0.0669 QAI 0.88319 54 0.601 QXA 0.291

3-arm signal sites20 58 0.0255 QN 0.73821 94 0.00712 QM 0.97522 95 0.00767 QN 0.666 exp{0.835 PT03}23 56 0.00673 QN 0.92324 140 0.0375 QT 1.034 exp{0.747 ISL}

4-arm signal sites25 137 0.181 QTH 0.75026 115 0.049 QTH 1.076 exp{0.782 LON)27 119 0.000962 QN 0.732 exp{2.723 PT01}28 91 0.0255 QT 0.971 exp{0.960 LON}29 204 0.0138 QN 0.97330 101 0.129 QD 0.66731 116 0.0684 QD 0.842

29

Table D3 Pedestrian accident models by junction type and operational form

Number of Vehicle Exponent of Pedestrian1 Exponent ofOperational form accidents Constant (k) flow1 vehicle flow (α) flow pedestrian flow (b)

3-arm priority sites1 24 0.0317 QPR 0.5562 32 0.145 QPV 0.1933 35 0.0637 QPW 0.5044 47 0.0944 QPU 0.3336 104 0.0133 QT 0.939 PK 0.6447 25 0.0351 QD 0.419 PT 0.2958 10 0.0121 QPR 0.7989 34 0.0731 QR 0.411 PK 0.43610 42 0.00156 QTA 1.695 PT 1.141

4-arm priority sites11 14 0.0220 QPW 0.77612 19 0.0330 QPU 0.48113 37 0.0419 QPW 0.70514 33 0.0950 QPV 0.54016 24 0.0995 QPV 0.49618 23 0.0227 QPW 0.85019 21 0.0984 QPW 0.479

3-arm signal sites20 35 0.00412 QPV 1.00921 59 0.0843 QPR 0.40922 61 0.0662 QPR 0.52423 71 0.121 QPR 0.35624 86 0.0534 QPU 0.523

4-arm signal sites25 71 0.0405 QPQ 0.59926 52 0.171 QPU 0.26227 77 0.0727 QPQ 0.60228 69 0.00481 QT 1.061 PT 0.64129 72 0.397 QPW 0.16230 66 0.0663 QPQ 0.60331 48 0.00292 QN 0.782 PK 0.910


30

Abstract

The report gives the findings of a study of accident risk based on a national stratified sample of 433 3-arm and 4-arm, priority and signal junctions having one or more one-way arms on 30 miles/hr urban roads. A total of 3,622personal injury accidents were recorded at the junctions over the period 1987-1994. Tabulations are given showingaccident frequencies, severities and rates by junction type. The main objective of the study was to developrelationships between accident frequency and vehicle and pedestrian flows, features, layout, and signal controlvariables, using the technique of generalised linear modelling.

Related publications

TRL281 Accidents at urban mini-roundabouts by J V Kennedy, R D Hall and S R Barnard. 1998 (price £50, code P)

TRL185 Accidents at urban priority crossroads and staggered junctions by R E Layfield and I Summersgill(TRL Limited), R D Hall and K Chatterjee (University of Southampton). 1996 (price £50, code P)

TRL184 Accidents at three-arm priority junctions on urban single-carriageway roads by I Summersgill,J V Kennedy and D Baynes. 1996 (price £50, code L)

TRL183 Non-junction accidents on urban single-carriageway roads by I Summersgill and R E Layfield.1996 (price £35, code J)

TRL135 Accidents at 3-arm traffic signals on urban single-carriageway roads by M C Taylor, R D Hall andK Chattergee. 1996 (price £50, code N)

CR65 Accidents at four-arm single carriageway urban traffic signals by R D Hall. 1987 (price £25, code F)

RR65 Accidents at rural T-junctions by D Pickering, R D Hall and M Grimmer. 1986 (price £20, code C)

LR1120 Accidents at 4-arm roundabouts by G Maycock and R D Hall. 1984 (price £20)

Prices current at November 2001

For further details of these and all other TRL publications, telephone Publication Sales on 01344 770783, or visitTRL on the Internet at www.trl.co.uk.

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