TRAFFIC STUDY & TRAFFIC FLOW THEORY

7/27/2019 TRAFFIC STUDY & TRAFFIC FLOW THEORY

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By Ikhwan Zainuddin

CHAPTER 1 TRAFFIC STUDYTRAFFIC VOLUME

the number of vehicles passing a given point during a

specified period of time

IMPORTANCE TRAFFIC VOLUME STUDY Traffic planning, design & research

Determine the efficiency of a traffic system

Evaluate the quality of service of a transport facility

Design roads no. of lanes & pavement thickness.

VOLUME STUDY METHODS Manual Count Methods

Automatic Count Methods

Count periods

2 hours (peak period), 4 hrs (morning & evening

peaks), 6 hrs (morning, afternoon & evening peaks),

12 hrs (daytime)

MANUAL COUNT METHOD

1. To determine

Vehicle classification

Turning movements

Direction of travel

Pedestrian movements

Vehicle occupancy

2. Method

Applied when small sample of data is required.

For periods less than one (1) day.

Normal intervals are 5, 10 or 15 minutes.

Counts are not usually taken on Mondays, Fridays

and weekends.

3. Recording Method

Tally Sheets

Mechanical Counting Boards

Electronic Counting Boards

AUTOMATIC COUNT METHOD

1.To determine

Vehicle hourly patterns

Daily or seasonal variations and growth trends

Annual traffic estimates

2. Method

Applied when large sample of data is required.

For periods more than one (1) day

Normal intervals are 1 hour for a 24 hour period

Counts may extend to a week, month or year

3. Recording methods

Portable Counters

Permanent Counters

Videotape

EXAMPLES OF TRAFFIC VOLUME STUDIES

INTERSECTION COUNT

1. Required for

timing traffic signals designing channelization

planning turn prohibitions

computing capacity

analyzing high crash intersections

evaluating congestion

2. Uses manual count method.

3. Simple intersections, a single observer - sufficient

4. Complex intersections, several observers required

PEDESTRIAN COUNT

1. Required for evaluating sidewalk and crosswalk needs

justifying pedestrian signals

timing traffic signals

2. Counts are conducted at intersection crossings,

midblock crossings or along sidewalks.

3. When pedestrians are tallied, those 12 years and

above are classified as adults.

VEHICLE CLASSIFICATION COUNT

1. Used to

establish structural & geometric design criteria

compute estimated highway user revenue

compute capacity

2. Classification counts should be conducted

if the percentage of heavy trucks is high

if vehicle mix at a crash site is a contributing factor

to the crash problem

3. Cars, MPVs, SUVs, 4WDs may be classified aspassenger cars

4 Heavy vehicles such as trucks and buses may be

classified as trucks

ADT & AADT COUNT

1. ADT Average Daily Traffic

Represents 24-hr count.

2. AADT Average Annual Daily Traffic

Represents 24-hr traffic volume averaged over a full

365-day year.

3. Requires the use of an automatic counter.4. Accuracy depends on existing roadway, weather

and traffic demand conditions.

5. AADT volume count has the following uses:

Measuring or evaluating the present demand for

service by the roadway or facility.

Developing the major or arterial roadway system.

Locating areas where new facilities or

improvements to the existing facilities are needed.

Programming capital improvements.


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By Ikhwan Zainuddin

By Ikhwan Zainuddin

SPOT SPEED STUDY A study of speed of traffic at one point or spot on a

trafficway.

Consists of a series or sample of observations of

individual speeds of vehicles approaching an

intersection or passing a point at a non-

intersection.

Used to estimate the speed distribution of an

entire traffic stream at that location under current

traffic conditions.

1. Carried out at

Trend locations straight, level, open sections of

rural highways, and midblock at urban streets.

Problem locations accident blackspots, and

locations where traffic signals and signs are

contemplated.

Representative locations for basic data surveys.Locations of before-and-after studies.

2. When carrying out a spot speed study, make sure

the observer and the measuring instrument does not

influence drivers speed.

The measuring instrument must be consealed from

the approaching drivers.

Recording of data should be made as

inconspicuous as possible.

Accumulation of on-lookers must be avoided.

3. Recommended study periods:

In three parts during off-peak periods: 1 hour

between 6 am 12 pm, 1 hour between 3 pm 6

pm, and 1 hour between 8 pm 10 pm. whereby

observations must be done in an hour, or not less

than 50 vehicles for each period.

4. Course length of a spot speed study depends on :

average speed of the traffic stream

5. Application of Spot Speed Data

For trends in operating speeds of different vehicle

types, which are obtained

from data collected through periodic sampling at

selected locations

For speeds at problem locations, to determine whether

speeds are too high

and if complaints received are justified

For traffic operation (regulation and control)

a. Establishing speed limitsb. Determining safe speeds at curves and at approaches

to intersections

c. Establishing lengths of no-passing zones

d. Locating traffic signs

e. Locating and timing traffic signals

f. School zone protection

g. Establishing speed zones

For accident analysis, to determine the relationship of

speed to accidents which may help in developing

corrective measures

For before-and-after studies, to evaluate the effect ofsome change in controls or conditions

For geometrical design features,

a. Designs assume uniform speeds, and it is important

to evaluate the effects of actual speed distributions

on design features

b. Length of speed change lanes, curvature, super

elevation, and sight distance are directly related to,

and vary appreciably with speed

To evaluate capacity in relation to desired speeds. If all

vehicles travelled at the same speed, capacity would be

at a maximum and certain types of accidents, such asovertaking, passing or rear-end collisions, would be

eliminated

As aid to enforcement,

To determine the effect of speed control measures

To measure the effectiveness of changes in

enforcement programs

Research studies

6. Recommended course lengths

Average speed (mph) Course length (ft)

< 25 88

25 40 176

> 40 264

Recommended sample size: at least 50, preferably

100

Like volume studies, spot speed studies are also not

taken on Mondays, Fridays and weekends.7. Spot Speed Study methods

Stopwatch Method

Radar/Meter Method

Pneumatic Road Tube Method

8. Calculation Step

Create Table Speed Class (km/h)Upper Limit

(km/h)Mid Point, x (km/h) Number of

Observation, ffxfx2 Presentage Total

Observation %Cummulative Percentage

Graph Histogram, Distribution Curve,

Cummulative Distribution Curve


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Find Mean (km/h)

Find Median (km/h)

By calculation, by graph (P50)

Find Pace

Pace = the 10 km/h range in speed in which the highest

number of observation was recorded

Find range of the pace using graph total distribution

Find percentage of vehicle in pace using graph

cumulative percentage total distribution

85th Percentile Speed (By graph P85) The 85th percentile speed is the speed at or below

which 85 % of the motorists drive on a given road

unaffected by slower traffic or poor weather

This speed indicates the speed that most motorists

on the road consider safe and reasonable under

ideal conditions

It is a good guideline for the appropriate speed limit

for that road

The 85th percentile speed as obtained from the

cumulative frequency distribution curve shown

below is 88 km/h Standard Deviation

By calculation

By graph (Cumulative Distribution Curve)


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CHAPTER 2 TRAFFIC FLOW

THEORYTRAFFIC FLOW PARAMETERS

SPEED

defined as rate of motion, or distance per unit time

Space Mean Speed, vs

is the average travel speed

VS = nL

ti

L = length of the highway segment (km)

ti = travel time of the ith vehicle to traverse the section

(hours)

n = number of travel times observed

Time Mean Speed, Vt

is the arithmetic mean of the measured speeds of all

vehicles passing a fixed roadside point during a given

interval of time

Vt = Vi

n

Vi= spot speed (km/hr)

n = number of vehicles observed

Relationship Vs and Vt

VOLUME (V)

actual number of vehicles observed or predicted tobe passing a point during a given time interval

RATE OF FLOW (q) (veh/hr)

q = v x k

represents the number of vehicles passing a point

during a time interval less than 1 hour. Thus, a

volume of 200 vehicles observed in a 10-minute

period implies a rate of flow of (200 60)/10 = 1200

veh/hr.

DENSITY (k)(veh/km)

Number of vehicles occupying a given length of lane or

roadway, averaged over time, usually expressed as

vehicles per kilometer (veh/km)

direct measurement of density (k) can be obtained

through aerial photography, but more commonly it is

calculated from the following equation if speed (v) and

rate of flow (k) are known: q = v k

SPACING (s)(sec/veh)distance between successive vehicles in a traffic stream

as measured from front bumper to front bumper

can be generally observed from aerial photographs

HEADWAY (h)

is the corresponding time between successive vehicles

as they pass a point on a roadway

can be measured using stopwatch observations as

vehicles pass a point on a lane

LANE OCCUPANCY (LO)

the ratio of the time that vehicles are present at a

detection station in a traffic lane compared to the time

of sampling

is a measure used in freeway surveillance

if one could measure the lengths of vehicles on a given

roadway section and compute the ratio:

RATIO (R)

LANE OCCUPANCY (LO)

DENSITY (k)

CLEARANCE (c)

is defined as the distance between successivevehicles in a traffic stream as measured from front

bumpe to back bumper

GAP (g)

Corresponding time between successive vehicles as

they pass a point on a roadway

The difference between spacing and clearance, and

headway and gap are as follows:


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CATEGORIES OF TRAFFIC FLOW

UNINTERRUPTED FLOW

Occurs on facilities that have no fixed elements (such

as traffic signals or stop signs) external to the traffic

stream, that cause interruptions to traffic flow.

Traffic flow conditions are thus the result of

interactions among vehicles in the traffic system and

between vehicles and the geometric characteristics

of the roadway/guideway system.

The driver of the vehicle does not expect to be

required to stop by factors external to the traffic

stream

Uninterrupted Flow facilities: Freeways, Multilane

Highways, Two-lane Highways

INTERRUPTED FLOW

Occurs on facilities that have fixed elements causing

periodic interruptions to traffic flow.

Traffic is stopped or significally slowed down

periodically irrespective of how much traffic exists.

The driver expects to be required to stop as and

when required by fixed elements that are part of the

facility

Interrupted Flow facilities: Signalized streets,

Unsignalized streets with stop signs, Arterials,

Transits, Pedestrian walkways, Bicycle paths.

P/S: Uninterrupted flow and interrupted flow are

terms that describe the facility and not the

quality of flow!

TRAFFIC FLOW MODEL FOR

UNINTERRUPTED FLOW

As vehicles speed and spacing increases, the speeds

approach the free speed, and drivers

adopt their own speed when uninfluenced by other

vehicles in the traffic stream (point C).

The dashed curve represents the normal flow

behaviour if all drivers were to have the same free

speed (point D).

It has been observed that drivers are uninfluenced byother vehicles in the traffic lane at flows of about 900

veh/hr or less, which is about half the capacity flow

(point B).

Maximum traffic density occurs (point A) when traffic

has virtually come to a complete stop.

In the forced flow region, each vehicle adopts its

minimum spacing and clearance distance.

SPEED, FLOW AND DENSITY RELATIONSHIPS

SPEED-DENSITY (v-k) RELATIONSHIPA linear relationship exists between the speed of

traffic on an uninterrupted traffic lane and the traffic

density

As density increases:

flow increases to an optimum with more vehicles on

the road speed decreases due to the interaction of vehicles


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FLOWD

ENSITYRELATIO

NSHIP

SPEEDFLOWRELATIONSHIP

Capacity (qmax) is the maximum flow for the facility:

when area under the v-k curve is at maximum

reached when the product of increasing density and

decreasing speed result in the maximum number of

vehicles passing a certain point

Congested unstable flow = high density, low speed:

no available gaps for entering vehicles

any vehicle that do enter create a disturbance that is

not easily dissipated

Uncongested stable flow = low density, high speed: plenty of gaps available for merging traffic

Note that the maximum flow occurs at of the free

mean speed (Vf). This is also called the optimal speed,

which is denoted by Vm. The value of qmax is determin ed

as follows:


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CHAPTER 3 CAPACITY &

LEVEL OF SERVICECAPACITY

the maximum hourly rate at which persons or vehicles

reasonably can be expected to traverse a point or a

uniform section of a lane or roadway during a given

time period under prevailing roadway, traffic and

control conditions

LEVEL OF SERVICE (LOS)a qualitative measure describing operational

conditions within a traffic stream and their perception

by motorists and/or passengers

6 levels of service:

LOS A Very Good

LOS B

LOS C

LOS D

LOS ELOS F Very Poor

Parameters which determine the LOS of a highway:

Flow

Speed

A MULTILANE HIGHWAYHas a posted speed limit of 60 to 90 km/h.

Has a total of 4 to 6 lanes.

Often is divided (has a median), can also be

undivided.

Is situated in suburban communities, leading into

cities.

Is also situated along high-volume rural corridors

connecting two cities.

Has traffic signals spaced at 3 km or less.

Accommodates 15,000 40,000 vehicles per day.

May accommodate as high as 100,000 veh/day when

access across the median is restricted and when all

major crossings are grade separated.

THE BASE CONDITIONS FOR A MULTILANE

HIGHWAYUnder BASE CONDITIONS,

full speed and capacity can be achieved.

3.6 m minimum lane width.

3.6 m minimum total lateral clearance in the direction

of travel.

Only passenger cars in the traffic stream.

No direct access points along the roadway.

Highway is divided.

Free flow speed (FFS) is greater than 100 km/h.

Total Lateral Clearance=Median Lateral+Shoulder Lateral

Clearance Clearance

Total Lateral Clearance = 1.6 + 1.2 = 2.8 m

* If lateral clearance (shoulder or median) is greater than

1.8 m, the lateral clearance is taken as 1.8 m.

THE PREDICTION OF LOS FOR A MULTILANE

HIGHWAY INVOLVES 3 STEPS:

DETERMINATION FREE FLOW SPEED (FFS)

(a) Field Measurement

Average of all passenger car speeds measured in field

under low volume conditions can be directly used as the

FFS if such measurements were taken at or below 1400pc/hr/lane

(b) Estimation

FFS = BFFS fLW fLC fM fA

where BFFS = base free flow speed

fLw = adjustment for lane width

fLC = adjustment for lateral clearance

fM = adjustment for median

fA = adjustment for access point density

BFFS is assumed to be 3 km/h lower than the 85th

percentile speed.

Recent studies suggests that BFFS is approximately

11 km/h higher than the speed limits of 65 km/h

and 70 km/h, and it is 8 km/h higher for 80 km/h

and 90 km/h speed limits.

The BFFS is then reduced by the adjustment factors

for lane width, lateral clearance, median type and

access point density.

The adjustment factors can be determined from thetables in HCM2000.

DETERMINATION OF FLOW RATE (vp)

vp = V .

PHV x N x Fhv x fP

vP = 15-min passenger-car equivalent flow rate

(pc/hr/ln)

V = hourly volume (veh/hr)

PHF = peak hour factor

N = number of lanesfHV = heavy vehicle adjustment factor

fP = driver population factor

Peak-Hour Factor

PHF = V .

4 x V15

where V = peak hourly volume

V15 = highest 15-minute volume

The PHFs for multilane highways have beenobserved to range from 0.75 to 0.95.

Lower values are typical of rural or off-peak

conditions, whereas higher factors are typical of

urban and suburban peak-hour conditions.

Where local data are not available, 0.88 is a

reasonable estimate of the PHF for rural multilane

highways and 0.92 for suburban facilities.


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Heavy Vehicle Adjustment Factor

ET , ER = passenger car equivalents for trucks or buses

(T) and recreational vehicles (RV) in the traffic stream

(refer either Tables 6, 7 or 8)

PT , PR = proportion of truck/buses and RVs in the

traffic stream

Note: In Malaysia there are no recreational vehicles.Therefore, neglect PR and ER

Driver Population Factor

When the traffic stream is made up of vehicles driven

by regular drivers (commuters, or drivers familiar with

the highway), the driver population factor, fP is taken

as 1.00. Driver population factor may range between

0.85 and 1.00

DETERMINATION OF LEVEL OF SERVICE

LOS is determined using the FFS and vP values. Look up the Speed-Flow curve.

Locate the vP value on the x-axis and draw a vertical

line upwards.

Determine the average speed, S.

(S=FFS if vP 1400 pc/hr/ln)

Calculate density, D = vP / S.

Determine the LOS on basis of density region in

which the point is located.

NOTE:

D = density (pc/km/ln)vp = flow rate (pc/h/ln)

S = average passenger-car speed (km/h)

TRAFFIC STUDY & TRAFFIC FLOW THEORY

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