KSR Transportation Traffic

8/8/2019 KSR Transportation Traffic

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Indian Institute of Technology, Kharagpur

Transportation Engineering

Lesson 2Traffic Engineering

K. Sudhakar ReddyDepartment of Civil Engineering

IIT Kharagpur

Email: [email protected]


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General

Traffic Engineering deals with the knowledgerequired for evaluation and design of differenttransportation facilities such as

Mid-block sections of highways Intersections / junctions

Cycle tracks Pedestrian Pathways Parking facilities Bus Terminals


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General

The Transportation facilities are designed to

PERFORM satisfactorily

The Measure of Performance (MOP) may be

different for different facilities

Speed of Operation, Safety, Comfort and

Economy are the major considerations fordesign of transportation facilities

Environmental Issues also play a significant role


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General

Traffic Inputs required for design and evaluation

of Transportation facilities

Characteristics of Road User Drivers,

Pedestrians

Characteristics of Vehicles

Traffic


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Traffic Characteristics

Traffic flow is characterized by (a) traffic volume

(b) traffic speed and traffic density

Traffic volume is the number of vehicles (or

persons) crossing a given section per unit time

Different units of traffic volume are used

depending on the unit of time considered Yearly Volume, Daily Volume, Hourly Volume


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Traffic Volume

Yearly (Annual) Volume Number of vehicles per year

Average Yearly (Annual) Volume = Total number ofvehicles counted / number of years of counting

Annual Average Daily Traffic (AADT) = Total no. ofvehicles counted /365 (if the traffic is counted for all365 days)

Average Daily Traffic (ADT) = Total No of vehiclescounted / No of days of count (less than 365)

ADT is used if it is not possible to count for all 365days


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Traffic Volume

AAWT or AWT Average Annual Weekday Traffic

(AAWT) is obtained by dividing the total number ofvehicles counted during all the weekdays (excludingweekends) in a year by the total number of weekdays

in a year

Average Weekday Traffic (AWT) is obtained by dividingthe total number of vehicles counted during the

weekdays by the total number of weekdays(considering a period that is shorter than a year(

These units are relevant for locations where there issignificant difference between the week-day and week-end traffic recreational locations etc


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Traffic Volume

Hourly Flow rates Useful for design of most

transportation facilities (capacity analysis, LOSanalysis, intersection design, etc)

Selection of Design Hourly Volume Many agenciesuse peak hourly flow rates for the design oftransportation facilities

In the case of existing facilities, peak hours of trafficcan be observed and the condition during the peakhours can be evaluated


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Traffic Volume

For new facilities (which are to be created) the design

hourly volumes are estimated from average dailyvolumes (ADT or AADT) using empirical adjustmentfactors. As mentioned earlier, planning exercise yields

Daily traffic volumes which need to be converted intounits useful for different design exercises.

Practical observations have resulted in useful

information which can be used to estimate peak(design) hourly volume from daily volume (ADT orAADT)


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Traffic Volume

For new facilities (which are to be created) the design

hourly volumes are estimated from average dailyvolumes (ADT or AADT) using empirical adjustmentfactors. As mentioned earlier, planning exercise yields

Daily traffic volumes which need to be converted intounits useful for different design exercises.

Practical observations have resulted in useful

information which can be used to estimate peak(design) hourly volume from daily volume (ADT orAADT)


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Selection of Design Hourly Volume

Hourly Volume (Veh/h)

Noof

Horsduringwhich

VolumeonX

-Axisis

Exceed

ed

Point of Diminishing Returns Corresponding Hourly Volume isgenerally selected as Design Volume


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Traffic Volume

The point of diminishing returns is generally observed

to correspond to 15th to 30th highest hourly volumes.30th highest hourly volume is usually selected fordesign. This means that facilities designed for this

volume may have congestion in 29 hours in a year

A Design Hour Volume Factor (k) is generally used toexpress design hourly volume as proportion of the

AADT (ADT)

DHV (Design Hourly Volume) = AADT X k

Where k = proportion of AADT during the 30th (or anyother selected) highest hour


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Traffic Volume

Directional Design Hourly Volume (DDHV) Since

traffic flows are directional and can have significantdifferences in terms of the traffic volumes expected inboth the directions, it is necessary to select the traffic

in the busier direction Directional Distribution (Split) Factor is used for this

purpose.

DDHV = AADT X k X D

Where D = Directional Split in Peak Direction


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Traffic Volume

Design Directional Sub-hourly Flow Rate

= AADT.K.D/PHF

Typical values of k, D and PHF factor are obtained from

practical observations. These values are usuallyavailable for different conditions :- urban, sub-urban,rural, etc


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Traffic Volume

Design Directional Sub-hourly Flow Rate

= AADT.K.D/PHF

Typical values of k, D and PHF factor are obtained from

practical observations. These values are usuallyavailable for different conditions :- urban, sub-urban,rural, etc


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Traffic Volume - Measurement

Traffic volumes are measured by (a) manual and (b)

automated methods

The counting (survey) can be conducted for a shortduration or continuously

Information regarding number of motorised vehicles,non-motorised vehicles, pedestrians etc is collected

In the case of intersections, turning volumes are alsosurveyed

Vehicles are counted category-wise

Temporal variation of traffic is obtained


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Traffic Volume - Measurement

Manual count is laborious and can be carried out only for

shorter duration (one or two weeks or shorter period)

Number of surveyors can be selected from generalguidelines available for the number of vehicles that can

be counted by a person (trained) per hour without fatigue

Automated counting of vehicles can be done by usingdifferent sensors that can detect the presence or absenceof a vehicle at a location by the signal produced. Variousprinciples (variation in magnetic field, heat sensing,optical, variation in electrical resistively, etc) are used for

automatic counting

I di I i f T h l Kh


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Traffic Speed

We are interested in two types of average (mean)

speeds that provide us information about the (1)instantaneous (spot) speeds at a location over sometime period (2) condition of the traffic over some space

(length of road) Spot speed of a vehicle is its instantaneous speed

measured at a location

Spot speed can be measured by observing the timetaken by the vehicle to cover a short distance (trap

length). The trap length can be selected depending onthe method of measuring time.

I di I tit t f T h l Kh


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Traffic Speed

For manual method of time measurement (using stop

watches) the trap length can typically be 25 to 30m.However, if electronic and automated method of timemeasurement is used, the trap length can be very small

to make the speed measured instantaneous. Spot speed study is conducted to identify the design

speeds that can be selected for typical highway

sections

Spot (instantaneous) speed information is necessary

to examine the safety, accident proneness of alocation.



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Traffic Speed

Spot speed study can also be used to assess the effect

of improving the road condition (where increasedspeeds are expected) or that of a speed limit restriction(speed reduction expected). Mean spot speed obtained

before and after the activity can be compared forthis purpose.



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Traffic Speed Design Speed

The design speed for which all the geometric features

of the road are designed can be selected as the 98th

percentile speed (speed which is expected to beexceeded 2% of time)

Speed (km/h)%of

timeSpee

disLess

than

thatgiven

onX-axis 100%98%

98 Percentile Speed

Indian Institute of Technology Kharagpur


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Traffic Speed

The mean of spot speeds collected at a location is

called time mean speed (TMS) as the speeds collectedover time (at a location) are averaged

Journey (Travel) Speed = Distance Travelled / Total

Travel time

Running Speed = Distance/ Travel time excluding

stoppages (if any for fuel at intersections, etc) Comparison of Journey speed with running speed

gives an idea about the extent of stopped delay on the

stretch.



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Traffic Speed

Free Speed Speed of the vehicle which is in a free

condition (not following other vehicles in the trafficstream, i.e. the vehicle in front of it is at such adistance the driver of this vehicle can choose the

speed he desires and governed by the road andneighboring conditions

As traffic volumes increase on the stretch, the journey

speeds decrease

Comparison of journey speed with free speed can give

us an idea about the degree of congestion



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Traffic Speed

Average Journey Speed Representative of the travel

condition on a stretch can be obtained by dividing thelength of the stretch by the average travel time

Average travel speed will vary with time depending on

the traffic volume, densities and the correspondingtravel times observed at different points of time.

The Average travel (journey) speed is Space meanSpeed (SMS) as it is obtained by considering the traveltimes of vehicles occupying some space at a given

time



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Traffic Speed

Average Journey Speed (SMS) of a stretch of road can be

obtained by identifying the vehicles that are there in thestretch and by averaging the travel times of those vehicles.Distance divided by the average travel time gives SMS.

TMS will be greater than SMS

If the spot speeds of 5 vehicles at a location on a stretch of5km length are 50, 60, 40, 60, 70 km/h, the TMS is

(50+60+40+60+70)/5 = 56km/h. If it is assumed that thevehicles cover the stretch at constant speed (equal to spotspeed), the travel times of the vehicles will be 0.1, 0.0833,

0.125, 0.0833, 0.0714 hours. Average travel time = 0.0926hours. SMS = 5/0.0926 = 53.98 km/h (< 56 km/h)



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Traffic Speed

The following empirically developed equation can be

used to estimate SMS from TMS (which is easy tomeasure)

SMS = TMS ((standard deviation of spot

speeds)2)/TMS

A number of procedures are available for measuring

travel times and delays using which travel speeds canbe obtained



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Traffic Density

Density (Concentration) is defined as the number of

vehicles occupying unit length of road (vehicles/km)

Density can be measured by conducting densityexperiment in which the times of entry into and exit

from the stretch are observed for all the vehicles. Fromthis data, the number of vehicles occupying the stretchat a given point of time can be obtained. Number of

vehicles divided by length gives us the density

Other procedures also can be used to obtain densities



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Time and Space Headways

Space (distance) headway is the space (distance

measured centre to centre or front bumper to frontbumper) separating vehicles on a stretch of road

Average distance headway (s), if available can be used

to obtain the density as 1000/s (vehicles /km)

Similarly, the difference between the arrival times of

successive vehicles at location is called as timeheadway. If average time headway (average of timeheadways of a number of vehicles arriving at alocation) can be obtained as t, flow rate can beobtained as 3600/t (veh/h)



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Fundamental Flow Diagrams

Flow (Q), speed (Space Mean Speed) V and Density (D)

are related as Q = V X D

It can generally observed that Speed depends ondensity. As densities increase, speeds of vehicles

decrease due to the closer proximity of other vehiclesand because of reduced availability of spaces formaneuvering



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Typical Speed-Density Relationship (Greenshields)

Density

Speed(SMS)

Free Speed

Jam Density



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Linear relationship between Speed and density will

yield Sped-Flow and Density-Flow relationships asgiven below (parabolic)

Density

Flow

Capacity

Jam Density

Speed

Flow

Capacity

FreeSpeed



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Capacity is the maximum flow possible (on a regular

basis) at the selected location If the relationship between Density and speed is linear,

the corresponding relationships between speed and

Flow and Density and Flow will be parabolic.

For these conditions, It can be shown that capacity

flow occurs at a density of Jam Density /2 and at aspeed of Free Speed /2.

Capacity (C) can be obtained as C = (VfXDj)/4



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Extreme Flow Conditions


Density

Speed(SMS)

Free Speed

Jam Density

HypotheticalContinuous Train

CaseRa

ce-track

Case

Parking



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gy, gp


Draw Speed-Flow and density Flow diagrams for the

three extreme flow conditions given in the previousslides

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