Design Principles of a Traffic Signal

8/13/2019 Design Principles of a Traffic Signal

1/22

Design Principles of a Traffic Signal

Lecture notes in Traffic Engineering And Management

Overview

Traffic signals are designed to ensure safe and orderly flow of traffic,Protect pedestrians and vehicles at busy intersections and reduce theseverity and frequency of accidents between vehicles enteringintersections. Generally 9 stages are followed for design of trafficsignal at particular intersection as per Indian standards. Those arefollowing,

1. Phase Design2. Cycle Time3. Effective Green Time4. Cycle Length5. Green Splitting6. Pedestrian Crossing Requirements7. Interval Design8. Performance Evaluation9. Special Cases (Effect of Turning Vehicles, Effect of Lane

Distribution)

First Five Stages are discussed in Chapter 34 (Design Principles of

Traffic Signal I). In this chapter we will discuss remaining four stagesdetailed.

Pedestrian crossing requirements

Pedestrian crossing requirements can be taken care by two ways; bysuitable phase design or by providing an exclusive pedestrian phase.It is possible in some cases to allocate time for the pedestrianswithout providing an exclusive phase for them. For example, consideran intersection in which the traffic moves from north to south andalso from east to west. If we are providing a phase which allows thetraffic to flow only in north-south direction, then the pedestrians cancross in east-west direction and vice-versa.


2/22

However in some cases, it may be necessary to provide an exclusivepedestrian phase. In such cases, the procedure involves computationof time duration of allocation of pedestrian phase. Green time for

pedestrian crossing can be found out by,

where is the minimum safe time required for the pedestrians to

cross, often referred to as the ``pedestrian green time", is the

start-up lost time, is the crossing distance in metres, and is thewalking speed of pedestrians which is about 15th percentile speed.

The start-up lost time can be assumed as 4.7 seconds and thewalking speed can be assumed to be 1.2 m/s.

Interval Design

There are two intervals, namely the change interval and clearanceinterval, normally provided in a traffic signal. The change interval or

yellow time is provided after green time for movement. The purposeis to warn a driver approaching the intersection during the end of agreen time about the coming of a red signal. They normally have avalue of 3 to 6 seconds. The design consideration is that a driverapproaching the intersection with design speed should be able to stopat the stop line of the intersection before the start of red time.Institute of transportation engineers (ITE) has recommended amethodology for computing the appropriate length of change intervalwhich is as follows:

where y is the length of yellow interval in seconds, t is the reaction

time of the driver, is the percentile speed of approaching

vehicles in m/s, a is the deceleration rate of vehicles in , g isthe grade of approach expressed as a decimal. Change interval canalso be approximately computed as y = SSD/v, where SSD is the

stopping sight distance and v is the speed of the vehicle. Theclearance interval is provided after yellow interval and as mentioned


3/22

earlier, it is used to clear off the vehicles in the intersection.Clearance interval is optional in a signal design. It depends on thegeometry of the intersection. If the intersection is small, then there isno need of clearance interval whereas for very large intersections, itmay be provided.

Change interval

Change interval or yellow or amber time is given after GREEN andbefore RED which allows the vehicles within a 'stopping sight distance'from the stop line to leagally cross the intersection. The amber time Yis calculated as

where t is the reaction time (about 1.0 sec), v is the velocity of theapproaching vehicles, g is the acceleration due to gravity (9.8

), n is the grade of the approach in decimals and a is the

deceleration of the vehicle (around 3 ).

Clearence interval

The clearence interval or all-red will facilitate a vehicle just crossedthe stop line at the turn of red to clear the intersection without beingcollided by a vehicle from the next phase. ITE recommends thefollowing policy for the design of all read time, given as Where w isthe width of the intersection from stop line to the farthest conflictingtraffic, L is the length of the vehicle (about 6 m), v is the speed of thevehicle, and P is the width of the intersection from STOP line to thefarthest conflicting pedestrian cross-walk.

Performance Evaluation

Performance measures are parameters used to evaluate theeffectiveness of the design. There are many parameters involved toevaluate the effectiveness of the design and most common of theseinclude delay, queuing, and stops. Delay is a measure that mostdirectly relates the driver's experience. It describes the amount oftime that is consumed while traversing the intersection. The figure 2shows a plot of distance versus time for the progress of one vehicle.The desired path of the vehicle as well as the actual progress of thevehicle is shown. There are three types of delay as shown in the

figure. They are stopped delay, approach delay and control delay.Stopped time delay includes only the time at which the vehicle is


4/22

actually stopped waiting at the red signal. It starts when the vehiclereaches a full stop, and ends when the vehicle begins to accelerate.Approach delay includes the stopped time as well as the time lost dueto acceleration and deceleration. It is measured as the timedifferential between the actual path of the vehicle, and path had there

been green signal. Control delay is measured as the differencebetween the time taken for crossing the intersection and time takento traverse the same section, had been no intersection. For asignalized intersection, it is measured at the stop-line as the vehicleenters the intersection. Among various types of delays, stopped delayis easy to derive and often used as a performance indicator and willbe discussed.

Figure 1:Illustration of delay measures

Figure 2:Graph between time and cumulative number of vehicles at an

intersection

Vehicles are not uniformly coming to an intersection. i.e., they arenot approaching the intersection at constant time intervals. They


5/22

come in a random manner. This makes the modelling of signalizedintersection delay complex. Most simple of the delay models isWebster's delay model. It assumes that the vehicles are arriving at auniform rate. Plotting a graph with time along the x-axis andcumulative vehicles along the y-axis we get a graph as shown in

figure 3. The delay per cycle is shown as the area of the hatchedportion in the figure.

Webster derived an expression for delay per cycle based on this,which is as follows.

Where gi is the effective green time, C is the cycle length, Vi is thecritical flow for that phase, and S is the saturation flow.

Delay is the most frequently used parameter of effectiveness forintersections. Other measures like length of queue at any given time(QT ) and number of stops are also useful. Length of queue is used todetermine when a given intersection will impede the discharge froman adjacent upstream intersection. The number of stops made is animportant input parameter in air quality models.

Special cases

Effect of Turning Vehicles

Right Turning Vehicles

Right-turn signal phases facilitate right-turning traffic and mayimprove the safety of the intersection for right-turning vehicles.However, this is done at the expense of the amount of green time

available for through traffic and will usually reduce the capacity of theintersection. Right-turn arrows also result in longer cycle lengths,which in turn have a detrimental effect by increasing stops anddelays. While phases for protected right-turning vehicles are popularand commonly requested, other methods of handling right-turnconflicts also need to be considered. Potential solutions may includeprohibiting right-turns and geometric improvements.

Right -Turn Phase Criteria

The three right -turn phase criteria presented below are the result ofconsiderable research and study.


6/22

1. Traffic Volumes2. Delay

Separate right -turn phasing may be considered if:

o The average delay for all right -turning vehicles on theapproach is at least 35 seconds during that same peakhour.

3. Collision ExperienceSeparate right -turn phasing may be considered if the criticalnumber of reportable right -turn collisions has occurred. Theseare:

o For one approach to the intersection, the critical numberis five l right -turn collisions in one year, or seven in twoyears.

o For both approaches to an intersection, the criticalnumber is seven right -turn collisions in one year, oreleven in two years.

So the right turning vehicles affected saturation flow based onadjusted saturation headway. Finally actual values of right turning arecalculated from right turn adjustment factor. The adjustments factoris calculated by following equations.

Adjusted Saturation headway,

Adjusted Saturation flow,

Multiplicative Right turn adjustment factor,


7/22

Example: 1

If there is 15 percent right turning movement, eRT (through-carequivalent for permitted left turns) is 3, saturation headway is 2 sec;Find the value of Adjusted Saturation flow.

Solution:

Given , , ,

Case 1

To find Adjusted headway, from that to calculate the reducing orincreasing saturation flow.

Adjusted Saturation flow

The Adjusted Saturation flow = 1385 Veh/hr

Case 2

To find the adjustment factor, then to calculate adjusted saturationflow based on ideal saturation flow (=1800)

The Adjusted Saturation flow = 1386 Veh/hr

So case 1 and case 2, the value of saturation flow adjusted are same.


8/22

Left Turning Vehicles

Adjustment Factor for Left turn:

In left turn adjustment factor for Saturation flow rate is as follows,

Exclusive Lane:

Shared Lane:

Where, pLT = Proportions of left turns in lane group.

Normally in left turn, separate signal phase are not provided atintersection as per Indian standard. But following facilities areprovided at intersection to prevent from some effects from leftturning vehicles.

Left-Turn LaneAllows left-turn-on-red (unless prohibited), reducingleft-turn queues.

Shared Lane with Island

Provision of islands permits its use for placement of traffic controldevices or as a pedestrian refuge.

Left Turn Lane with Island

Left-Turn Lane with Island and Dedicated Downstream Lane

Effect of Lane Distribution

Congestion and Delay at intersection particularly formed by to toomany vehicles are moving same lane. So reduce that problem, weneed to provide lane distribution. The lane distribution at intersectionnormally followed two categories.

First one is the total volume of given approach are distributed byproviding separate lane for left, right and through movement. Forthat individual movement, we need to fix some percentage of totalflow at that particular approach. This type clearly defined in Figure 5and following example.


9/22

In second type, the given approach total volumes are separated byindividual lane for left, right and straight. And straight movingvehicles also distributed into left and right turn lanes for unavoidablecondition. If through movement vehicles are high, we need to followsecond type distribution. Second type is explained in Figure 6 and

example. Normally high straight cases we followed second method. Inthat second type divided into two distribution methods. First one is,through movement distributed into left, right and straight lanes.Second is, extra separate lane provide for through movement. Soeach cases some lane distribution factors are followed. Thatimportance points are shown in following examples.

Example: 2

For a given intersection, Traffic flow Proportion of Left and Right turnare 10% and 20% respectively. Find the traffic flow for givenapproach. Volume 2500veh/hr.

Figure 3:Individual Lane distribution

From North to South, Total movement is 2500 veh/hr

Left turn Traffic movement from total directional movement = 10%

Right turn Traffic from total directional movement = 20%


10/22

Through Traffic from total directional movement = 70%

Left turning Vehicles =

Right turning Vehicles =

Through Movement Vehicles =

Example: 3

For a given intersection, Traffic flow Proportion of Left and Right turnare 10% and 20% respectively. Left and Right turn Lane utilization

factors are 0.2 and 0.3 respectively. Find the traffic flow for givenapproach. Volume is 2500veh/hr.

Figure 4:Through movement distributed in left and right turn lanes also

From North to South,

Left turn Traffic movement from total directional movement = 10%

Right turn Traffic from total directional movement = 20%

Through Traffic from total directional movement = 70%

Left turning Vehicles =

Right turning Vehicles =

Through Movement Vehicles =


11/22

Lane Distribution

Left turn utilisation factor = 0.2

Right turn utilisation factor = 0.3

Through traffic in Left turn Lane =

Through traffic in Right turn Lane =

Through traffic in Median Lane =

Table 1:PCU Values:

Vehicle Type PCU Value

Car / Taxi 1

Auto 0.75

2W 0.5

Buses 3

LCV 1.5

2 Axle Trucks 3

Bicycle 0.5

Conclusion:

Generally intersection problems are unavoidable Elevated Expressways or freeway systems, grade separated

intersection Efficiency, safety, speed, cost of operation and capacity of road

system very much depends on the intersection design. In this chapter covered pedestrian phase, interval design,

performance evaluation, turning effects and lane distribution

Assignment problem:

Find Critical Volume (Vi) for a Given 4 arm Intersection. Traffic flowProportion of Left and Right turn are 10% and 20% respectively (Forall approach). Left and Right turn Lane utilization factors are 0.2 and0.3 respectively.


12/22

Use following Phase Plan:

Solution:

From West to East,

Left turn Traffic movement from total directional movement =10%

Right turn Traffic from total directional movement = 20% Through Traffic from total directional movement = 70% Left turning Vehicles = Right turning Vehicles = Through Movement Vehicles =

Lane Distribution

Left turn utilisation factor = 0.2 Right turn utilisation factor = 0.3 Through traffic in Left turn Lane =



13/22


From East to west,



Lane Distribution

Left turn utilisation factor = 0.2 Right turn utilisation factor = 0.3 Through traffic in Left turn Lane =



From North to south,



From south to North,



14/22


Performance measures

Performance measures are parameters used to evaluate the

effectiveness of the design There are many parameters involved toevaluate the effectiveness of the design and most common of theseinclude delay, queuing, and stops. Delay is a measure that mostdirectly relates the driver's experience. It describes the amount oftime that is consumed while traversing the intersection. The figure1shows a plot of distance versus time for the progress of one vehicle.The desired path of the vehicle as well as the actual progress of thevehicle is shown. There are three types of delay as shown in thefigure. They are stopped delay, approach delay and control delay.Stopped time delayincludes only the time at which the vehicle is

actually stopped waiting at the red signal. It starts when the vehiclereaches a full stop, and ends when the vehicle begins to accelerate.Approach delayincludes the stopped time as well as the time lost dueto acceleration and deceleration. It is measured as the timedifferential between the actual path of the vehicle, and path had therebeen green signal. Control delayis measured as the differencebetween the time taken for crossing the intersection and time takento traverse the same section, had been no intersection. For asignalized intersection, it is measured at the stop-line as the vehicleenters the intersection. Among various types of delays, stopped delay

is easy to derive and often used as a performance indicator and willbe discussed.

Vehicles are not uniformly coming to an intersection. i.e., they arenot approaching the intersection at constant time intervals. Theycome in a random manner. This makes the modeling of signalizedintersection delay complex. Most simple of the delay models isWebster's delay model. It assumes that the vehicles are arriving at auniform rate. Plotting a graph with time along the x-axis andcumulative vehicles along the y-axis we get a graph as shown infigure2.The delay per cycle is shown as the area of the hatched
http://www.civil.iitb.ac.in/tvm/1111_nptel/573_SignalDes/plain/plain.html#qfperhttp://www.civil.iitb.ac.in/tvm/1111_nptel/573_SignalDes/plain/plain.html#qfperhttp://www.civil.iitb.ac.in/tvm/1111_nptel/573_SignalDes/plain/plain.html#qfperhttp://www.civil.iitb.ac.in/tvm/1111_nptel/573_SignalDes/plain/plain.html#qfwebhttp://www.civil.iitb.ac.in/tvm/1111_nptel/573_SignalDes/plain/plain.html#qfwebhttp://www.civil.iitb.ac.in/tvm/1111_nptel/573_SignalDes/plain/plain.html#qfwebhttp://www.civil.iitb.ac.in/tvm/1111_nptel/573_SignalDes/plain/plain.html#qfwebhttp://www.civil.iitb.ac.in/tvm/1111_nptel/573_SignalDes/plain/plain.html#qfper


15/22

portion in the figure. Webster derived an expression for delay percycle based on this, which is as follows.

(1)

where is the effective green time, is the cycle length, is thecritical flow for that phase, and is the saturation flow.

Delay is the most frequently used parameter of effectiveness forintersections. Other measures like length of queue at any given time

( ) and number of stops are also useful. Length of queue is used todetermine when a given intersection will impede the discharge froman adjacent upstream intersection. The number of stops made is animportant input parameter in air quality models.

Overview

Topic that will be covered in this chapter are:

1. Effect of right turning vehicles2. Adjustments on saturatin flow3. Clearence and change interval4. Various delay models at signalized intersection5. HCM procedure on signalized intersection capacity and level of

service analysis

Effect of right-turning vehicles

1. A right-turnings vehicle will consume more effective green timetraversing the intersection than a corresponding through

vehicle.2. Applicable especially at permitted right movements3. right turn has great difficulty in manneouring and find a safe

gap4. right turn vehicle may block a through vehicle behind it5. right turn vehciles may take 2, 4, or even 10 times the time to

that of a through movement6. The equivalency concept will answer how many through

vehicles could pass the intersection during the time utilized by athrough movement.

7. If 3 through and 2 right turn movement takes place at sometime duration in a given lane. Assume at the same time


16/22

duration in another identical lane if 9 through vehicles moved,then vehicles, then

(2)

8.9. Therefore, the right-turn adjustment factor under the current

prevailing condition is 3.0.10. This factor is normally applied in the saturation flow by

adjusting its value.

(3)

11.12. For example, if there is 15 percent right-turn movement,

is 3, and saturation headway is 2 sec, then the adjustedstaturatin headway is computed as follows:

(4)

13.14. The saturation head way is increased thereby reducing

the saturatin flow veh/hr.

15. The adjested saturation flow can be written as(5)

16.17. From the Equation3and5,following relation can be

easily derived:

(6)
http://www.civil.iitb.ac.in/tvm/1111_nptel/573_SignalDes/plain/plain.html#qtSatHwayAdjhttp://www.civil.iitb.ac.in/tvm/1111_nptel/573_SignalDes/plain/plain.html#qtSatHwayAdjhttp://www.civil.iitb.ac.in/tvm/1111_nptel/573_SignalDes/plain/plain.html#qtSatHwayAdjhttp://www.civil.iitb.ac.in/tvm/1111_nptel/573_SignalDes/plain/plain.html#qtSatFlowAdjhttp://www.civil.iitb.ac.in/tvm/1111_nptel/573_SignalDes/plain/plain.html#qtSatFlowAdjhttp://www.civil.iitb.ac.in/tvm/1111_nptel/573_SignalDes/plain/plain.html#qtSatFlowAdjhttp://www.civil.iitb.ac.in/tvm/1111_nptel/573_SignalDes/plain/plain.html#qtSatFlowAdjhttp://www.civil.iitb.ac.in/tvm/1111_nptel/573_SignalDes/plain/plain.html#qtSatHwayAdj


17/22

18.

19. where is the multiplicative right turn adjustmentfactor to the ideal stauration flow.

20. In the above example,

(7)

21.

22. Therefore the adjusted saturatin flow isveh/sec.

Change interval

Change interval or yellow or amber time is given after GREEN andbefore RED which allows the vehicles within a 'stopping sight distance'from the stop line to leagally cross the intersectin. The amber timeis calculated as

(8)

where is the reaction time (about 1.0 sec), is the velocity of the

approaching vehicles, is the acceleration due to gravity (9.8m/sec2), is the grade of the approach in decimels and is thedeceleration of the vehicle (around 3 m/sec2).

Clearence interval

The clearence interval or all-red will facilitate a vehicle just crossedthe stop line at the turn of red to clear the intersection with out beingcollided by a vehicle from the next phase. ITE recomends thefollowing policy for the design of all read time, given as

(9)

where is the width of the intersection from stop line to the farthestconflicting trafic, is the length of the vehicle (about 6 m), is the


18/22

speed of the vehicle, and is the width of the intersection from STOPline to the farthest confliting pedestrain cross-walk.

Problem

The traffic flow for a four-legged intersection is as shown in figure5.

Figure 5:Traffic flow for a typical four-legged intersection

Given that the lost time per phase is 2.4 seconds, saturation headwayis 2.2 seconds, amber time is 3 seconds per phase, find the cycle

length, green time and performance measure(delay per cycle).

Assume critical ratio as 0.9.

Solution

The phase plan is as shown in figure6.

Figure 6:Phase plan

Sum of critical lane volumes is the sum of maximum lanevolumes in each phase, = 433+417+233+215 = 1298vph.
http://www.civil.iitb.ac.in/tvm/1111_nptel/573_SignalDes/plain/plain.html#qflastprobhttp://www.civil.iitb.ac.in/tvm/1111_nptel/573_SignalDes/plain/plain.html#qflastprobhttp://www.civil.iitb.ac.in/tvm/1111_nptel/573_SignalDes/plain/plain.html#qflastprobhttp://www.civil.iitb.ac.in/tvm/1111_nptel/573_SignalDes/plain/plain.html#qfphasehttp://www.civil.iitb.ac.in/tvm/1111_nptel/573_SignalDes/plain/plain.html#qfphasehttp://www.civil.iitb.ac.in/tvm/1111_nptel/573_SignalDes/plain/plain.html#qfphasehttp://www.civil.iitb.ac.in/tvm/1111_nptel/573_SignalDes/plain/plain.html#qfphasehttp://www.civil.iitb.ac.in/tvm/1111_nptel/573_SignalDes/plain/plain.html#qflastprob


19/22

Saturation flow rate, from equation= = 1637 vph. == 0.793.

Cycle length can be found out from the equation as C=

= 80.68 seconds 80 seconds. The effective green time can be found out as

= 80-(4 2.4)= 70.4 seconds, where is

the lost time for that phase = 4 2.4.

Green splitting for the phase 1 can be found out as = 70.4[ ] = 22.88 seconds.

Similarly green splitting for the phase 2, =22.02 seconds.



The actual green time for phase 1 from equationas = 22.88-3+2.4 23 seconds.

Similarly actual green time for phase 2, = 22.02-3+2.4 23seconds.



Pedestrian time can be found out from as = 21.5seconds. The phase diagram is shown in figure7.
http://www.civil.iitb.ac.in/tvm/1111_nptel/573_SignalDes/plain/plain.html#qfphhttp://www.civil.iitb.ac.in/tvm/1111_nptel/573_SignalDes/plain/plain.html#qfphhttp://www.civil.iitb.ac.in/tvm/1111_nptel/573_SignalDes/plain/plain.html#qfphhttp://www.civil.iitb.ac.in/tvm/1111_nptel/573_SignalDes/plain/plain.html#qfph


20/22

Figure 7:Timing diagram

The actual cycle time will be the sum of actual green time plusamber time plus actual red time for any phase. Therefore, forphase 1, actual cycle time = 23+3+78.5 = 104.5 seconds.

Delay at the intersection in the east-west direction can be foundout from equationas

Delay at the intersection in the west-east direction can be found

out from equation,as

(10)

Delay at the intersection in the north-south direction can be

found out from equation,

(11)


21/22

Delay at the intersection in the south-north direction can be


(12)

Delay at the intersection in the south-east direction can be


(13)

Delay at the intersection in the north-west direction can be


(14)

Delay at the intersection in the west-south direction can befound out from equation,

(15)

Delay at the intersection in the east-north direction can befound out from equation,

(16)

Summary

Green splitting is done by proportioning the green time amongvarious phases according to the critical volume of the phase.Pedestrian phases are provided by considering the walking speed and


22/22

start-up lost time. Like other facilities, signals are also assessed forperformance, delay being th e important parameter used.

References

1. L. R Kadiyali. Traffic Engineering and Transportation Planning.Khanna Publishers, New Delhi, 1987.

2. William R McShane, Roger P Roesss, and Elena S Prassas.Traffic Engineering. Prentice-Hall, Inc, Upper Saddle River, NewJesery, 1998.

Acknowledgments

I wish to thank several of my students and staff of NPTEL for theircontribution in this lecture.

Prof. Tom V. Mathew 2013-02-09

Documents

Design Principles of a Traffic Signal