Traffic Signals  

Traffic signals are control devices which could alternately direct the traffic to stop and proceed at intersections using red and green traffic light signals.

The main requirement of traffic signals are

• It should draw the attention of road user.
• It should provide meaning & time to respond.
• It should have minimum waste of time.

  Type of Traffic Signals  

The signals are classified into the following types:

1. Traffic Control Signals
   1. Fixed-time Signal
   2. Manually Operated Signal
   3. Traffic Actuated (automatic) Signal
2. Pedestrian Signal
3. Special traffic Signal

Traffic Control Signals

Traffic Control Signals have three colored light glows facing each direction of traffic flow. The Red light is meant for “Stop“, Green light indicates “GO” and the Amber light allows the clearance time for the vehicles which enter the intersection area by the end of green time.

Traffic control signals are of three types:

Fixed-time Signal

In Fixed Time Signal the timing of each phase of the cycle is predetermined base on the traffic studies. The main drawback of this is that some times the traffic flow on one road may be almost nil and traffic on cross road may be quite heavy but signal operates with fixed timings.

Traffic Actuated (automatic) Signal

Traffic actuated signals are those in which the timings of phase and cycle are changed according to traffic demand.

• Semi-actuated Signal is a signal whose timing (cycle length, green time, etc.) is affected when vehicles are detected (by video, pavement embedded inductance loop detectors, etc.) on some, but not all, approaches. This mode of operation is usually found where a low-volume road intersects a high-volume road. In such cases, green time is allocated to the major street until vehicles are detected on the minor street: the the green indication is briefly allocated to the minor street and then returned to the major street.
• Fully Actuated Signal is a signal whose timing (cycle length, green time, etc.) is completely influenced by the traffic volumes, when detected, on all of the approaches. Fully actuated signals are most commonly used at intersections of two major streets.

Manually Operated Signal

These signals are operated manually and not commonly used. In these types of signals, the traffic police watches the traffic demand from a suitable point during the peak hours at the intersection and varies the timings of these phases and cycle accordingly.

Pedestrian Signal

It is used to give the right of way to pedestrians to cross a road when the vehicular traffic shall be stopped by stop signal.

Special Traffic Signal

Special traffic signal such as “FLASHING BEACONS” are meant to Warn the traffic. When signal is flashing red then the vehicles shall stop before entering the nearest crosswalk at an intersection.

While flashing yellow signals are caution signals meant to signify that drivers may proceed with caution.

  Type of Co-ordination of Traffic Signal System  

There are four general types of co-ordination of Traffic signals system

1. Simultaneous System
2. Alternate System
3. Simple Progressive System,
4. Flexible Progressive System

Simultaneous System

• In this system all the signals along a given road always show the same indication (green, red etc.) at the same time.
• As the division of cycle in also the same at all intersections, this system does not work satisfactorily

Alternate System

• It shows opposite indications in a route at the same time.
• This system generally in considered to be more satisfactory than the simultaneous system

Simple Progressive System

• Signals used: fixed time signals
• A time schedule is made to permit, as nearly as possible a continuous operation of groups of vehicles along the main road at a reasonable speed.
• Disadvantage: Cannot co-ordinate a large series of signals. As problem may be created for other routes.

Flexible Progressive System

• In this system it is possible to automatically vary the length of cycle, cycle division and the time schedule at each signalized intersection with the help of a computer.
• This is the most efficient system of all the four types described above.

  Important Terminology of Traffic Signals  

1.Cycle

A signal cycle is one complete rotation through all the indications provided. (Red, Green, Amber)

2.Cycle Length (C)

It is the time in seconds that it takes a signal to complete one full cycle of indication ie. the time interval between the starting of green for one approach till next time the green starts.

3.Interval

It indicates the change from one stage to another. There are two types of interval:

1. Change Interval: It is also called yellow time and it indicates the interval between green and red Signal.
2. Clearance Interval: It is also known as all red time and is included after each yellow interval indicating a period during which all signal phases shows red and it is used for clearing of vehicles at the intersections.

C = G_A +A_A + R_A 

or C = G_B +A_B + R_B

G_A +A_A = R_B

G_B +A_B = R_A

Hence C = G_A +A_A + G_B +A_B

or  C = R_A +R_B

Note: Clearance interval is provided after yellow interval & is optional, hence if the intersection is small then there is no need of clearance interval.

4.Green Time

It is provided to allow traffic flow through the intersection. It is designed on the basic of traffic volume of given road. It is the actual time duration the green signals is turned on.

5.Red Time

It is provided to stop the traffic flow on the intersection. It is designed on the basic of traffic volume of cross road. It is the actual time duration the red signals is turned on.

6.Amber Time

It is clearance time provided after the green time, just for clearance of traffic so that traffic on cross road can be started.

7.Phase

8.Lost Time (t_L)

• It is the time during the phase which is not utilized effectively for vehicle movement.
• Total lost time is combination of
  1. Startup Lost Time (t_SL)
  2. Clearance Lost Time (t_SL)

Startup Lost Time (t_SL)

• It is the lost at startup at green time.
• At the start of green time the driver standing very near to the queue usually take sometime to react & some accelerate the vehicle. This lost time is know as Startup Lost Time (t_SL).

Clearance Lost Time (t_CL)

• There is a psychological tendency of driver not to use later portion of the amber, which is called clearance lost time.

9.Effective Green Time (g_i)

Effective green time is the actual time available for vehicle to cross the intersection.

g_i= Gi + A_i – t_L

where,

Gi = Actual green time

A_i= Amber time

t_L =Lost time

10.Lane Capacity

If C = cycle length (in second), then number  of cycles in an hour is = 3600/C

Total effective green time in an hour = (3600/C) ×g_i

If h = time headway is seconds for the vehicles crossing the intersection, then total no. of vehicles crossing the intersection in one hour = {(3600/C) ×g_i} ÷ h

=\(\frac{3600}{h}\frac{g_i}{C}\)

capacity of a lane in veh/hr =\(\frac{3600}{h}\frac{g_i}{C}\)

\(\frac{3600}{h}\) = Saturation Capacity, \(\frac{g_i}{C}\)=green ratio.

  Method of Signal Designing  

There are various method of signal designing:

1. Trial Cycle Method
2. Approximate Method
3. Webster Method
4. IRC Method

For Gate and IES point of view only Webster Method in syllabus.

Webster Method

In this method the optimum cycle length (C_O) calculated on the basis of least total delay to the vehicles at the signalized intersection.

Optimum cycle time, C_O= \(\frac{1.5L+5}{1-Y}\) seconds

where,

L = Total time lost = nt_L + R

n = Number of phase

t_L= Lost time = Startup Lost Time (t_SL) + Clearance Lost Time (t_SL)

R = All red time

For the average signal cycle, the lost time (t_L) is taken to be 2 seconds.

Total time lost,

L = 2n+R  (∴t_L=2)

and,

Y= y_A+y_B

\(y_A=\frac{q_A}{S_A}\).

\(y_B=\frac{q_B}{S_B}\)

where,

q_A or q_B= Normal flow in road A/B in veh/hr/lane

S_A or S_B = Saturation flow in road A/B in veh/hr/lane

Green time for road A is given by,

G_A = \(\frac{y_A}{Y}\) (C_O-L) sec

and

Green time for road B is given by,

G_B = \(\frac{y_B}{Y}\) (C_O-L) sec

Example.1

Design two phase traffic signal by webster’s method using the following data:

The all red time required for pedestrian crossing is 12 sec. Design two phase traffic signal by webster’s method.

Solution

C_O= \(\frac{1.5L+5}{1-Y}\)

L = nt_L + R = 2n +R = 2×2+12

L = 16sec

\(y_A=\frac{q_A}{S_A}\).

\(y_A=\frac{400}{1250}\)

y_A = 0.32

\(y_B=\frac{q_B}{S_B}\).

\(y_B=\frac{250}{1000}\)

y_B = 0.25

Y= y_A +  y_B =0.32 + 0.25 = 0.57

C_O= \(\frac{1.5\times 16+5}{1-0.57}\)

C_O = 67.5sec

G_A = \(\frac{y_A}{Y}\) (C_O-L) sec

G_A = \(\frac{0.32}{0.57}\) (67.5-16) =28.9sec

G_A ≈ 29 sec

G_B = \(\frac{y_B}{Y}\) (C_O-L) sec

G_B = \(\frac{0.25}{0.57}\) (67.5-16) = 22.58sec

G_B = 22.6sec

Providing amber time of 2 sec for each phase for clearance.

For Phase A

G_A = 29 sec

A_A = 2 sec

R_A = G_B +A_B = 22.6 + 2 = 24.6 sec

Red time for pedestrian crossing is 12 sec.

For Phase B

G_B = 22.6 sec

A_B = 2 sec

R_B = G_A +A_A = 29 + 2 = 31 sec

Red time for pedestrian crossing is 12 sec.

Example.2

A fixed time two phase traffic signal is to be designed for an urban intersection using Webster’s approach. The intersection is having N-S and E-W roads, where any straight ahead traffic is permitted. The design hourly traffic flow from various arms & the corresponding saturation flow are given as follow compute optimum cycle length by assuming all red period per phase & time lost per phase due to starting delay as 3 second and 2 second respectively.

Solution

C_O= \(\frac{1.5L+5}{1-Y}\)

L = nt_L + R = 2×2 + 3×2 =10 sec

y₁ = (1000/3000=0.33 or 600/2400= 0.25)max

y₁ = 0.33

y₂ = (950/3600=0.33 or 800/2000= 0.4)max

y₂ = 0.4

Y= y₁+y₂ =0.733

C_O= \(\frac{1.5\times 10+5}{1-0.733}\)

C_O = 75sec