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Traffic Signal Phasing: How Intersections Manage Vehicle and Pedestrian Movement

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 Traffic signal phasing is one of the most important concept in traffic Engineering. The traffic signal phasing concept define how the traffic signal divide the time among the different traffic movement at an intersection. On other word, it determines who moves, when moves and who will wait.

Well deisgn signal phasing plan will enhace safety, reduce the delay and improve the traffic flow at an intersection. Furthermore,it will help in organizing the traffic movement at an intersection and prevent the conflict between vehicles, pedestrian and cyclist.

-What Is a Traffic Signal Phase?

Traffic signal phase is a specific part of the signal cycle which allows one movement or more to take place. 

For example, in a four-leg intersection. The traffic could allow the traffic movement from north to south. While the traffic movement must be idle from the east to west. Then another phase would take place and allow the traffic movement from east to west, while traffic movement stopped from north to south. 

Each phase usually includes three main indications:

1-Green time (green light): allow vehicles and pedestrian to move. 

2-Yellow time (yellow light) : it is a warning that the green time about to finish. 

3-red time (red light): movement must stop. 

All the phases forms togather one complete traffic signal cycle. 

Why Traffic Signal Phasing Is Important? 

Traffic signal are not limited only to changing ligth from green to red. It is designed to reduce the conflict between the different road users,without the proper phasing, vehicles that turn left, vehicles that moves straight, the pedestrian and cyclist who is crossing the road could conflicts with each other which may result in chaos. 

The proper phasing will achieve the following goals:

-improves traffic intersection safety

-reduce traffic congestion 

-provide a higher safety standards to pedestrian 

-organize the turning movement 

-reduce the accidents 

-improve the traffic intersection capacity 


Common Types of Traffic Signal Phasing

1. Two-Phase Signal Control

It is the simplest type of traffic signal phasing. It allows a single traffic movement in one direction and in second phase it allows the traffic movement in the crossing direction. An example is:

1-phase 1: North-South traffic movement 

2-phase 2: East-west traffic movement 

This system is common in simple intersection with low traffic volumes and limited turning conflicts. 

2. Protected Left-Turn Phasing

In this type, vehicles that are turning left given a separate green arrows and during this time the opposite traffic movement stopped. This type of phasing will be useful when the left turning movement is heavy. 

3-permissive left turn phasing: during this pahse, vehicles that are turning left allowed to turn during the circular green light but it should give priority for the traffic movement of the opposite direction. This type of phasing will be effective for the moderate traffic volumes and when the drivers can find safe gaps to turn left. 

4-protected-permissive left turning phasing: this type of phasing combine the previous two type. Vehicle turning left gets a green arrow to turn left and it can turn left during the circle green light when there is a suitable gaps. This method balance between effective and safety. 

5-split phasing: thid type of phasing gives each approach it separate green lighting. For example, traffic movement in the north moves separately. Then traffic moves in the douth direction separately. This type of phasing used usually in intersections with heavy traffic or where thesr is safety concerns. This phasing can increase the delay as it adds more phases to traffic signal cycle

6-pedestrian phasing: 

The pedestrian phasing controls the time that allows people to cross the road. This phase could work simultaneously with vehicle phases or it could work as a separate phase for pedestrian only. In some intersections, the pedestrian phasing stops all the vehicles and allows pedestrian to cross in all directions and this known as pedestrian scramble. 

Important Terms in Signal Phasing

-Signal Cycle: it is the total time required for all phases to take place one time. For example, if all the intersection phases occurs during 90 seconds. Then the signal cycle is 90 seconds. 

-Green time: it is the time that allow a specific traffic movement to take place and usually the green time is more for heavier traffic movement. 

-Yellow Interval: it warns the drivers that the signal about to turn red which provides the drivers with additional time to safely stop or cross the intersection. 

-All-Red Interval: it is short time where the signal becomes red for all the traffic movements in order to help in clearing the intersection from vehicles before other movement phase take place. 

-Lost Time: this occurs when vehicles is not effectively moving. An example of this, the delay in vehicles movements at the beginning of the green light or the clearance time between different phases. 

Factors affecting the design of signal phasing:

-Traffic volume on each approach

-Number of turning vehicles

-Pedestrian demand

-Road geometry

-Crash history

-Speed limits

-Nearby intersections

-Public transport movement

-Emergency vehicle needs

The purpose is to establish a safe and effective phasing plan that is suitable for the relas traffic movement. 

Example of a Basic Four-Phase Intersection

It could works as follows:

-Phase 1: North-south through traffic and right turns

-Phase 2: North-south protected left turns

-Phase 3: East-west through traffic and right turns

-Phase 4: East-west protected left turns

This phasing separates the left turns from the opposite traffic movement which reduces the conflicts and improve thw safety. 


Benefits of Proper Traffic Signal Phasing

The appropriate phasing can effectively improve the performance of the intersection, protect pedestrian, improve traffic flow ans reduce unnecessary stopping. On the other hand, inappropriate phasing can cause long delays, confusions, unsafe turns and increases possibility of accidents. 


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