Traffic Signal Operation - Signal Phases

What is a traffic signal phase?

A phase is a specific movement that has a unique signal indication.

If you have a four legged intersection, with protected left turns in all directions, the signal would be called an "8-phase intersection".

If the intersection has four legs, with protected left turns on the main street, but permissive left turns on the side street, the signal would be called a "6-phase intersection".

IF the intersection has four legs, with permissive left turns on all approaches, the signal may either be called a "4-phase intersection", or possibly a "2-phase intersection". In most cases, calling this type of intersection a 2-phase intersection is an oversimplification of how the signal actually operates.

What does this mean?

The traffic signal controller processes the request for green in a very specified, logical, manner. The logic is not always apparent, especially when you want to go, and you keep getting a red light.

Modern traffic signal controllers are relatively powerful control devices, but they are... computerized control devices, and they are specifically designed to operate as safely as possible in a very ordered fashion.

The traffic signal controller doesn't know if you are in a hurry, or if you are busy changing a CD in your car's radio, or for that matter, both at the same time. The signal processes the input information and makes decisions on a industry standardized method of decision making.

The following diagram shows the basic phase sequence diagram for a specific signal.

Straigh forward phase sequence diagram

A little explanation is in order.

Most modern traffic signals operate with what is sometimes termed a 2-ring, 8-phase dual quad operation.

In essence, phases (the funny looking o with the slash in the middle) 1, 2, 3, and 4 are in ring 1. Phases 5, 6, 7 and 8 are in ring 2. The traffic signal can present a green to only specific movements at the same time.

Say what?

The traffic signal is capable of showing any combination green indication to any single movement in ring 1, and any single phase in ring 2 at the same time. The traffic signal is prohibited from showing more than one phase a green indication in the same ring at a time.

Between phases 2 and 3 (and 6 and 7), there is a barrier. The barrier is a programmed requirement. The traffic signal must terminate the phases in ring 1 and ring 2 on one side of the barrier before continuing to the next side of the barrier. Likewise, there is a barrier on the right side of phases 4 and 8.of green indications to the following at the same time:

• Phases 1 and 5 (usually opposing left turns)
• Phases 1 and 6 (usually one left turn, and the adjacent thru movement)
• Phases 2 and 5 (one left turn and one adjacent thru movement)
• Phases 2 and 6 (two opposing thru movements)

Both the phases on the left side of the barrier in Ring 1 and Ring 2 must transition through the barrier simultaneously to continue

• Phases 3 and 7 (usually opposing left turns)
• Phases 3 and 8 (usually one left turn, and the adjacent thru movement)
• Phases 4 and 7 (one left turn and one adjacent thru movement)
• Phases 4 and 8 (two opposing thru movements)

Both the phases on the right side of the barrier in Ring 1 and Ring 2 must transition through the barrier simultaneously to continue with phases 1, 2, 5 and 6 again.

Obviously, phases 1 and 2 could not operate simultaneously, as this would be a conflicting set of movements (a left turn plus an opposing thru movement simultaneously would be bad).

In most cases, traffic signals want to go from the left side of the diagram to the right side of the diagram. In most cases, traffic signals don't want to back up.



This particular intersection has the main street movements on phases 2 and 6. Phases 1 and 5 are the main street left turns. The side street movements are phases 4 and 8, with the left turns being permissive (That's why phases 3 and 7 say "Future".

The large single headed arrows denote the vehicle movements. The smaller double arrow heads with the slashes in the middle show the pedestrian movements.

This is a relatively common type of intersection, you probably have a lot of intersections just like this in your town.

The following intersection has what is called split phase. Phases 7 and 8 operate sequentially.

NEMA phase sequence diagram - split phase 7 and 8

The phasing can be simple to complex.