Basic traffic signal operation Pretimed Operation

Pretimed operation is the most basic type of operation. In this mode of operation, the signal will alternate the green, yellow and all-red time without regard to the traffic flow.


A signal may have one, some, or all phases on pre timed mode. For many downtown grids, most of the signals are on pre timed mode, and in many cases, the signals are in pedestrian recall.


Some signals are temporarily essentially set to pre timed mode if the vehicle detection fails.


The amount of time for each movement may be programmed to change by time of day.


The minimum amount of time for each phase should equal the sum of WALK, Flashing DONT WALK, Yellow and Al-Red. In some cases, some agencies allow the flashing DONT WALK to extend through the Yellow time. This can shave a few seconds of time off the cycle length of the signal.


Pretimed operation with the signals in pedestrian recall can be a very effective way of dealing with a grid network of signals. Many agencies don't put pedestrian pushbuttons in downtown grid signal networks, as the typical pedestrian volumes would have the signals constantly serving pedestrians anyway.

A pretimed signal operates on a user specified amount of time for each specific movement. For instance, if the signal is a 2-phase signal (2 specific signalized movements), like at the intersection of two one-way streets, and is operating in pretimed mode, the signal will serve one movement for a specified amount of time, followed by the second movement.

In this example, the first movement may be programmed for 20 seconds of green, followed by 3 seconds of yellow, followed by 3 seconds of all-red. The second movement would then commence, programmed for 30 seconds of green, then 3 seconds of yellow and 3 seconds of all-red. The total cycle time would be 62 seconds (20+3+3+30+3+3=62).

There are advantages, and disadvantages to this type of operation. The advantage is, it is pretty straightforward to set up, and the specific amount of green assigned to each phase (unique signalized movement) can be varied by time of day.

The variation by time of day allows for the cycle length to be adjusted to lengthen when necessary, and shorten when necessary.

The downside to this type of operation is that the signal is essentially a dumb box, without any way for it to modify operations by what is actually happening with the traffic. If the second movement has no cars, why serve it? If the 2nd movement has only 1 car waiting, why serve it for the entire time?

The fact of the matter is that with some situations, pretimed works pretty well, like in a downtown central business district grid. It also is a lot cheaper than installing vehicle or pedestrian detection systems in the intersection. It is also a pretty good way to deal with failed detection, as a temporary measure until the detection can be fixed.

It is possible that specific movements are timed, and other movements have vehicle detection, which would make that type of a signal semi-actuated.