During peak hours lesser movements (like one lane left turns) may seem short. This is often because larger movements (like three lane through) may need the time to match the traffic demand. Yes, sometimes 2-5 vehicles may wait a second cycle for a left turn; while 8-18 through vehicles pass on the first green. The same number of seconds accomplishes much more for three lanes of through vehicles than it did for the one left turn lane.
There are three basic restrictions in adding more time to a movement:
- In order to coordinate with adjacent signals we must make the sum of all movements equal to the sum of all movements at all adjacent signals. Therefore we are working with a fixed amount of time. We cannot simply ADD time without taking it away from another movement.
- The arterial direction needs to be provided coordination in at least two directions (four directions where two arterials cross). This means the arterial needs green at a predetermined time. If both directions along the arterial arrive at the same time, it is easier to provide lengthy time to the side street. When one arterial platoon arrives significantly sooner than another, it take longer to service both directions of the arterial coordination. Since you are dealing with a fixed amount of time (cycle length); this leaves less time for other movements.
- When one direction is green several other directions are red. During heavy traffic periods, extending the green for one direction stacks up vehicles in other directions. Then those directions need longer greens. That of course makes the red at the first direction longer, more vehicles stack up and you end up in an ever increasing demand for green time.
Why do I wait so long to get a green light off the side street?
The traffic signals in our city run within coordination overlays for the majority of each day. In order to coordinate a group of signals together, the sum of all movements at any signal in the coordinated group must be equal to the sum of all movements at any other signal in that same coordinated group. This sum of all movements at a signal is called the cycle length. Therefore the large signals in a coordination group dictate the cycle length for the smaller signals in that group. As an example when you are waiting on a side street like Waterview, to cross Belt Line, you are waiting on large signals like Belt Line Rd. at US75 or Belt Line Rd. at Coit Rd. to accomplish all their movements. It is simply not possible to coordinate signals along an arterial if you do not obey this rule.
In addition, part of the City’s neighborhood traffic calming efforts include using signal timing to make smaller streets less attractive to cut-through traffic when there are larger arterials available that are more appropriate for that traffic to take. Part of those efforts can include deliberately higher delays on the side street to discourage that cut-through traffic. This may be an inconvenience to the residents of that neighborhood, but the benefits of lower traffic through the neighborhood have compensated for that inconvenience.
What determines how much time each movement gets?
The coordination overlay is different for each signal and changes several times each day. It is an apportionment of time based on historical vehicle counts for the appropriate time of day and day of week. It is our sincere desire to allow all vehicles which are stopped on the cross street at a signal to pass on the first green cycle and to pass as many vehicles as possible along the arterials without stopping. There are detectors in the roadway, which tell the signal controllers coordination overlay when a vehicle is waiting on the side street and needs a green service. The coordination overlay determines when an appropriate window of opportunity is, and allows the side street to be serviced at that point. It makes this determination based on when vehicles are expected to pass from adjacent signals and how many vehicles are anticipated to need service off the side street. Once the green is allowed for the side street, the duration of that green is determined by the vehicle detector until one of the following limits is reached:
- The time gap between vehicles exceeds the ‘GAP’ limit (usually 2.5 seconds).
- The maximum time for the movement is reached. This maximum changes by time of day and by type of movement. In general for left turns it ranges from 12-20 seconds, and for side street through it ranges from 12-30 seconds.
- The coordination overlay determines when the next movement must be serviced in order to get back in green for the coordinated directions at the appropriate time.
- An emergency vehicle desires passage along the opposing roadway. The emergency vehicle has a preemption device which takes priority over the signal.
If a particular movement is repeatedly shorter (day after day) than what is needed to pass all vehicles present; the coordination overlay may need adjustment, or the location may be over capacity.