During peak periods, the green time for vehicles waiting on the side street or arterial street left turn lanes (called “minor movements”) may seem too short. This often occurs because larger movements (like heavy three lane through traffic) may need a lot of green time to match the traffic demand. Sometimes a few left turn vehicles may wait for a second cycle to complete a left turn, while all through movement vehicles clear the intersection on one green service. The same number of seconds moves many more vehicles in three lanes of through traffic than it does for the one left turn lane. In other words, the needs of the many can outweigh the needs of the few.
There are three basic restrictions when adding more time to a movement:
- In order to coordinate with adjacent signals we must make the total time needed to serve all movements at one intersection equal to the sum of all movements at each signal in the coordinated system. Therefore we are working with a fixed amount of time, called the “cycle length”. We cannot simply ADD time to a movement without taking it away from another movement.
- Coordination needs to be provided for both directions on the arterial street. This means that each through movement needs green at a predetermined time in each cycle of the signal. If both directions on the arterial arrive at an intersection at the same time, then they are both using the same green time at that signal. It is then easier to provide lengthy green time to the side street. However, when one arterial platoon of vehicles arrives significantly sooner than the other, it takes longer to service both directions of the arterial coordination. Since there is a fixed amount of overall time (cycle length), this leaves less time for other movements.
- More green time for one approach means more red for all other approaches. During periods of heavy traffic, extending the green for one direction stacks up vehicles even more in other directions. Then those directions need more green time. That in turn makes the red at the first approach longer and more vehicles stack up there. This results in an ever increasing demand for green time.
Why do I wait so long to get a green light off the side street?
Most traffic signals in our city are timed to coordinate with each other for the majority of each day. The goal is to allow as many cars as possible to travel through multiple signals along an arterial street without stopping. To accomplish coordination, the cycle length at all signals in the coordinated group must be the same. (The cycle length is the sum of the time given to all movements- i.e. the time it takes to serve every direction and get back to the starting point.) The busiest intersections in a coordination group dictate the cycle length for the smaller signals in that group. As a result, smaller intersections like Campbell Rd. at Nantucket Dr. will have excess green time, because busier signals like Campbell Rd. at Coit Rd. need more time to complete all their movements. It is simply not possible to coordinate signals along an arterial if this rule is not obeyed.
What determines how much time each movement gets?
The coordination timing is different for each signal and changes several times each day. Green time is distributed to left turn, through, and right turn movements based on historical vehicle counts for the given time of day and day of week. It is our sincere desire to allow all vehicles that are stopped at a signal to clear the intersection on the first green service.
There are detectors in the roadway which tell the signal controller when a minor movement vehicle (on the side street or arterial street left turn lane) is waiting and needs a green light. The signal controller also locks in a call for the WALK and DON’T WALK time when a pedestrian pushes a button to cross. The signal controller determines when the best window of opportunity occurs during the cycling of the signal, and allows the minor movements to be serviced at that point. It makes this determination based on when arterial through movement vehicles from adjacent signals are expected to arrive and clear, and how many minor movements are anticipated to need service. Once the green begins for each minor movement, the duration of that green is extended by the vehicle detector until one of the following limits is reached:
- The time gap between vehicles exceeds the programmed limit.
- The maximum time allowed 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 10-20 seconds and for side street through it ranges from 12-30 seconds.
- The coordination timing requires a return to the main street green for the coordinated directions.
- An emergency vehicle approaches the intersection with lights and sirens on. The emergency vehicle has a preemption device which takes priority over the signal.
If the green time for a particular movement is repeatedly (day after day) shorter than what is needed to clear all vehicles present, the coordination timing may need adjustment, or the location may be over capacity. A traffic engineer needs to observe the traffic flow and assess the signal timing.