Ever wonder what the heck the city is thinking about when it programs its traffic lights? One frustrated reader did and sent us the following question, which we forwarded to Santa Rosa’s traffic engineer, Rob Sprinkle, for a response, which is below:
The question: As a Santa Rosa driver, I’m amazed and frustrated by the traffic lights in town. They seem designed to impede traffic, not facilitate it. In most cities, if you are driving on a major thoroughfare, going the speed limit, you make a number of lights and then all the lights turn red at once and the cross street lights all turn green. In Santa Rosa, it’s possible and frequent to miss two, three, four lights in a row on major streets like College Avenue, Fulton Road, Guerneville Road. Is there something about the lights in Santa Rosa I don’t know? Is there something the traffic engineers don’t know? Could you investigate? Robert D.
The answer: It is hard to explain how each signal in the city is timed and why, but I will share some of the challenges we have and hopefully help explain why signals operate the way they do.
At each intersection there is a traffic signal controller (computer) with instructions programmed on how to operate. The instructions (I will list just a few here) tell the controller whether to work in conjunction with other traffic signal controllers, if so, at what times during the day; how long the yellows must be; how long the pedestrian crossing times need to be; whether there are left turn arrows that need activation, and a host of other parameters.
Typically a controller uses detectors as inputs in order to determine if a pedestrian would like to cross or if vehicles and bikes are waiting to proceed at the signal. The pedestrian uses the buttons on the poles to input the detection, and either in-ground loops or camera detectors are used for vehicle and bike detections. One of the challenges we have is stuck detectors. A stuck detector tells the controller that there is a car, or it may “think” there is a car, waiting on a specific approach. If the detector is broken, or if a pedestrian button is broken, then the signal will continue to “think” there is a car or pedestrian waiting and will default back to that approach in the absence of any other detections. These issues are often difficult to track down as they can be intermittent but do cause unnecessary delay.
That aside, the majority of arterial corridors do have coordinated signal timing plans throughout various times of day. Our older systems use the actual time of day to set a group of signals into a synchronization plan to help move traffic more efficiently on a corridor. Even so, depending on when and where vehicles enter the system and whether it is peak commute times, delays may be experienced. We often do get inquiries regarding signal operations or malfunctions and it is often due to a malfunctioning detector. Another issue that occurs is when a signal transitions from one timing plan to another timing plan; the controller can short cycle and not give the same amount of time to an approach that it had in the previous cycle. The controller is doing this to try to get into step or sync with the other signals in the system.
Another system the City has been installing is an adaptive traffic control system. This system uses the detectors to determine how busy a signal, or group of signals, is and automatically adjusts the cycle length of that group to best utilize the time in the cycle. College Avenue between North Dutton Avenue and Brookwood Avenue has seen a 49% reduction in stops and a 33% decrease in travel time from end to end along this corridor compared to the non-adaptive traffic control system. Similarly, Guerneville Road/Steele Lane has seen an increase in efficiency following the implementation of the adaptive traffic control system.
This is just a quick explanation of some of the issues related to signal timing. It is much more involved and complicated than discussed above but I hope this helps explain some of the issues.
The city continually reviews the signal timing and operations throughout the city in order to achieve the greatest safety and efficiency.