Traffic signal delays

Last week we got a traffic related question/comment on the Accident Analysis entry on this blog. This particular resident pointed out that the traffic light intersections in WDM are all equipped with sensor systems and asked why they are not activated. They pointed out that sometimes when approaching a red light going north/south, with little to no traffic coming from the other direction, the light remains red until someone comes along from east or west and the light turns red on them so you can go. The question was, why is this so? Does it make sense from a traffic standpoint? I hope to explain to you how it does.

It is true that all of the signalized intersections in West Des Moines do utilize some method of detection of traffic at the intersection, either with loops in the pavement or with video detection cameras. This provides the intersection with what we call fully-actuated traffic operation so that the signal controller knows which movements have traffic on them and can skip movements that do not have traffic. For example, if there is no traffic in a left turn lane then the green arrow would not be displayed. Our goal using this system is to promote the smooth flow of traffic along the city’s streets in order to minimize delay at the signalized intersections. When a light turns red on someone just as they pull up to it, that is not a result of entrapment or the intersection detecting their presence and forcing them to stop; it is usually part of a larger coordination of traffic flow--we are constantly monitoring and responsible for the collective street network and not just one location.

Minor streets will experience an increase in delay when a traffic signal is placed into coordination with a major road in order move the larger volume of vehicles along the major street. This is because we need to force the signal to remain on the major street for a specified time period as well as be able to return to the major street within a specified time period to move the group of vehicles along the roadway from intersection to intersection. Traffic signal coordination plans change several times a day depending on the traffic volumes.

A malfunction with a detector loop or detector camera could also result in an extended wait time at an intersection. When detectors fail, they fail in a constant call or “fail-safe” mode. This will cause the green signal to return to a movement that does not have a vehicle there placing a call. During periods of heavy traffic, a detector malfunction is difficult to spot if one is driving through the intersection. These malfunctions are more evident during the light traffic periods since a green signal would be displayed for an extended time on a movement with no traffic.

Not all signal malfunctions can be spotted by the city personnel. We also rely on input from the public to assist us in this task. If you see a problem, or believe there is a problem, please contact the Public Works Department (222-3480 or via the Online Service Desk on our homepage at www.wdm-ia.com). We will need to know the intersection where the problem was observed, what the problem was, and the time of day that you observed the problem.

Sometime in the next couple weeks I will be providing a more comprehensive analysis of the equipment we utilize in our traffic operation system and what its capabilities and uses are. So watch for that!