Actuated Traffic Signals Vs Yellow-Red Flashing

We have been asked why the city of West Des Moines does not use yellow-red flashing of traffic signals during the night when traffic volumes are low (the comment on my last traffic-related blog is one example). Yellow-red flash at a traffic signal is typically used when a traffic signal has a pretimed controller. This type of signal control gives a preset amount of time to one traffic movement and then a preset amount of time to the next traffic movement. The signal controller does not know whether there is a vehicle waiting for the green signal or not. This causes vehicles to stop on the major street when there is no traffic on the side street needing the signal.

The traffic signals in West Des Moines are fully traffic actuated. This means the vehicle detectors for each traffic movement let the signal controller know when a vehicle is present and needs a green signal. During the night, the traffic signals are taken out of coordination and placed into free operation. The signal is set to dwell, or rest in green, on the major street until a vehicle is present on another movement (an intersecting minor street, for example). When the signal controller receives the call that a vehicle is present, it will end the green for the major street and give the green to the movement requesting it. After that traffic has cleared, the signal will go back to green on the major street.

The problem with placing traffic signals into flash at night is that studies have shown that the number of traffic crashes occurring during the night flash operation increases, especially right-angle crashes which are the most serious. With the fully traffic actuated method of traffic signal control, we are providing a combination of safe and positive method of traffic control.

If a signal is observed cycling between the major street and side street or left turn lane during the night without a vehicle present, please contact our Public Works Department. We will need to know the intersection where the problem was observed, what you observed occurring, and the time of day you observed the problem. Thanks again for your interest in these types of issues. It is definitely worth asking why cities operate differently, and we want to let our residents know why we have made the choices we do regarding these kinds of operations in West Des Moines.