Traffic Signals

A traffic signal is any power-operated device by which traffic is warned or directed to take some specific action. The Washtenaw County Road Commission adheres to the regulations and guidelines provided by the Michigan Manual of
Uniform Traffic Control Devices (MMUTCD). The MMUTCD
specifies the size, shape and color of all traffic signs and
signals. It also provides guidelines for installing signals.


The decision to install a traffic signal is based upon the
following factors:


  • Vehicular and pedestrian volumes
  • Crash history
  • Vehicle delay
  • Vehicle progression


Traffic signals operate on one of three principles: pre-timed, semi-actuated and fully-actuated. Below is a brief summary of how the three operate.


  • Pre-timed Signal: A pre-timed signal runs a set timing plan independent of the existing traffic. It also has the capability of running different cycle lengths depending on the time of day.
  • Semi-actuated Signal: A semi-actuated signal includes a mechanism installed on the minor road that detects when traffic is present. This detection switches the green phase to the minor road to allow traffic to clear.
  • Fully-actuated Signal: A fully-actuated signal includes mechanisms installed on both the major and minor roads that detect the volume of traffic present. Based on the amount of traffic, the signal provides enough time to accommodate all of the vehicles.


The Road Commission receives many requests for traffic signals based on safety concerns. However, traffic signals do not always prevent crashes. In many instances after the signal is installed, the total number of crashes increases while the severity of the crashes decreases. While a properly placed traffic signal improves the flow and decreases serious injury crashes, an unnecessary one can be a source of danger and annoyance to all who use an intersection: pedestrians, cyclists and drivers.


Intersection Control Beacons (Flashers)

An intersection control beacon is a standard traffic signal head having flashing Yellow or Red indications in each face. They are intended for use at intersections where traffic or physical conditions do not justify conventional traffic signals but where high crash rates or potential exists and indicate a special need.