Signal Synchronization, Timing and Triggers

Document Center   Community Safety   Online Payments   Events Calendar   City Services   Action Center   City Code   Map Center

Contact Us

226 West 4th Street
Davenport, IA 52801
Get Directions

Signal Synchronization, Timing and Triggers

There are many different types of traffic signals and uses for them. Traffic Engineering follows the Manual on Uniform Traffic Control Devices (MUTCD) and industry guidelines for placement and other characteristics of traffic signals. The progression of vehicles along a roadway is a guiding factor in defining how the signal will operate. In general, traffic signals are timed to reduce the delay for the most vehicles and help traffic flow more smoothly.

A standard for most signalized intersections is that the
  • collector streets, which enter onto or off of major arterial streets, have shorter green light cycles because of lighter traffic volume, while;
  • major arterial streets have longer green light cycles to move a greater volume of traffic efficiently.
This signal timing minimizes the delay for the traffic on the main arterial but sometimes causes larger delays for the traffic on the minor streets. If signals are timed correctly and the appropriate type of signal controller is used, minimum total delay (for major and minor street traffic) should be achieved.

Signal timing and trigger mechanisms used to move traffic through signalized intersections are as follows:

Pre-timed signals are those signals that have a preset cycle length for specified times of the day or for the entire day. Unlike actuated signals, a pre-timed signal cannot adjust to traffic flow. Therefore, the optimum cycle lengths for these intersections must be determined. Factors considered in designing a cycle length include the number of phases, the largest number of vehicles that can use a green light, and the number of lanes having right-of-way, just to name a few. Pedestrians are also a major consideration and their presence could increase the side-street green times and consequent cycle length at an intersection.  All of the downtown signals in Davenport and a few outside of downtown are pre-timed signals.

Some pre-timed traffic signals are timed as a system.  This synchronization of signals is often implemented on major thoroughfares where space between signals is close. Synchronization on major thoroughfares moves traffic more efficiently with generally fewer stops.   When a system is synchronized if you hit a green light at the first signal in the system and you are traveling the speed limit you will hit another green light at the next intersection, and so on.  Current lengths of roadway that feature synchronization during heavy traffic periods are: 

Actuated signals are those signals that can vary their cycle length (i.e., they vary the length of the green lights given) to the traffic flow that uses the intersection. Detectors register vehicles that approach these intersections and send the information to a signal controller. The controller adjusts the length of green light for the current traffic conditions. There are two types of actuated signals: fully actuated and semi-actuated.

  • Fully actuated signals are found at intersections that exhibit large fluctuations of traffic volumes from all of the approaches during the day. There is a set minimum and maximum green time for these signals. If there are no opposing vehicles stopped at the intersection, the moving traffic will receive additional green time. The minimum green time is often set equal to the time required for a pedestrian to safely cross the intersection.
  • Semi-actuated signals have detectors only on the minor street approaches to an intersection. They are often found at the intersections of main arterials and minor low-volume roadways. The main arterial has a green light until a vehicle is detected on the minor street. When the traffic volume is high on the side streets (during peak travel times) the semi-actuated signal acts as a pre-timed signal.