Traffic Signal Timing

The following general description will provides a summary of the process involved in the establishment of traffic signal synchronization plans.

In order to implement a timing pattern (or plan), the Traffic Engineering Division first collects and collates data on traffic flow, distance between signals, traffic volumes (both turning movement counts as well as continuous arterial counts) and the like. This information then forms the basis for the establishment of signal sections. These sections consist of a certain number of adjacent signalized intersections that have similar characteristics with respect to traffic volume, traffic directional flow, geographic location, and so forth. Subsequent to the collection and collation of this traffic data, the Division's Signal Timing Engineer then inputs this information into traffic signal timing software (such as SYNCHRO). The outputs from these programs provide the preliminary signal progression data like cycle length, offsets and splits.

The cycle length is the time that elapses for the signal to service all the signal phases between the beginning of green for the main street to the return of the beginning of green for the main street; that is, a complete sequence of signal indications. This time, expressed in seconds, is dependent upon the traffic volume to be serviced at any given intersection (main street and side streets) and therefore varies by the time of day.

The offset, which is the number of seconds that a defined time reference point at a traffic signal occurs after the time reference point of an adjacent traffic signal, determines the progression direction. For example, when the heavier flow is westbound during a certain time of the day, the progression will favor that direction.

The split (or phase, or phase green) defines the amount of green time allocated for each movement which is dependent on the amount of the traffic by approach and turning movement.

However, it should be noted that the offsets and splits are also dependent on overall vehicle delay (both arterial and intersection) as well as the posted speed of the roadway. The signal timing software takes this into account when minimizing the delay and stops for all traffic movements and establishing the best progression for the roadway.

Once the signal timing data has been entered into the signal controllers, the Signal Timing Engineer then observes the progression and overall traffic conditions to fine tune the patterns established by the computer programs. Please note that, during periods of congested traffic flow which may not be characteristic of the pattern being evaluated, the operating speed of the roadway may be more or less and thus progression suffers.

There are some problems that even the best signal timing can not prevent. One such problem is the saturation of the roadway network during certain times of the AM or PM peak. This saturation is characterized by the fact that there are more cars on the different approaches to a signalized intersection than can be handled by the available lanes and green times (splits) of the signal. This is comparable to the flooding of land adjacent to canals when the entering water volume exceeds the outflow or when a temporary blockage occurs (such as a traffic crash). This type of congestion can have a long term effect on the arterial traffic flow and can still be present even after the accident has cleared and the vehicles removed.

Another problem or constraining factor in obtaining optimal traffic flow is the operation of the loop detectors. These devices are sensors embedded in the roadway that provide the traffic signal with data that determines how much green time is allocated to the minor movements (that is, side street and/or separate left turn phases). If any one of them is damaged or broken, the movement associated with that loop must be placed on recall until the loop can be repaired. The net result is that the amount of green time is reduced for the other movements, thus further impairing operation.

The study of the timing for arterials in Broward County is an ongoing process. In addition to our efforts in always seeking to optimize arterial progression, several solutions have also been reviewed in the past. These include such things as increasing the number of lanes on a highway, providing for more controlled access along the corridor at more minor side street locations, and the like.

Click here for some frequently asked questions about signal timing.