Signal Coordination Concepts
Traffic signal coordination is normally implemented to improve the level of service of a road or a network of roads, where the spacing of signals is such that isolated signal operation would cause excessive delays, stops and loss of capacity. The popular concept is that coordinating traffic signals is simply to provide green-wave progression whereby a motorist travelling along a road receives successive green signals. While this is one of the aims, the principal purpose of coordination is to minimise overall delay and/or number of stops. This can be achieved using fixed-timing plans or using adaptive technology.
The three main components of coordinated timings are:
- • Cycle time
- - the time to complete all phases in a timing plan (a phase is any period in a cycle where non-conflicting traffic movements may run);
- • Stage splits
- - the amount of time allocated to a phase in a cycle
- • Offsets
- - green signals at adjacent intersections are set to occur at a given time, relative to that at a reference intersection. It depends on the distance between signals, the progression speed along the road between the signals and the queues of vehicles waiting at red signals.
Traffic delay without coordination
In this figure you can see that the vehicle is delayed at the second intersection due to an uncoordinated signal time offset.
Traffic delay with coordination
In this figure you can see that the vehicle is not delayed at the second intersection due to a coordinated signal time offset.
Without coordination, there are frequent stops and unnecessary delays, for example:
- • Vehicles pass through a green light at one intersection only to be stopped by a red light at the next intersection, causing inconsistent travel;
- • Vehicles must wait through more than one green signal at an intersection due to blockages ahead;
- • Vehicles must stop at a red light when there are no other vehicles or pedestrians at the cross street.
- • In addition to extending travel time these situations also increase fuel consumption and emissions, as a stationary vehicle is much less efficient than a vehicle in motion.
Fixed-time plans, while allowing for traffic signal coordination as described, have the major disadvantage of being unable to adapt to changing conditions in real-time. At best they can be manually updated using accumulated traffic data however apart from being costly, this is a time-consuming exercise and could not be carried out to allow for unpredictable incidents e.g. accidents or breakdowns. For these reasons, an adaptive control system is preferable.