How do Traffic Signals Work?

There is a lot of mystery surrounding how traffic signals work, however they are relatively simple.

In a nutshell, a traffic signal installation is comprised of a controller, traffic light heads and detection.

The controller is the ‘brains’ behind the installation and contains the information required to force the lights through various sequences.

Traffic signals run under a variety of different modes which can be dependant on location and time of day.

Fixed Time

Under fixed time operation the traffic signals will display green to each approach for the same time every cycle regardless of the traffic conditions.  This may be adequate in heavily congested areas but where a lightly trafficked side road is included within the sequence it is very wasteful if in some cycles there are no vehicles waiting as the time could be better allocated to a busier approach.

Vehicle Actuation (VA)

This is one of the most common modes of operation for traffic signals and as the name suggests it takes into account the vehicle demands on all approaches and adjusts the green time accordingly.

The traffic demands are registered through the detection installed either in the carriageway or above the signal heads.  The controller then processes these demands and allocates the green time in the most appropriate way. Minimum and maximum green times are specified in the controller and cannot be violated.

A vehicle passing a detector will demand a certain phase and once that phase is green any additional vehicles passing the detector will cause the phase to extend.  Traffic continues extending the green until either the traffic demand ceases and another approach gains green, or a conflicting demand causes the maximum green timer to count down.

At some junctions, if the side roads are lightly trafficked they are programmed to be called only when there is a traffic demand on street.  This approach is classed as being demand dependent. Pedestrian crossing facilities within junctions are often demand dependent to ensure that traffic is only stopped when a pedestrian is waiting to cross.

Whilst VA mode is more responsive than fixed time it can still be inefficient if there are long queues building up on conflicting approaches. The setting of maximum timers can be difficult due to changes in traffic patterns through junctions over time therefore to maintain effective operation the maximum timings should be regularly updated.  This is a labour intensive task for a Local Highway Authority and is often not undertaken thus leading to the signals becoming less and less effective over time.

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How do Traffic Signals WorkFixed TimeVehicle Actuation, VA, Demand Dependant