When it comes to the question of ‘How do traffic signals work?’ there is more to it than meets the eye, however, in a nutshell, a traffic signal installation comprises a controller, traffic light heads and detection.
The controller is the ‘brain’ of the installation and contains the information required to force the lights through various sequences.
Traffic signals can run under a variety of different modes which can be dependent on location and time of day.
Under fixed time operation the traffic signals will display green to each approach for the same amount of 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 can be 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)
Vehicle Actuation is one of the most common modes of operation for traffic signals, and as the name suggests, it takes into account the variations in vehicle demand 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 time 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 one of the side roads is lightly trafficked, the signals will be programmed to only call a green light 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 and is not appropriate for all types of signal controlled junctions. 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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