Traffic signals and signal controlled crossings

Traffic signals

The use of traffic signals to control traffic movements can reduce congestion, improve road safety and enable the introduction of specific strategies which regulate the use of the road network.

Example of such strategies might be:

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  • to reinforce the designated route hierarchy
  • to give priority to public transport
  • to provide crossing facilities for pedestrians and cyclists, or
  • to maximise traffic flow

The Pelican is a signal controlled pedestrian crossing which uses far side pedestrian signal heads and has a vehicle red/pedestrian green crossing period followed by a flashing amber/flashing green man where pedestrians, who are still on the crossing continue to have right of way.

The Puffin is a signal controlled pedestrian crossing but unlike to the Pelican does not use far-side pedestrian signal heads but relies on near-side pedestrian signals. The Puffin crossing also employs pedestrian detection which is used to vary the length of the pedestrian crossing period (red signal to vehicular traffic). The system does not employ flashing amber/flashing green man periods.

On crossing detection is used to extend the all red period to allow slower pedestrians to safely cross the road.

The Toucan crossing is a signal controlled crossing to assist both pedestrians and cyclists. The crossing has far side or near side pedestrian /cyclist heads and employs pedestrian / cyclist on crossing detection in a similar way to the pelican crossing. The Toucan does not employ flashing amber/green man but has a blackout period similar to the pedestrian phase at traffic signals.

The pedestrian / cyclist detection varies the length of the all red period ( i.e. red signal to vehicular traffic / red to pedestrians / cyclists)

Phase - The word phase is used to describe an approach or pedestrian movement.

Stage -  A stage is that part of a cycle during which a particular set of phases receives green. In each signal controller there is a normal sequence in which the various stages receive green and one repetition of this sequence is called a cycle.

Intergreen Period -   The period between the end of the green for one phase and the start of green for another phase gaining right of way. In the United Kingdom this period includes an amber for one phase which last for 3 seconds and a red and amber period for the next phase which last 2 seconds. These may overlap by up to 1 second, run consecutively, or be separated by an all red period during which both phases receive red signals.

Vehicle Actuation - Traffic signals working under vehicle actuation give green signals for time periods which are effected by traffic demands. Traffic demands are measured by vehicle loops which are installed on the approaches to the signals. For each phase there will be a minimum green period which is normally seven seconds.

Vehicle Detectors - Vehicle detectors are linked to the loops and are used to:

  • Demand and/or extend the green periods for a phase
  • Extend an all red period
  • Demand and/or extend a green for a particular category of vehicle (eg: buses, emergency vehicles)
  • Introduce a ‘hurry’ call
  • Measure speed of approaching vehicles
  • Detect queues

Fixed time operation - Traffic signals can operate to a pre-determined stage order and duration of green periods. This arrangement is known as fixed time operation and does not generally respond to changes in traffic demand.

Signal Aspects - Signal heads facing drivers have 3 signal aspects – red, amber and green. A green arrow may be fitted in place of a full green aspect and further green arrows can be added to assist traffic direction and control when used in conjunction with the appropriate regulatory signs and road markings.