A standard set of traffic signals consists of:
- a traffic signal controller
- vehicle detector loops and pedestrian push buttons
- traffic signal lights
- posts, pits and underground electrical cables that connect all the components together.
The traffic signal controller
The traffic signal controller is a computer controlled device that processes information received from the detector loops and pedestrian push buttons. It changes the lights based on its programming.
The controller sets how long the light is green for each traffic movement, and controls the change from one combination of lights (known as a phase) to the next.
It can operate in a 'standalone' way, or it can be programmed to coordinate with a series of nearby traffic signals.
Vehicle loop detectors and pedestrian push buttons
Vehicle loop detectors and pedestrian push buttons make the controllers 'see' them, and change the signal to give them right of way.
Vehicle loop detectors are loops of wire that work as a metal detector, buried in the road near the stop line at the intersection. When a vehicle is passing over the loop the magnetic field (inductance) of the loop changes. The controller is told that a vehicle is waiting to go through the intersection.
When the pedestrian push button is pressed the controller knows that a pedestrian is waiting to cross.
Traffic signal lights
Traffic signal lights are how the controller directs traffic. They tell the road users when to go and when to stop.
Tasmanian traffic signal lanterns follow universal traffic signal colour conventions.
- GREEN = Go if it is safe to do so.
- YELLOW = Stop if it is safe to do so
- RED = Stop.
Signal phases and cycles
Each combination of green, yellow and red lights is called a phase. Each phase has a programmed minimum time. Once the signals have entered a phase they can’t change again until the minimum time has ended.
One full sequence of all the vehicle and pedestrian movements (phases) at an intersection is known as the signal cycle. The cycle time varies by location and time of day.
The yellow light
Traffic lights change from green to yellow to warn people that the signal is about to turn red.
The length of the yellow light depends on the speed limit of the road.
The yellow light means stop if it is safe to do so. Any vehicle travelling at the speed limit toward a green signal that changes to yellow should have enough time to stop safely. If the driver has entered the intersection, they should have time to leave the intersection before the signal changes to red.
All-red time
The all-red time is the time between the end of the yellow light on one phase and the start of the green light on the next phase.
All-red time provides a safe clearance for people that cross the stop line towards the end of the yellow light, as they may be in danger of colliding with vehicles or pedestrians starting in the following phase.
The all-red time is based upon the physical size of the intersections and speed limit of the road. Similar to the length of the yellow light, the all-red time does not change throughout the day.