How do traffic signals work?
How do traffic signals work?
A standard set of traffic signals consists of:
a traffic signal controller;
vehicle detector loops and pedestrian push buttons;
traffic signal lanterns; and
posts, pits and underground electrical cables that connect all the components together.
The traffic signal controller
Housed in a grey metal box on a corner of the intersection, the controller is the 'brain' of the system. It is a computer that processes information received from the detector loops and pedestrian push buttons and changes the signal lanterns in accordance with its programming. Based upon the prevailing demands, the controller determines the length of the green signal for each traffic movement and controls the transition from one combination of green and red signals (known as phase) to the next. It can operate in a 'stand alone' manner or be programmed to coordinate with a series of adjacent traffic signals.
Vehicle loop detectors and pedestrian push buttons
Vehicle loop detectors and pedestrian push buttons are the 'eyes' of the system. They are mechanisms motorists and pedestrians use to make the controllers 'see' them and change the signal to give them right of way.
Vehicle loop detectors are loops of wire buried in the road leading up to the stop line at the intersection. When a vehicle is passing over the loop the magnetic field (inductance) of the loop changes. The controller detects that a vehicle is waiting to proceed through the intersection.
Likewise, when the pedestrian push button is pressed the controller knows that a pedestrian is waiting to cross.
Traffic signal lanterns
Traffic signal lanterns are the means by which the controller directs traffic. They tell the road users when to go and when to stop. Queensland 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; and RED = Stop.
Modern pedestrian signals use the symbolic walking green and standing red figures figures although there are still older lanterns around that display WALK and DON'T WALK.
Over time, the department is converting standard signal lanterns to LED (Light Emitting Diode) lanterns which are very energy efficient and long lasting.
Signal phases and cycles
Each combination of green and red signals that the controller is programmed to display is called a phase. Each phase has a programmed minimum time so that once the signals have entered a phase they cannot change again until the minimum time has expired. One complete sequence of all the vehicle and pedestrian movements (phases) at an intersection is known as the signal cycle. In Queensland, the duration of a cycle is usually between 50-150 seconds. The cycle time varies by location and time of day.
The yellow signal
Traffic signals change from green to yellow to warn approaching motorists that the signal is about to turn red. The length of the yellow signal depends on the speed limit of the road. Most urban intersections have a yellow time of four to six seconds. The yellow signal 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 sufficient time to stop safely or clear the intersection before the signal changes to red if the driver has entered the intersection.
The all-red time is the time between the end of the yellow signal on one phase and the commencement of the green signal on the next phase. All-red time is used to provide a safe clearance for vehicles that cross the stop line towards the end of the yellow signal 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 signal, the all-red time does not change throughout the day.