How traffic lights work

Posted 7 September, 2017 Jane Woodhams

There are many things we don’t take much notice of in day-to-day life – traffic signals are among them.   In the video below, we take a look at this complex web of technologies as we go behind the scenes at Adelaide’s Traffic Management Centre to show you how traffic lights actually work. Then, read on as we look at some of the most commonly asked questions about SA’s traffic system.  

How are traffic lights and other signals managed?

Traffic lights are connected to a computerised system called SCATS – an Australian program used in 27 countries – which co-ordinates and measures traffic flow through in-road sensors and adjusts signals to ensure traffic moves efficiently. This intelligent traffic system also records the density of traffic sitting at intersections and then alters the timing and sequence of the next phase of lights to get vehicles moving quickly and safely.

Does the weight of my car trigger a green light at an intersection?

No. The detectors behind the stop line are loops of wire that produce a magnetic field. The change in the magnetic field when something metallic passes over it – like a car, motorbike or pushbike – is what activates the detector and triggers the signal to change. The signal also calculates the gap between cars and how long the vehicle has been at the intersection.    

Can you be too far behind the white line?

Yes – you need to be over the loop to be detected. For this to happen, you need to be immediately behind the painted white stop line. However, keep in mind that it’s against the law to have any part of your car over the white line.

Is it possible to get a ‘green run’?

Whenever you want to get somewhere quickly, it seems like the lights inevitably turn red as you approach. However, the SCATS system generally aims to get motorists through two to five sets of lights without stopping. During peak times, the groupings of lights that are being coordinated are bigger, but in the off-peak hours, the system automatically adjusts the links to smaller groups. This means that you may stop more often; however, you won’t be stopped for as long.