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Seeing red, triggering change

1 April, 2011

Ever been left stranded at a red light for what seems like hours, the green a distant memory? Emma Clark takes a close look at traffic light sensors.

As a solidly responsible rider you’ve pulled up at a red light and are waiting patiently, but as time ticks by, and the light refuses to budge, frustration is mounting. There’s no other traffic about. It’s tempting to just run the red. But should you? What if the lights haven’t registered your presence; how do you get them to ‘see’ you?

“It has to do with the amount of metal in the bike,” says Steve Bean, keen bike rider and manager of Intelligent Transport Systems at VicRoads. “Traffic signals have inductor wires embedded in the road surface which are connected to an electronic receiver in the nearby traffic signal controller.

“A current is passed through the loop wire, creating an electromagnetic field. When a metallic body such as a bicycle or car passes over the detector, it will produce a change to the electromagnetic field of the loop, registering its presence. Weight doesn’t affect the detection.”

The wires are embedded into the road and sealed with an epoxy compound (see figure 1) and you can usually see the telltale grooves in the surface of the road. All the wires connect to a detector pit at the kerb.

Because bikes contain less metal than cars, to be detected they should be situated directly over the area of greatest sensitivity. The wires in each lane are looped in a figure eight shape, with the centre line of the box a crossover point, and the most sensitive location. To maximise your chances of triggering the light, run your wheel along this centre line (see figure 2).

“Most of the issues with the system have to do with tuning the loops,” says Steve. “If they are tuned too sensitively, cars in other lanes, or even wheelchairs, can trigger the loops. If they are tuned too low, then bikes, motorbikes and scooters can have trouble triggering them.”

Bikes made from carbon fibre or aluminium may have more difficulty registering their presence, but most will contain enough metal parts to trigger a response.

There are some intersections where riders have reported they cannot change the lights no matter how many times they roll over the embedded wires. In these situations, it is best to assess the risk and decide whether to jump off the bike and press the pedestrian button, wait for a car, or get off and walk. Don’t run the red: it antagonises the public and you risk being hurt, or hit with a hefty fine.

Always report a faulty induction loop to your local roads authority; that’s the only way they will know there is a problem. Steve told Ride On: “There is usually a phone number located on the traffic controller box near the traffic lights. You can call this number and someone will visit the site and retune the system.”


We have heard of all sorts of tips on how to trigger traffic lights. What’s your best advice?

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