A wire loop under the tarmac
How Does the Road Know You Are Waiting?
A loop of wire in the road carries a magnetic field that your car's metal disturbs.
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Step 1 of 5
Look for the rectangle cut into the tarmac.
Just before the stop line you can usually see thin saw cuts forming a rectangle, and a loop of wire sits in those slots under the surface.
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Step 2 of 5
A loop of wire makes a magnetic field.
Running a current round a loop creates a weak magnetic field through it, and the strength of that field depends on what is sitting nearby.
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Step 3 of 5
Metal above it changes that field.
A large lump of metal parked over the loop alters how easily the field forms, and the controller can measure that change electrically.
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Step 4 of 5
The change is counted as a vehicle.
Once the controller registers a change it knows somebody is waiting, and your direction joins the queue for the next available green.
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Step 5 of 5
Bicycles are hard to see, so buttons exist.
A slim aluminium frame disturbs the field far less than a car does, which is why cyclists and pedestrians are given a button instead.
Road loops are usually buried about 5 centimetres below the surface and can notice metal up to roughly a metre above them.
The short version
Traffic lights know you are there because a wire loop in the road carries a magnetic field that the metal of your car disturbs.
Try it yourself
Look down at the tarmac next time you stop at lights. The thin rectangular saw cuts mark the loop buried underneath.
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