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.
-
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 a couple of metres across. A loop of wire sits in those slots, under the surface.
-
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.
-
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.
-
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.
-
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
The saw cuts a couple of metres across, just before the stop line, hold a loop of wire whose magnetic field changes when metal parks over it. A bicycle's slim aluminium frame barely disturbs it, which is why cyclists get a button.
Try it yourself
Look down at the tarmac next time you stop at lights. The thin rectangular saw cuts mark the loop buried underneath.