How does a traffic light know you’re waiting?
Move the car over the buried wire. Watch what the road notices.
Change detected No vehicle detected
Find the sensor
There’s a wire under the road.
A wire sits in a shallow groove cut into the road, then sealed over. It goes around several times to make a loop. Move the car toward it and watch the detector.
Some traffic lights use loops like this one. We’ve opened up the road so you can see the wire. The meter gives a simplified picture of how the loop reacts to nearby metal.
Wait, what?
It can still see a stopped car.
The electricity in the wire keeps changing even when the car is still. The metal keeps changing how the loop behaves, so the detector still notices it. The road doesn’t need to bend. This sensor isn’t weighing the car.
The metal changes the circuit.
Electricity in the wire keeps reversing direction. That makes a changing magnetic field around the loop.
When a car is above it, that field produces small circulating currents in the car’s metal. Those currents make their own magnetic field, which changes how the loop behaves electrically.
The detector measures that change and tells the traffic controller a vehicle is here. It can do that while the car is stopped.
The dashed marks stand for a changing magnetic field, not visible rays. The arrows in the metal show circulating currents. Both views are still snapshots.
Why the light can stay red.
The detector tells the controller a vehicle is waiting. The controller decides when that lane gets a green light. It has to account for other lanes and people crossing, too. Being detected doesn’t mean it’s your turn yet.
Other intersections use cameras or radar to notice vehicles. Some lights run on a set schedule instead.
Put a name to the hidden sensor
Inductive loop
A wire wound into a loop below the road. A detector powers it and watches for the electrical change caused by a vehicle’s metal.
“The car stopped over the inductive loop.”Eddy currents
Small electrical currents that circulate inside metal when a changing magnetic field reaches it. Their own magnetic effect changes how the road’s loop behaves.
“The loop makes eddy currents in the car’s metal.”Go down the rabbit hole
ReadTechnical reading: see how loop detectors workThe Federal Highway Administration’s detector handbook, with cutaways and electrical theory.ReadTechnical reading: explore other traffic sensorsThe handbook compares loops with cameras, radar and other ways to notice traffic.ReadTechnical reading: how sensors help control the lightsThe traffic signal timing manual explains how detection feeds into a controller’s decisions.
TryStop over the wireMove the Car position slider until the detector says “Vehicle detected.” Leave it there. The detector keeps noticing the car even when it stops.