I know a lot of you have been waiting with bated breath since my last post on grocery stores using radar for automatic doors. At the end of the post I mentioned looking into stop lights, and whether they use radar to see when a car is waiting for the light to change.
Short answer: No!
It turns out the way a traffic light senses that a car is waiting for a green arrow or light is an inductive loop. So what’s that?
An inductive loop is a coil of wire embedded in the road. After the asphalt is laid, road workers come back and cut a groove in the road and install the wire. Often the disturbed piece of road is covered by white paint, but you can often see it, with a big rectangular loop in the middle of the road.
An inductor (the wire) is an electromagnet. But instead of being designed to draw in metal, it is there to see IF there is a big metallic object is nearby. Like, for instance, a car. The loop in the ground is a magnet, and if you measure the amount of inductance it will be far higher when a car is present. It doesn’t take very much energy, so these magnets are left on all the time.
Don’t worry, they won’t mess with your electronics.
The reason for this, rather than a radar gun, is that it is more accurate at determining if a car is still waiting, and hasn’t turned right. While a radar gun would be capable of measuring if a car has turned, it would be a more complex process than using an inductive loop.
These loops can be tricky for motorcycles and bikes. They may not have enough metal to trigger then, so I’ve seen bike-mounted magnets that will produce a greater amount of inductance than the bike alone.
So it turns out that traffic lights don’t use radar guns to sense if a car is waiting, but they do have a pretty cool piece of technology for tracking cars.