Embedded Traffic Light Sensors


My neighbor owns a motorcycle and sometimes has to sit through several rounds of lights at the intersection near our house because the traffic light sensor embedded in the asphalt doesn't pick up enough steel to let the traffic light computer know someone is waiting at the intersection.

What? You didn't know that's how a lot of lights work these days? Yep, other than during rush hour (when they're usually on simple timers like they were in the old days), many traffic lights, especially those with the arrows that allow you to turn left, work off some kind of sensor. The most common is the inductive loop under the road.

When you're in a car (which has a lot of steel in it), idling right behind the crosswalk, the car acts like the core of an inductor, causing the loops of wire under the asphalt to trigger the computerized traffic light box at the intersection. Basically, your car is telling the light, "Hey, I'm here, let me through."

But my neighbor's motorcycle doesn't have enough umph, and he'll often get shut out of an entire sequence of red-green. Even the cars coming in the opposite direction get the green, but he sees only red.

Bicyclists have the same problem. True story: I was once on a bike in Irvine, California (don't ask) and made a left turn without the arrow (because it wasn't turning green!) and got pulled over by a police car for running the red. Luckily, he let me go with a warning, but it really wasn't my fault. If motorcycles don't have enough steel in them to trigger the sensors, how could bicycles? (Actually, I've recently found a couple patents for devices that would hook up to bike's for just the reason. Check this one out.)