The other day I was driving and I came to an intersection and stopped for a red light. There were no other cars in sight in any direction and I wondered if I should just run the red light. I know that would be breaking the law, but the laws are only there to protect us and give us a more productive and efficient society anyway and in this particular instance I was at a perpendicular intersection with good sight lines in all directions in the middle of the day and there wasn't so much as a pedestrian around. I'm sure everyone who drives regularly has encountered this situation before. And it occurred to me that at some intersections this never really happens. At some intersections they have some sort of detector that knows your car is there and the light automatically changes. But how does it detect your car, I wondered. Thoroughly lost in thought at this point, all considerations of becoming an outlaw for the day faded. I imagined all sorts of sensors they might employ for such a task... lasers, pressure plates, leprechauns... I couldn't be sure... The light changed to green and I drove on, but the thought stayed with me so when I got home I was compelled to google it. I was surprised to discover that while there are a variety of techniques they use to accomplish essentially the same goal, by far the most common detection system they use is the inductive loop traffic detector. Briefly, it is a coiled wire embedded in the surface of the road that, when electricty is passed through it, creates an electromagnetic field. The inductance is constantly measured and any changes are detected. When a large metallic object, such as a car or truck is placed in close proximity (i.e., parked overhead), the inductance is greatly increased, the meter measures the change, and the traffic light turns green.
Later, while searching youtube for something entirely unrelated, I came across this video (THOSE WHO RIDE SCOOTERS, MOTORCYCLES, BICYCLES, OR SMALL CARS TAKE NOTICE!):
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I hate when you get stuck, late at night, behind someone at one of these lights. The person in front of you has no idea about the wires and just sits there. I used to have a turn light pulling in to where I lived off of 280, that would only work at night when it was triggered. I got stuck behind folks for five to ten minute intervals all the time. the light would only change when they would final just decide to run the it.
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