General Question

rowenaz's avatar

How does a traffic light change?

Asked by rowenaz (2436points) August 6th, 2007

Around here, when I drive up to an intersection with a traffic light, there are large rectangular shapes indented into the paved road. I know that I need to drive onto one of them, in order to get the light to change to green - but what is happening? Is it the weight of my car or something magnetic??

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10 Answers

glial's avatar

I would suspect a laser is detecting a shorting of it's length by your car blocking it. Or it could be Elfin Magic.

kevbo's avatar

I would say "mass" in either case as evidenced by my traffic light troubles during my motorcycle riding days (i.e. the light wouldn't turn for me).

hearkat's avatar

I agree with kevbo, as I have heard other motorcycle riders complain about this. More and more the motion detectors are being used instead. They are cameras mounted on the traffic light post.

thetmle's avatar

From what I have heard the boxes on the ground are the "trip wires" for the light. When your car's tires on on the strips the light senses a change in resistance and it trips the light. When I was riding motorcycles I was sure to stop on one of those lines and I never had a problem with a traffic light not noticing me!

hossman's avatar

The light sensors are magnetic and are triggered by changes in the magnetic field, just like the anti-shoplifting sensors at stores. Sometimes motorcycles and kit cars do not have enough metal to disrupt the magnetic field and trigger the light change.

Mrs_Dr_Frank_N_Furter's avatar

I used to think that there were people far away watching it all and changing the lights. then I fould out that it was a senor

claysmith's avatar

I can second glial and kevbo on the motorcycle troubles bit, but from what I’ve read the answer given by hossman is most correct. The Seattle Times has an article that indirectly answers the question. From the article, “As part of the sensor system, a wire is cut into the pavement in the shape of a circle or a rectangle near the stop line, Cunningham said. It senses the metal of vehicles and signals a control box on the side of the road to change the light.”

evelyns_pet_zebra's avatar

I’m going with elfin magic myself.

rowenaz's avatar

I like that one. My daughter said it was the fairies,

IchtheosaurusRex's avatar

A common misconception is that these sensors, called vehicle detectors, are tripped by weight. The loops buried in the pavement are actually metal detectors. The loop is part of a resonant circuit, an inductor (the loop) in parallel with a capacitor, and an oscillator tuned to the resonant frequency of the circuit. The presence of metal in or near the loop changes the inductance, and thereby the resonant frequency. The impedance of the circuit drops. This alerts electronics in the traffic light to the presence of a vehicle, so it knows to change.

Some of these sensors are not tuned particularly well, so they won’t pick up a small vehicle like a motorcycle or especially, a bicycle. Positioning the bike over the edge of the loop gives you the best chance of being detected.

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