# Real World Application: Intersection Signal Sensors

## Main Question or Discussion Point

Hey everyone!

I recently picked up a motorcycle for class/work commutes because walking home at 11pm from the lab is not fun. But I have come across something of interest.

Some intersections keep a green light on the main street and red on cross streets until a car pulls up. Before observing that I had sat at intersections forever, finally opting to "right on red" around until I got heading in the right direction again.

So clearly something senses a vehicles presence at the intersection, I ruled out weight sensors because that seems too easy to "fool" (ie have a mob of people stand on the spot and could get the light to change maybe). So I'm going to guess something such as how metal detectors work with detecting magnetic fields, which does make sense, a car does have a lot of iron on it. But what would you think the detecting threshold of something like this be? Or in other words, if I bought a couple rare-earth magnets and put them on the bottom of the motorcycle would this create enough signal to trigger the stop lights?

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I wondered this question too. I always thought they were pressure sensors but you're right that they detect for a change in magnetic field:
http://auto.howstuffworks.com/question234.htm

I've heard of motorcyclists who have the same problem as you (where the signals don't change) and am interested in your rare-earth magnet solution. I think the size of the inductors under the road are fairly large (in comparison to a bike), so I don't know if those magnets would sustain a big enough change.

mgb_phys
Homework Helper
They are generally induction sensors so any lump of metal will do.
It's a problem for a cycle - you can sometimes trigger them by aligning the bike exactly over the edge of the sensor.
The other solution is to get off and press the pedestrian cross walk button.

They are generally induction sensors so any lump of metal will do.
It's a problem for a cycle - you can sometimes trigger them by aligning the bike exactly over the edge of the sensor.
The other solution is to get off and press the pedestrian cross walk button.
I had thought about that too but my friend recently got a ticket for "abandonment" for doing so, he's trying to fight it right now. I think I'll go ahead with the magnet idea, if it doesn't work I can always use them for other things.

clem