# What mechanism detects cars at a stop light?

**URL:** <https://boards.straightdope.com/t/what-mechanism-detects-cars-at-a-stop-light/74601>\
**Category:** Factual Questions\
**Created:** [August 2, 2001, 3:56am UTC](https://boards.straightdope.com/t/what-mechanism-detects-cars-at-a-stop-light/74601 "2001-08-02T03:56:20Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![super\_edo](https://avatars.discourse-cdn.com/v4/letter/s/dfb087/32.png) [@super\_edo](https://boards.straightdope.com/u/super_edo)\
**Post date:** [August 2, 2001, 3:56am UTC](https://boards.straightdope.com/t/what-mechanism-detects-cars-at-a-stop-light/74601/1 "2001-08-02T03:56:20Z")

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Some stop lights do not change to green unless a car is detected as waiting, particularly at intersections where nearly all the traffic is going across one street and not the other. So what mechanism triggers the light to change? It’s something under the concrete, right? But how does it work? I’ve been wondering about this for years. Anyone know?

Matthew

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**Author:** ![yabob](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/yabob/32/2821_2.png) [@yabob](https://boards.straightdope.com/u/yabob)\
**Post date:** [August 2, 2001, 4:05am UTC](https://boards.straightdope.com/t/what-mechanism-detects-cars-at-a-stop-light/74601/2 "2001-08-02T04:05:42Z")

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Been there, driven over it:

[http://boards.straightdope.com/sdmb/showthread.php?threadid=39443](http://boards.straightdope.com/sdmb/showthread.php?threadid=39443)

Most modern ones are induction loops, sort of like big metal detectors. Some of us can remember ones operated by a mechanical pressure plate.

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**Author:** ![super\_edo](https://avatars.discourse-cdn.com/v4/letter/s/dfb087/32.png) [@super\_edo](https://boards.straightdope.com/u/super_edo)\
**Post date:** [August 2, 2001, 4:08am UTC](https://boards.straightdope.com/t/what-mechanism-detects-cars-at-a-stop-light/74601/3 "2001-08-02T04:08:47Z")

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Thanks. Huh. I searched for it in the messageboards, but nothing came up.

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**Author:** ![Hilow](https://avatars.discourse-cdn.com/v4/letter/h/c4cdca/32.png) [@Hilow](https://boards.straightdope.com/u/Hilow)\
**Post date:** [August 2, 2001, 4:32am UTC](https://boards.straightdope.com/t/what-mechanism-detects-cars-at-a-stop-light/74601/4 "2001-08-02T04:32:27Z")

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I Agree that this topic is mostly covered by the previous link, but one thing I believe is missing is pulling too far into the intersection to wait for the light.

In my town I often notice the cars in the left turn lane have pulled into the crosswalk and adjusted the position of all cars behind them to the point that the left turn signal doesn’t even fire.

It is very important to pay attention to the lines that are painted to indicate the stop point and to stop behind that line. If you pass it, you may be in for a long wait.