# trafic light triggers **URL:** \ **Category:** Factual Questions\ **Created:** [February 24, 2003, 5:13am UTC](https://boards.straightdope.com/t/trafic-light-triggers/157103 "2003-02-24T05:13:10Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![King\_Friday](https://avatars.discourse-cdn.com/v4/letter/k/43a26b/32.png) [@King\_Friday](https://boards.straightdope.com/u/King_Friday)\ **Post date:** [February 24, 2003, 5:13am UTC](https://boards.straightdope.com/t/trafic-light-triggers/157103/1 "2003-02-24T05:13:10Z")
What exactly triggers a traffic light to change? It’s easy to see _where_ the device is, the big square right before the intersection, but what is it? I always thought it was some kind of weight sensor, but I was recently told that it something that detects an electricial current (from a vehicle engine). My father always told me that when he rode his motorcycle to work early in the mornings, he had to go through red lights because they would never change for him which makes me think it’s a weight thing, so…what is it? ---
**Author:** ![Blown\_Injected](https://avatars.discourse-cdn.com/v4/letter/b/a87d85/32.png) [@Blown\_Injected](https://boards.straightdope.com/u/Blown_Injected)\ **Post date:** [February 24, 2003, 5:16am UTC](https://boards.straightdope.com/t/trafic-light-triggers/157103/2 "2003-02-24T05:16:42Z")
I have activated such devices by waving metal over the area - leads me to believe that it is a magnetic switch ---
**Author:** ![King\_Friday](https://avatars.discourse-cdn.com/v4/letter/k/43a26b/32.png) [@King\_Friday](https://boards.straightdope.com/u/King_Friday)\ **Post date:** [February 24, 2003, 5:21am UTC](https://boards.straightdope.com/t/trafic-light-triggers/157103/3 "2003-02-24T05:21:57Z")
> [@](#): > > \*Originally posted by Blown & Injected \* > \*\*I have activated such devices by waving metal over the area - leads me to believe that it is a magnetic switch \*\* Now that I think about it, the power gates at work ( BWI airport) work with metal detectors. Those gates can be opened by sliding a nice sized piece of metal under the gate. ---
**Author:** ![bradwalt](https://avatars.discourse-cdn.com/v4/letter/b/858c86/32.png) [@bradwalt](https://boards.straightdope.com/u/bradwalt)\ **Post date:** [February 24, 2003, 5:22am UTC](https://boards.straightdope.com/t/trafic-light-triggers/157103/4 "2003-02-24T05:22:57Z")
Good question! And is it the same type of switch that controls those parking lot gates? I’m always afraid the swinging gate is going to come crashing down on my roof if I don’t clear it in time! ---
**Author:** ![Happy\_Lendervedder](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/happy_lendervedder/32/8488_2.png) [@Happy\_Lendervedder](https://boards.straightdope.com/u/Happy_Lendervedder)\ **Post date:** [February 24, 2003, 5:24am UTC](https://boards.straightdope.com/t/trafic-light-triggers/157103/5 "2003-02-24T05:24:34Z")
I always thought they were just run on timers. Huh. Learn something new every day. Happy ---
**Author:** ![PearlJamDW](https://avatars.discourse-cdn.com/v4/letter/p/6de8d8/32.png) [@PearlJamDW](https://boards.straightdope.com/u/PearlJamDW)\ **Post date:** [February 24, 2003, 5:27am UTC](https://boards.straightdope.com/t/trafic-light-triggers/157103/6 "2003-02-24T05:27:42Z")
They actually operate with an electrical component known as an inductor. As your car approaches the light, the steel chassis (which has a high iron content), increases the inductance of the inductor past a critical value, notifying the traffic light that you are impatiently waiting for it to change. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [February 24, 2003, 5:29am UTC](https://boards.straightdope.com/t/trafic-light-triggers/157103/7 "2003-02-24T05:29:37Z")
It’s just a big metal detector search coil. Not all traffic lights use these, but when they’re present, you can see them as lines cut into the pavement in a rectangular shape, unless the roadway was repaved after the coil was installed. [Here’s](http://www.howstuffworks.com/question234.htm) some good information.