# Traffic Light Sensors **URL:** \ **Category:** Factual Questions\ **Created:** [May 5, 2003, 6:16pm UTC](https://boards.straightdope.com/t/traffic-light-sensors/173233 "2003-05-05T18:16:09Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![cbpiz](https://avatars.discourse-cdn.com/v4/letter/c/e9c0ed/32.png) [@cbpiz](https://boards.straightdope.com/u/cbpiz)\ **Post date:** [May 5, 2003, 6:16pm UTC](https://boards.straightdope.com/t/traffic-light-sensors/173233/1 "2003-05-05T18:16:09Z")
Question for anyone would knows about this type of thing… Does every left turn signal have a “sensor” and if so, where is it? My husband has a habit of cussing out the car in front of us when we are stopped at a light and that car has not pulled up far enough to trigger this mysterious sensor. If it is under the concrete, why don’t I ever see workers digging up the road to repair these things? Nothing works that perfectly and is never in need of repair. Do all lights have a sensor or just some? Will the light never change if the bonehead in front of us never moves up a bit? I have been wondering about this for years now. Please help! ---
**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:** [May 5, 2003, 6:36pm UTC](https://boards.straightdope.com/t/traffic-light-sensors/173233/2 "2003-05-05T18:36:43Z")
Some have them some don’t. When they’re there, it’s nothing more than a big coil of wire buried under the road; a giant metal detector. You don’t see them digging them up for repair, because there’s virtually nothing to fail. The electronics for them are under an access panel in the roadway, or a box on the side of the road. There’s usually more than one for each lane being monitored, so triggering it is seldom a problem. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [May 5, 2003, 6:45pm UTC](https://boards.straightdope.com/t/traffic-light-sensors/173233/3 "2003-05-05T18:45:32Z")
Sometimes you can see a big rectangular cut in the ashpalt, sometimes it’s a sensor on the traffic light poll (getting more popular around here). If they repaved the road you won’t see the asphalt cut though ---
**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:** [May 5, 2003, 6:47pm UTC](https://boards.straightdope.com/t/traffic-light-sensors/173233/4 "2003-05-05T18:47:58Z")
I often see where the road was cut as if a sensor was placed in the road but the light does not seem to react as if the sensor is working - I yell at the road and the establishment for going through the expense and not making use of the time and trouble. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [May 5, 2003, 7:07pm UTC](https://boards.straightdope.com/t/traffic-light-sensors/173233/5 "2003-05-05T19:07:42Z")
**Blown & Injected** the senson doesn’t always give you a priority to go it just allow the turn signal to activate in it’s next cycle. Some do however give you a priority, some places have 4 way red lights (going straingt - too little traffic for turn signals) until someone hits the sensor, 1st one to hit it gets the green. This can get intertesting when you can see the car on the cross street and both know that whoever gets the sensor 1st will have the green. The sensor is usually close to the light so if you don’t get the green you have to lay on the brakes really hard to stop in time. ---
**Author:** ![El\_Smasho](https://avatars.discourse-cdn.com/v4/letter/e/f9ae1b/32.png) [@El\_Smasho](https://boards.straightdope.com/u/El_Smasho)\ **Post date:** [May 5, 2003, 7:08pm UTC](https://boards.straightdope.com/t/traffic-light-sensors/173233/6 "2003-05-05T19:08:08Z")
Those coils don’t always recognize motorcycles. I was behind a cop once and the turn light was not coming up fast enough for him, so he signalled me to pull alongside him to trigger 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:** [May 5, 2003, 7:20pm UTC](https://boards.straightdope.com/t/traffic-light-sensors/173233/7 "2003-05-05T19:20:31Z")
> [@](#): > > \*Originally posted by kanicbird \* > \*\ ***Blown & Injected** the senson doesn’t always give you a priority to go it just allow the turn signal to activate in it’s next cycle. > > Some do however give you a priority, some places have 4 way red lights (going straingt - too little traffic for turn signals) until someone hits the sensor, 1st one to hit it gets the green. This can get intertesting when you can see the car on the cross street and both know that whoever gets the sensor 1st will have the green. The sensor is usually close to the light so if you don’t get the green you have to lay on the brakes really hard to stop in time. \*\* Yeah, but I have been coming up to intersections that seem to have the sensor, there is no car there and the light comes anyway??? It would seem to me that if there is no car there, especially late at night, the sensor should say, no car waiting to turn so no light - We should go find the traffic light programer and beat’em up ---
**Author:** ![GaryM](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/garym/32/241_2.png) [@GaryM](https://boards.straightdope.com/u/GaryM)\ **Post date:** [May 5, 2003, 7:25pm UTC](https://boards.straightdope.com/t/traffic-light-sensors/173233/8 "2003-05-05T19:25:00Z")
Sometimes the sensors are used to ensure the light stays green until all the vehicles in the feeder lane have had a chance to turn. If the guy ahead of you leaves too big a gap, you might get stuck at the light. ---
**Author:** ![dwc1970](https://avatars.discourse-cdn.com/v4/letter/d/a183cd/32.png) [@dwc1970](https://boards.straightdope.com/u/dwc1970)\ **Post date:** [May 5, 2003, 7:34pm UTC](https://boards.straightdope.com/t/traffic-light-sensors/173233/9 "2003-05-05T19:34:28Z")
[This link](http://auto.howstuffworks.com/question234.htm) explains how the sensors work in more detail. > [@](#): > > There are all sorts of technologies for detecting cars – everything from lasers to rubber hoses filled with air! By far the most common technique is the inductive loop. An inductive loop is simply a coil of wire embedded in the road’s surface. To install the loop, they lay the asphalt and then come back and cut a groove in the asphalt with a saw. The wire is placed in the groove and sealed with a rubbery compound. You can often see these big rectangular loops cut in the pavement because the compound is obvious. ---
**Author:** ![NinetyWt](https://avatars.discourse-cdn.com/v4/letter/n/cc9497/32.png) [@NinetyWt](https://boards.straightdope.com/u/NinetyWt)\ **Post date:** [May 6, 2003, 1:32pm UTC](https://boards.straightdope.com/t/traffic-light-sensors/173233/10 "2003-05-06T13:32:45Z")
Plus sometimes the wire loop _is_ broken and must be replaced. If the light refuses to work **Blown** I would suggest calling the road department.