# Traffic light sensors **URL:** \ **Category:** Factual Questions\ **Created:** [August 19, 2003, 7:47am UTC](https://boards.straightdope.com/t/traffic-light-sensors/196163 "2003-08-19T07:47:06Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [August 19, 2003, 7:47am UTC](https://boards.straightdope.com/t/traffic-light-sensors/196163/1 "2003-08-19T07:47:06Z")
Most traffic lights these days have sensors that detect the presence of cars waiting for the light to turn green. These sensors are buried in the lanes, one at the head of each lane, and usually one or more some ways further back. As I understand it, they detect cars by induction. The sensors are basically a magnetic loop and the presence of a large amount of conductor near by (the metal in a car) induces changes in that magnetic field. You can tell where the sensors in the road are by looking for a small black diamond shape (the older ones) or a large black circle with a black line running to the side of the road. However, I’ve noticed that at most (if not all) of the newest intersections, I can’t see where they put the sensors. There’s no black lines on the pavement. This is not just an academic exercise. I bicycle frequently and sometimes I need to trigger traffic lights. I can usually do this by laying my bike down on the sensor and it will detect it. But on these new intersections I can’t see a sensor. On the assumption that there must be a sensor there even if I can’t see it, I tried laying down my bike at the head of the lane, but it didn’t work. So what’s going on? I don’t think they are using optical detection of cars as I would expect it to detect me on my bike. Plus I don’t see any special cameras for that. Is there some other technology at work here? ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [August 19, 2003, 10:24am UTC](https://boards.straightdope.com/t/traffic-light-sensors/196163/2 "2003-08-19T10:24:02Z")
Is it possible that the road and pavement have been surface-dressed, obscuring the lines? Is there a small box on the top of the traffic light cluster? - if so, they may be using microwave doppler detectors. Alternatively, the lights might be on a pre-programmed timer. ---
**Author:** ![x-ray\_vision](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/x-ray_vision/32/351_2.png) [@x-ray\_vision](https://boards.straightdope.com/u/x-ray_vision)\ **Post date:** [August 19, 2003, 10:57am UTC](https://boards.straightdope.com/t/traffic-light-sensors/196163/3 "2003-08-19T10:57:43Z")
According to [this](http://fredericksburg.com/News/FLS/2002/042002/04152002/564781/printer_friendly) web page: > [@](#): > > QThe new light at Lichfield Boulevard and U.S. 17 [in Stafford] is not properly timed. Apparently, no sensors were installed in the pavement. Is this project finished? Also, what is the purpose of the cameras above this intersection and the others on U.S. 17? > > –David Clark, Stafford > > AThe cameras above the intersection are the sensors. Placing them in > the pavement becomes very costly because they have to be replaced every time any road work is done. Hence, all new signals installed on busy roads by the Virginia Department of > Transportation now have cameras mounted on them. Each camera controls the timing of the light based on how much traffic it detects in each direction. ---
**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:** [August 19, 2003, 12:45pm UTC](https://boards.straightdope.com/t/traffic-light-sensors/196163/4 "2003-08-19T12:45:01Z")
New roads also have the sensors placed _before_ the asphalt is poured and levelled. Thus, no lines. ---
**Author:** ![js\_africanus](https://avatars.discourse-cdn.com/v4/letter/j/87869e/32.png) [@js\_africanus](https://boards.straightdope.com/u/js_africanus)\ **Post date:** [August 19, 2003, 1:32pm UTC](https://boards.straightdope.com/t/traffic-light-sensors/196163/5 "2003-08-19T13:32:47Z")
> [@](#): > > \*Originally posted by Q.E.D. \* > \*\*New roads also have the sensors placed _before_ the asphalt is poured and levelled. Thus, no lines. \*\* The city engineers I worked with preferred to do this when possible. They were not fans of putting in saw-cuts in order to put in the sensor loops.