# How can they tell I ran a red light? **URL:** \ **Category:** Factual Questions\ **Created:** [June 16, 2003, 6:45pm UTC](https://boards.straightdope.com/t/how-can-they-tell-i-ran-a-red-light/182268 "2003-06-16T18:45:16Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![NFlanders](https://avatars.discourse-cdn.com/v4/letter/n/a9adbd/32.png) [@NFlanders](https://boards.straightdope.com/u/NFlanders)\ **Post date:** [June 16, 2003, 6:45pm UTC](https://boards.straightdope.com/t/how-can-they-tell-i-ran-a-red-light/182268/1 "2003-06-16T18:45:16Z")
I’ve been wondering about this ever since they installed a camera at an intersection near my house. When does the camera take a picture? If it takes it at the moment the light turns red, they will only photograph people who entered the intersection when the light was yellow. The most egregious offenders enter the intersection several seconds after the light has been red. How long would someone have to wait after the red light to enter the intersection with impunity? ---
**Author:** ![bordelond](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bordelond/32/150_2.png) [@bordelond](https://boards.straightdope.com/u/bordelond)\ **Post date:** [June 16, 2003, 6:54pm UTC](https://boards.straightdope.com/t/how-can-they-tell-i-ran-a-red-light/182268/2 "2003-06-16T18:54:37Z")
I’m pretty sure that these devices take several time-stamped pictures in a row, probably for several seconds after the light turns red. ---
**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:** [June 16, 2003, 6:58pm UTC](https://boards.straightdope.com/t/how-can-they-tell-i-ran-a-red-light/182268/3 "2003-06-16T18:58:00Z")
They probably have a sensor buried under the road, sort of like a big metal detector. When a car passes over it and the light is red, a picture is snapped. If it’s an older road, you might be able to see the lines cut in the pavement where the coils are buried. ---
**Author:** ![traindrain](https://avatars.discourse-cdn.com/v4/letter/t/e56c9b/32.png) [@traindrain](https://boards.straightdope.com/u/traindrain)\ **Post date:** [June 16, 2003, 7:46pm UTC](https://boards.straightdope.com/t/how-can-they-tell-i-ran-a-red-light/182268/4 "2003-06-16T19:46:59Z")
[This](http://electronics.howstuffworks.com/red-light-camera1.htm) describes how these cameras work. Short version: typically there are two induction loop triggers positioned under the road near the stop line. The cameras typically are meant to capture you entering the intersection _and_ mid-way through. ---
**Author:** ![NBIT33](https://avatars.discourse-cdn.com/v4/letter/n/71c47a/32.png) [@NBIT33](https://boards.straightdope.com/u/NBIT33)\ **Post date:** [June 16, 2003, 8:57pm UTC](https://boards.straightdope.com/t/how-can-they-tell-i-ran-a-red-light/182268/5 "2003-06-16T20:57:07Z")
Got a ticket this way. They mailed me two pictures; one shot of me not yet in the intersection, but with the light having already turned red, and another of me IN the intersection, light obviously still red. Had me dead to rights. ---
**Author:** ![NoGoodNamesLeft](https://avatars.discourse-cdn.com/v4/letter/n/8797f3/32.png) [@NoGoodNamesLeft](https://boards.straightdope.com/u/NoGoodNamesLeft)\ **Post date:** [June 16, 2003, 8:57pm UTC](https://boards.straightdope.com/t/how-can-they-tell-i-ran-a-red-light/182268/6 "2003-06-16T20:57:49Z")
There are also models that use a motion sensing beam that is focused in the middle of the intersection -at least that’s what is out front of my house. (Sort of like the thing at Radio Shack that goes !BING BONG! when you walk in the door). If a car breaks that ‘beam’, the photo is taken. Frankly, I can’t see how a sensor under the road would know the difference between traffic that is supposed to be stopped (say East/West) and traffic that is allowed to go thru the intersection (North / South). ---
**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:** [June 16, 2003, 9:05pm UTC](https://boards.straightdope.com/t/how-can-they-tell-i-ran-a-red-light/182268/7 "2003-06-16T21:05:03Z")
> [@](#): > > \*Originally posted by NoGoodNamesLeft \* > \*\*Frankly, I can’t see how a sensor under the road would know the difference between traffic that is supposed to be stopped (say East/West) and traffic that is allowed to go thru the intersection (North / South). \*\* There’s usually two sensors for each stop line, one at the stop line and another just past it. They don’t cross the entire roadway, so traffic turning onto the street that has the red light doesn’t trigger the camera. Yes, that means there’s a way around it, but don’t do that. ---
**Author:** ![manhattan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/manhattan/32/7_2.png) [@manhattan](https://boards.straightdope.com/u/manhattan)\ **Post date:** [June 16, 2003, 11:29pm UTC](https://boards.straightdope.com/t/how-can-they-tell-i-ran-a-red-light/182268/8 "2003-06-16T23:29:28Z")
> [@](#): > > \*Originally posted by NoGoodNamesLeft \* > \*\*There are also models that use a motion sensing beam that is focused in the middle of the intersection -at least that’s what is out front of my house. (Sort of like the thing at Radio Shack that goes !BING BONG! when you walk in the door). \*\* That’s gotta be the technology used here in NYC. I know they didn’t dig up intersections to install these things (in fact, they move 'em around from time to time). There must be some sort of radar trigger in the camera installation itself. ---
**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:** [June 16, 2003, 11:36pm UTC](https://boards.straightdope.com/t/how-can-they-tell-i-ran-a-red-light/182268/9 "2003-06-16T23:36:20Z")
Yes, i’d wager it’s similar to many supermarket automatic doors. They use a low-power doppler radar to detect motion. ---
**Author:** ![stockton](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@stockton](https://boards.straightdope.com/u/stockton)\ **Post date:** [June 17, 2003, 3:18am UTC](https://boards.straightdope.com/t/how-can-they-tell-i-ran-a-red-light/182268/10 "2003-06-17T03:18:08Z")
Ouch… they must work better than my WAG detector. Is this still GQ? “Probably, gotta be, pretty sure, might be, I’d wager?”… 5 out of 8 responses. Anyone have the actual dope? I’m curious about this too, and I have my own wild theories. I won’t puke them up as fact in case you might remember them as such. Kindly return the favor. ---
**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:** [June 17, 2003, 3:21am UTC](https://boards.straightdope.com/t/how-can-they-tell-i-ran-a-red-light/182268/11 "2003-06-17T03:21:32Z")
[Traffic Light Cameras](http://electronics.howstuffworks.com/red-light-camera.htm) on How Stuff Works. ---
**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:** [June 17, 2003, 3:24am UTC](https://boards.straightdope.com/t/how-can-they-tell-i-ran-a-red-light/182268/12 "2003-06-17T03:24:20Z")
As you can see there were reasons for my “probably” and “I’d wager”. There’s no one technology for these units. ---
**Author:** ![chaoticbear](https://avatars.discourse-cdn.com/v4/letter/c/f07891/32.png) [@chaoticbear](https://boards.straightdope.com/u/chaoticbear)\ **Post date:** [June 17, 2003, 4:41am UTC](https://boards.straightdope.com/t/how-can-they-tell-i-ran-a-red-light/182268/13 "2003-06-17T04:41:56Z")
On my way to the DMV to take my driving test, I actually ran a stoplight with some camera-ey things attached… (I didn’t notice, but my mom freaked…) Nothing ever happened, however… ---
**Author:** ![stockton](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@stockton](https://boards.straightdope.com/u/stockton)\ **Post date:** [June 18, 2003, 1:18am UTC](https://boards.straightdope.com/t/how-can-they-tell-i-ran-a-red-light/182268/14 "2003-06-18T01:18:20Z")
Thanks, Q.E.D., and no offense intended. I just noticed that this thread seemed to have a LOT of WAGs for a GQ, and was interested in the real dope. You came through, as usual. My hat’s off to you. Thanks for the slamdunk cite. ---
**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:** [June 18, 2003, 1:35am UTC](https://boards.straightdope.com/t/how-can-they-tell-i-ran-a-red-light/182268/15 "2003-06-18T01:35:51Z")
No offense was taken. 🙂 ---
**Author:** ![TheLoadedDog](https://avatars.discourse-cdn.com/v4/letter/t/35a633/32.png) [@TheLoadedDog](https://boards.straightdope.com/u/TheLoadedDog)\ **Post date:** [June 18, 2003, 2:09am UTC](https://boards.straightdope.com/t/how-can-they-tell-i-ran-a-red-light/182268/16 "2003-06-18T02:09:33Z")
> [@](#): > > \*Originally posted by NoGoodNamesLeft \* > **…(Sort of like the thing at Radio Shack that goes !BING BONG! when you walk in the door.** Those are called doorbells. 😃 ---
**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:** [June 18, 2003, 2:13am UTC](https://boards.straightdope.com/t/how-can-they-tell-i-ran-a-red-light/182268/17 "2003-06-18T02:13:53Z")
Those things us PIR (Passive Infrared), and are horribly subject to false triggering in an outdoors environment. None of the traffic light cameras that I’ve seen use this. ---
**Author:** ![Swoop](https://avatars.discourse-cdn.com/v4/letter/s/b77776/32.png) [@Swoop](https://boards.straightdope.com/u/Swoop)\ **Post date:** [June 20, 2003, 11:12am UTC](https://boards.straightdope.com/t/how-can-they-tell-i-ran-a-red-light/182268/18 "2003-06-20T11:12:02Z")
Interesting. The system described at howstuffworks, would that be effective for catching motorcycles? Cruising inbetween 2 lanes so they don’t actually ride over/through the coils/field? (cough) ---
**Author:** ![Mort\_Furd](https://avatars.discourse-cdn.com/v4/letter/m/8e8cbc/32.png) [@Mort\_Furd](https://boards.straightdope.com/u/Mort_Furd)\ **Post date:** [June 20, 2003, 11:41am UTC](https://boards.straightdope.com/t/how-can-they-tell-i-ran-a-red-light/182268/19 "2003-06-20T11:41:01Z")
> [@](#): > > \*Originally posted by Swoop \* > \*\*Interesting. The system described at howstuffworks, would that be effective for catching motorcycles? Cruising inbetween 2 lanes so they don’t actually ride over/through the coils/field? (cough) \*\* That is a good question. Years ago, I drove a motorcycle in Houston, Texas. I got trapped at left turns on intersections quite often. The left turn signal only got activated when there was vehicle in the left turn lane. They used induction loops (you could see the cuts in the road where they’d been installed) and it seems like my motorcyle didn’t have enough metal to trigger the sensor. I did a lot of waiting for cars to pull up behind me and trigger the sensors. There were some intersections where I never got trapped, though. Maybe they had those sensors set to trigger for smaller vehicles - and maybe they can adjust them easily. ---
**Author:** ![35340](https://avatars.discourse-cdn.com/v4/letter/3/8baadc/32.png) [@35340](https://boards.straightdope.com/u/35340)\ **Post date:** [June 20, 2003, 12:39pm UTC](https://boards.straightdope.com/t/how-can-they-tell-i-ran-a-red-light/182268/20 "2003-06-20T12:39:43Z")
I have to agree with Mort Furd … my dad has seen this problem, too. And in parts of El Paso where this was a big problem, there were no easy “three right turns” to avoid it.