# Turn Lane Sensors **URL:** \ **Category:** Factual Questions\ **Created:** [October 14, 2001, 10:24am UTC](https://boards.straightdope.com/t/turn-lane-sensors/87353 "2001-10-14T10:24:35Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![Hirka\_T\_Bawa](https://avatars.discourse-cdn.com/v4/letter/h/db5fbb/32.png) [@Hirka\_T\_Bawa](https://boards.straightdope.com/u/Hirka_T_Bawa)\ **Post date:** [October 14, 2001, 10:24am UTC](https://boards.straightdope.com/t/turn-lane-sensors/87353/1 "2001-10-14T10:24:35Z")
In the left hand turn lanes that have an arrow, how do they know there is a car waiting to make the turn? I assume it has something to do with the metal lines i see in the lane, but how does the car trigger it? ---
**Author:** ![Schnitte](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/schnitte/32/9033_2.png) [@Schnitte](https://boards.straightdope.com/u/Schnitte)\ **Post date:** [October 14, 2001, 10:36am UTC](https://boards.straightdope.com/t/turn-lane-sensors/87353/2 "2001-10-14T10:36:48Z")
The metal lines cover an electromagnetic coil through which a certain electric current is flowing, creating a magnetic field. If a car is standing on the lines, the car’s metal generates a magnetic field caused by the coil’s field, and this again induces changes in the original coil current. The sensor thus knows when a car is standing in the lane and can control the lights adequately. ---
**Author:** ![DVous\_Means](https://avatars.discourse-cdn.com/v4/letter/d/9fc29f/32.png) [@DVous\_Means](https://boards.straightdope.com/u/DVous_Means)\ **Post date:** [October 14, 2001, 1:26pm UTC](https://boards.straightdope.com/t/turn-lane-sensors/87353/3 "2001-10-14T13:26:08Z")
Too bad if your car is made out of fibreglass, and the engine is cast from non-ferrous metal - you could be stuck in the turning lane forever… ---
**Author:** ![ski](https://avatars.discourse-cdn.com/v4/letter/s/bbe5ce/32.png) [@ski](https://boards.straightdope.com/u/ski)\ **Post date:** [October 14, 2001, 3:04pm UTC](https://boards.straightdope.com/t/turn-lane-sensors/87353/4 "2001-10-14T15:04:26Z")
> [@](#): > > \*Originally posted by DVous Means \* > \*\*Too bad if your car is made out of fibreglass, and the engine is cast from non-ferrous metal - you could be stuck in the turning lane forever… \*\* Or on a motorcycle - this is a well-known problem among bike riders. I have had to just run the red light (at an empty intersection) instead of being stuck there my entire life. ---
**Author:** ![SirRay](https://avatars.discourse-cdn.com/v4/letter/s/cc9497/32.png) [@SirRay](https://boards.straightdope.com/u/SirRay)\ **Post date:** [October 14, 2001, 4:56pm UTC](https://boards.straightdope.com/t/turn-lane-sensors/87353/5 "2001-10-14T16:56:58Z")
In my area we have a left turn lane, in which too many people don’t pull up to the stop line, and consequently they don’t trigger the turn arrow. Hence, you can wait up to 5-10 minutes for the light to finally cycle or the idiot to run the light (ironically triggering the sensor, which is normally on a 20-25 second cycle). Since honking at these oblivious morons is useless, sometimes I have to drive around them and trigger the light myself (stopping over the stop line), which is kinda dangerous and probably illegal. Anyone else have a better solution to this problem? ---
**Author:** ![mmmiiikkkeee](https://avatars.discourse-cdn.com/v4/letter/m/edb3f5/32.png) [@mmmiiikkkeee](https://boards.straightdope.com/u/mmmiiikkkeee)\ **Post date:** [October 14, 2001, 9:13pm UTC](https://boards.straightdope.com/t/turn-lane-sensors/87353/6 "2001-10-14T21:13:58Z")
I thought there were pressure sensors at the heads of the intersections… oh well. ---
**Author:** ![Cardinal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cardinal/32/4000_2.png) [@Cardinal](https://boards.straightdope.com/u/Cardinal)\ **Post date:** [October 15, 2001, 7:32am UTC](https://boards.straightdope.com/t/turn-lane-sensors/87353/7 "2001-10-15T07:32:56Z")
definitely true about the motorcycles. I would try to stop right on top of a line, and then rock left and right to influence the magnetic field. It even worked sometimes. But way back when I had that Honda 50cc scooter, gooooood luck. I ran them all the time late at night. ---
**Author:** ![Barbarian](https://avatars.discourse-cdn.com/v4/letter/b/e19adc/32.png) [@Barbarian](https://boards.straightdope.com/u/Barbarian)\ **Post date:** [October 15, 2001, 2:18pm UTC](https://boards.straightdope.com/t/turn-lane-sensors/87353/8 "2001-10-15T14:18:02Z")
It helps to have a passenger. MrsB is designated “get off and hit the pedestrian button” pusher. ---
**Author:** ![Harvey\_The\_Heavy](https://avatars.discourse-cdn.com/v4/letter/h/c5a1d2/32.png) [@Harvey\_The\_Heavy](https://boards.straightdope.com/u/Harvey_The_Heavy)\ **Post date:** [October 15, 2001, 6:33pm UTC](https://boards.straightdope.com/t/turn-lane-sensors/87353/9 "2001-10-15T18:33:57Z")
Sometimes it helps to flash your headlights. I think it activates the same sensor that detects the stobe light on an ambulance. I think some intersections have a sensor that picks up the sound of a siren. I’ve seen the Highway Patrol change the light with a short burst on the siren. ---
**Author:** ![lawoot](https://avatars.discourse-cdn.com/v4/letter/l/35a633/32.png) [@lawoot](https://boards.straightdope.com/u/lawoot)\ **Post date:** [October 15, 2001, 6:40pm UTC](https://boards.straightdope.com/t/turn-lane-sensors/87353/10 "2001-10-15T18:40:04Z")
> [@](#): > > \*Originally posted by SirRay \* > \*\*Since honking at these oblivious morons is useless, sometimes I have to drive around them and trigger the light myself (stopping over the stop line), which is kinda dangerous and probably illegal. > Anyone else have a better solution to this problem? \*\* If the cycle goes more than twice I figure they’re idiot. I usually get out of my car, tap on their window (which is usually good for a cardiac inducing moment on their part, as I am a BIG man) and tell them they need to move forward to trigger the light. I then calmly stroll back to my car and buckle up before the light changes back. ---
**Author:** ![Hirka\_T\_Bawa](https://avatars.discourse-cdn.com/v4/letter/h/db5fbb/32.png) [@Hirka\_T\_Bawa](https://boards.straightdope.com/u/Hirka_T_Bawa)\ **Post date:** [October 16, 2001, 6:39am UTC](https://boards.straightdope.com/t/turn-lane-sensors/87353/11 "2001-10-16T06:39:41Z")
Thanks Schnitte, Answered my question really quick… Too bad I haven’t been around the past couple days to see it. 🙂 So its all done with magnets. Tell you the truth, when I was trying to figure it out, I never once thought it might be magnets.