This totally unrelated reply raised my question about traffic lights.

I am starting to think that my chinese head lamp is a magical instrument, every time I go near traffic lights they turn green. It is hilarious to drive in heavy-traffic and just that you get green lights! Perhaps just a placebo feeling so I want to know how your traffic lights behave to you when you are driving within other traffic? How do they notice you with/without lights?

Well, I need to explain how it works. I have head lamp with blinking white light and particularly when it is very dark, the traffic lights are clearly programmed somehow to detect the intensity variation of lights and then turn green. I have heard that emergency cars show some kind of special light to turn lights green for them. But I suspect whether the head-lamp contains some special light, it was the cheapest model I found from eBay.

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    My experience is that "they don't". We've got cameras and induction loops at various locations near me. None of them ever pick me up. Luckily most locations have a periodic timed switch as backup for when the detectors fail. One location does not, and if a car doesn't come along going the same direction I have to fall back on the "broken signal" law (where if it doesn't change for an extended period of time you are allowed to assume it's broken and go anyways (when safe of course)). Commented Jan 28, 2015 at 15:16
  • Some lights can detect the strobe effect of an emergency vehicle like an ambulance or fire engine. Sometimes lights with dedicated Tram signals have a similar detector. However its country and site-dependent, and (at least in my country) considered illegal to use a strobe on a regular vehicle. The vast bulk of traffic lights lack the strobe sensor, so emergency vehicles do have to slow to a safe speed on red and then nose through the intersection.
    – Criggie
    Commented Nov 25, 2016 at 23:51

2 Answers 2


There are two basic types of intersection detection systems out there:

  • Induction loop coils: These are basically a metal loop or figure 8 embedded in the pavement, and will detect anything that conducts electricity. Much like a metal detector, such as you might walk through at the airport. Sometimes you can see lines carved into the road surface. Getting your wheel over the lines will help you get picked up.
  • Cameras: You may be able to see the cameras, even. These are generally black and white cameras that are programmed with areas they pick up for each lane and do some sampling things and if the samples look different than the "baseline", that's a detection. If your headlamp is helping, this is the type of detector in use at that intersection. A bright light will definitely be picked up at night. Light clothes that contrast with the color of the road surface will also help.
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    @freiheit laying your bike over the coils will also help to trigger them. Their location can often be recognized by large rectangular cuts / repairs that have been made in the road Commented Feb 19, 2011 at 5:31
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    @David: I've never tried laying my bike down; the signals I have the most problems with are camera-controlled. Good point about the cuts, but hard to describe the right place to put your wheels and sounds like he might be talking about cameras. The cuts can be circular, rectangular, diamond, rectangle with corners cut, octagonal (rectangle with corners cut very far), or any of those with a line through the middle to make a sort of figure-8.
    – freiheit
    Commented Feb 19, 2011 at 6:48
  • I've never had to actually lay my bike down to trigger one of these, though I think it isn't the same everywhere. Usually just a wheel over them works, and of course you can always move up and let a car go over the coils if that doesn't work. I don't really worry about being able to trigger a green light, because where I live, state law allows you to disregard a traffic signal as malfunctioning if it does not turn green after an unusually long amount of time, provided it is safe to do so. Commented Feb 19, 2011 at 18:10
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    You can tape a big rare-earth magnet to your bottom bracket. That will increase your electro-magnetic signature. It'll be easier to trigger induction loops. Commented Oct 10, 2011 at 6:54
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    @TadeuszA.Kadłubowski: It's all about a conducting body, not magnetism. Big rare-earth magnets are dangerous, and that trick probably only works because the magnets are reasonably conductive. Use some coils of copper such as from a transformer or inductor.
    – freiheit
    Commented Oct 10, 2011 at 18:38

Some traffic lights in some cities have sensors which detect a strobe pattern emitted by emergency vehicles like ambulances, firetrucks, and police cars. I've heard that it's more of an urban myth that they would be triggered by devices not designed to be picked up by the sensors or by drivers flashing their high beams. However, it's possible that the flashing headlamp you use is bright enough and has a similar strobe pattern which triggers the preemptive mode on the traffic signals.

See more at wikipedia: http://en.wikipedia.org/wiki/Traffic_signal_preemption

  • Good point - modern cars are now more likely to have LED lights. They can flicker badly when viewed through a camera with a shutter - which may be able to trigger the strobe detector.
    – Criggie
    Commented Apr 4, 2018 at 19:59

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