This works for temporary traffic lights here in the UK (I've just googled and some people seem to think they are motion sensors, but from my experience they aren't)

If it's dark and quiet, I think both sides go to red, then when a light is detected, that side goes green. Quite common knowledge in the UK

No, sorry - they are motion sensors. I can trigger them on my bike, with no lights involved. You're suffering from confirmation bias, and not taking into consideration the fact that the lights change when they detect your vehicle, this is usually going to be coincident with you flashing your headlights, but the former is the cause, not the latter. It's a common misunderstanding, though.

I could understand why people would believe this.

http://en.wikipedia.org/wiki/Traffic_signal_preemption#Line-...

In the US at least, that is how a significant number of these systems work. It's rather unlikely they would get the right frequency to trigger it, but it's certainly possible.

This is spot on and how it was where I grew up and was a volunteer firefighter. IIRC, the visible strobes on our equipment we referred to as "opticoms"

Opticoms can be preempted with a strobe light using an IR filter; they don't need to receive visible light to activate.

If the Opticom sequence is trivial, the hardware is easy. If its a complex pattern, a 555 timer is required.

Sitting outside your local fire station recording video of the strobe pattern on an emergency vehicle should be sufficient to determine the necessary pattern (you step through frame by frame to identify the sequence).

The above is informational only. One should keep in mind that today, most traffic lights will log the preemption with their central controller, and that unauthorized use is illegal in most jurisdictions.

I doubt the relays that control the headlights are even capable of switching at the required frequency...

In the UK a lot of taxi drivers seem to believe this, the superstition is reinforced whenever they happen to flash when the lights were about to change anyway. It's also a comfort thing, similar to a placebo button[0] - people want to believe that they have some control over the mundane parts of their lives.

[0] http://en.wikipedia.org/wiki/Placebo_button

You guys make me feel old.

http://www.textfiles.com/phreak/BOXES/chrome

Camera based detection systems track and detect based on vehicle headlights at night. It's theoretically possible that flashing your lights will ensure that your vehicle is detected, but it won't make the lights change any faster as people presume.