traffic light sensors

[sophie_1003]

Do traffic lights detect that cars are there by a sensor that detects headlights? This is due to an argument between me and my peers at uni and one of our tutors who says that the only sensors that traffic lights have are a pressure pad in the road so any links proving otherwise would be lovely!

[ChuckFickens]

no they dont'

and it's not pressure pads either, it an induction loop that detects the car passing over it.

look just before most lights and you will see a dark thin ring of tarmac line which is where the sensor loop is buried such as in this pic
http://www.plan-bravo.com/photos/loop2.jpg

[sophie_1003]

aww, that sucks! we were so convinced we were right! thanks for your answer!

[Buenchico]

There are two main ways which arriving cars are detected at traffic lights. Firstly, there are sensors in the road surface. However they don't detect pressure. Instead, they use an inductive loop, which creates a magnetic field and then detects the change to that field when a vehicle passes over the loop.

Secondly, there are radar detection systems mounted on the top of the traffic lights. These transmit a beam, which is reflected back by oncoming vehicles.

Neither system has anything to do with headlights.

Chris

[yobanfa]

im sure some do detect car headlights as some of the ones round where i live, if you flash them as you get close to them they change

[ChuckFickens]

forgot about the radar sensors, though are they not more often used on temporary lights?

I believe that the myth about lights detecting flashing lights is due to a method that was trailed for a short while to turn lights green when emergency vehicle approached by picking up the flashing lights.

[sophie_1003]

ok, think the induction loop thing is what my tutor meant! So why do some lights seem to change quicker when you flash you're lights at them?

[sophie_1003]

sorry, hadn't seen the other answers before my last reply! but yeah, yobanfa, that was our argument!

[ChuckFickens]

how do you know they change quicker? you don't know when they where going to change if you didn't flash your lights.

[sophie_1003]

that's why I said "seem" lol! It's just a matter of comparison to when I've approached them and not flashed!

[sddsddean]

They 'seem' to because by the time you get round to thinking "I'll flash my lights" the detector loops or microwave detectors have all ready noticed your vehicle and put your circuit into demand (the proper term for making the lights go green on your approach), or to put it another way, the machinery is much quicker than your brain! I've installed several sets of lights and loads of sets of temporary lights, so I do know what I'm talking about.

[sophie_1003]

ok, but there are a set of lights near me that you can actually be already at the lights and then flash and then they change, is this just a delay in the technology working properly? not doubting you by the way, just wondering!

[Ethel]

This old chestnut again

http://www.snopes.com/autos/law/strobe.asp

[ChuckFickens]

it's not a delay in the technology working properly, assuming the other end of the lights are green when you approach them then you will probably have to stop at the lights, the lights will know your there so if nothing is coming the other way then they will change the other end to red. There will then be a preset delay to allow any traffic that may have already passed the far end to clear the lights before your end changes green.

Some lights default to red both ends if no traffic has passed the sensors for a while, then as soon as they detect a car pas approaching one end they can immediately turn that end green. there was a set such as this on a "temporary bridge" in place for nearly 5 years near where I live, if I was approching them at 30mph and could see no other traffic was about they would be red but I knew I could just keep driving without slowing down and the red light would turn green just as I got to it without fail. Freaked a few friends out on occasion though :)

[taichiperson]

Surerly theres a difference between permanent and temporary lights? Temporary lights have a sensor on top, I've definately had a faster response in low light conditions after flashilng my lights....

[sddsddean]

No you haven't! Temp lights always default to 'all red' (assuming they are traffic detection mode and not timed), so the detector will pick you up and 'demand' your circuit immediately. The microwave detectors on top of traffic light heads CAN NOT detect photons emitted by your headlights.

[raysparx1]

Temporary traffic lights have sensors that detect oncoming traffic but do not detect from side to side. so if you are stuck at temporary ones for ages keep rolling the car backwards and forwards, or get out and wave your arm towards them this will speed up the change rate. they can also be set on a timer and no amount of waving will make them change quicker.

[Lakitu]

I can flash certains sets of lights up here and they change for me...

[Lakitu]

With my headlights, that is, not my boobs.