Traffic lights operation

[Salad Dodger]

Most (if not all) traffic lights seem to have sensors but how do these work. I often travel early morning with very little traffic around yet if I approach a red light the lights still go through their complete sequence before changing in my favour even if no other traffic is waiting. Does that not make the role of the sensors useless or is it all controlled via computer?

[joko]

they are on a timer.

if they stayed on green, traffic crossing that road would have to be on red all the time.

they have to coordinate between themselves to ensure there all in sync so they have adjust to suit.

some are of course controlled by zebra crossings that also have to be taken into account

[Sasha13]

I was under the impression that the sensors detect flashing lights - i.e. police and ambulances etc and change in their favour to make it safer for them to cross at speed. I have approached traffic lights a couple of times at night when I'm the only car to be seen and flashed my head lights, whereupon the red has turned to green, but this may just be coincidence!!

[NORMANTHEDOG]

you girls believe anything you are told,dont you.

[Ethel]

Many traffic lights have simple motion detectors - when it detects something on the road, it changes to green. This is especially the case at night time and in rural areas.

The pedestrian crossing near me operates differently - after the button has been pushed, it changes the traffic lights when it detects a large break in the traffic.

[sddsddean]

Just so we can explode the myth...TL's will not change if you flash your lights at them! (even if you are driving with Blues and Twos!)

There are 2 types of detectors used on TL's. Motion sensors (the black box on top of the TL head) and loop detectors (coils of wire placed in the road prior to the junction, usually in the form of a diamond or lop sided diamond).
Motion detectors are microwave detectors and pick up a 'vehicle' moving towards it. They are active down to 2 mph so they can pick up bicycles. Loop detectors work on electromagnetic induction. A low voltage is passed through the loop and when a iron mass (such as a car) moves into the loop it induces a voltage rise in the loop which the electronics detect. These aren't as good at picking up bikes. When a TL system is not on constant demand (when a detector is tripped, it puts a 'demand' on the processor in the box of tricks for that route), it should default to keeping one particular direction green (usually the main road). When the demand is tripped it should change the lights.

[dzug]

A few of the ones round here go to an all red phase when idle, and turn green for the first road on which traffic approaches

And some are computer controlled - not so much to work out which ones need to change, but to allow optimum timings for each road under varying conditions.

But all these are essentially variants on the excellent explanation given by sdsddean

[heathfield]

And there are some in my area that will change to give priority to traffic on one road used by city buses over the other that has no bus traffic.

[markja]

my car has an ingenious device fitted that turns all traffic lights red when i approach!!!!!! i can't find the fuse to disable it.

[Sasha13]

I think my theory is better!!