I often cycle around the city, and sometimes when I need to turn left and I am the only thing in the left-turn lane, the intersection infrastructure senses that I am there and triggers the left turn arrow.
At some busy intersections that can have a long left-turn lane, there is even a sensor to sense how many people are trying to turn left and will adjust the length of the light based on this. Where are these sensors and how do they work?
In: Engineering
For some, it’s simple timers.
Where there are vehicle sensors, which you may be able to tell buy a rectangular crack where the first vehicle would be, there’s a loop of inductive wire that can detect the presence of a vehicle by a change in its electromagnetic field. (Some believe this is a scale of some kind, but that would require far too much maintenance.)
These inductive wires are usually under where the first vehicle’s motor tends to be, as that tends to both generate it’s own EM field, and has enough metal to cause disruption in the EM running through the wire. Some intersections may have more than one loop in a lane to help detect numbers of cars.
This sets a bit in the controllers for the intersection that helps its programming know whether to react to a car. Depending on the time of day and sophistication of the controller, this could shorten an opposing light, and even immediately change a signal.
Small cars, motorcycles, and bicycles can have problems as they might not cause enough disruption to the EM in the wire. People don’t stand a chance, and are only detected when they hit crosswalk buttons.
There are a few ways. Induction loops cut into the pavement to sense steel cars is the most common, but it’s going the way of the dodo with newer technology, like microwave, radar, and video detection taking its place. Pedestrians and cyclists are picked up using the newer stuff, which can distinguish between cars and people.
In longer turn lanes (and on higher speed through lanes) there is often a sensor placed well in advance of the intersection for two reasons: the controller can add time to a signal phase if it detects a long queue of cars, and it can add a few seconds if a vehicle arrives just before it was about to go red.
On the road, have you see the rectangular cutouts? Those are weight sensors.
Edit: I assumed they were weight based, ignore me and carry on!
There are a few different systems used at traffic lights.
Some just work on a timer – the lights just go through the same sequence repeatedly, whether cars are waiting or not.
A lot of US systems use inductive loops – this is a coil of wire set into the road that can detect a large lump of metal (such as a car).
Elsewhere it is also common to see radar detectors – basically the same system that is used to trigger automatic doors in shops, but pointing at the road to spot cars and cyclists as they approach. These can use a few different methods to do the actual detecting (radar, it, cameras, etc), but work in pretty much the same way.
It is also worth noting that traffic lights can be set to use multiple different systems at different times – so a timed dry of lights can use different time sequences to suit busy and quiet times, or switching between timed and sensor lights at different points to ensure traffic is kept flowing.
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