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
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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