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.
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