How do traffic signal sensors work?

Question

While it appears that during heavy traffic most traffic lights run accordingly to programmed intervals, At night, I've noticed signals tend to be triggered by my car's approach. Some lights will not turn, however. I've sat in a left turn lane at a particular intersection and watched the lights cycle through 3 or 4 cycle repetitions without ever giving me a left turn signal and with no other cars at the intersection. If I back up slightly and pull forward, the left turn light will then change. How do these traffic light sensors register my car's approach?

Answer 1

depends on which ones:

_1. The most common type is the inductance loops you see cut into the pavement._ Essentially we install wire to form a loop in the sawcut slot._ Once current is run through the wire, it creates an electro-magnetic field that can sense when a large metal object drives above it._ There are amplifiers located in the signal controller cabinet at the intersection that can be tuned to adjust the sensitivity, but we can only turn up the sensitivity so high before creating interference and false vehicle calls._ I would imagine that any of the products you listed would improve the ability to detect bikes with lower amounts of metal._ If you see in the pavement where the loop has been installed, you would probably be more easily detected if you stop on top of the middle line (there are normally three sawcut lines).

_2. The other type of detection used at some intersections is video detection._ Two of the MoDOT intersections you mentioned, Chestnut/Glenstone and 65/Chestnut, and one City-maintained intersection you mentioned, Campbell and Sunshine, are equipped with cameras that detect when vehicles are present on each approach._ The computer learns what the background image looks like for each approach and then whenever the image changes in certain zones (just behind the stop bars) it determines a vehicle has entered the zone._ These also need maintenance from time to time._ Unfortunately the products you listed would not be much help for these types of intersections._ If you sit at an approach with a camera mounted across the intersection, and you believe you are not getting detected, my recommendation would be to move around a little to change the video image within the zone._ (By the way, the video detection cameras are not the cameras that transmit video back to our traffic center—they are only used as detectors.)

Answer 2

Ya know that white line that most intersections have?? Well, I believe when you are at that line it trips the traffic light. OR~~ I always believed that flashing your brites will trigger the light since on some of them there is a sensor on the top.

Answer 3

With lazer & some kind of small clock i do suppose