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Detecting the Traffic at Signals
How does the controller at in intersection know you are there? (or does it...)
Loops
Nearly all of the intersections in Oak Ridge use a buried loop in the pavement to detect traffic. These loops create a magnetic field which is disturbed by the magnetic materials in a car passing over it. A special device in the traffic control cabinet monitors the buried loop and reports to the controller when it has been disturbed.
To make a loop, a groove is cut into the pavement. A thin wire is placed in the groove, and the groove is filled with a special material. In some intersections, you can see the groove that contains the loop. In other intersections, the loop has been covered by asphalt.
Loops took the place of the metal traffic plates that were in use at one time. These devices are seldom, if ever, in use today. Even in other Cities of which the author is aware, they exist in the intersection simply because they are so hard to remove.
Other Detectors
There are a couple of other devices in use to detect traffic. These are normally used in special circumstances where it would be difficult or impossible to make a loop. The City of Oak Ridge uses microwave detectors in some intersections. These devices resemble a closed circuit TV camera mounted on a pole or signal cross arm.
They function similar to police radar, sensing the presence and direction of traffic. It is possible for extremely slow moving vehicles not to be picked up by the devices, although this is seldom a problem.
Nothing at All
We would not be complete without adding that some movements at some intersections do not have any detection at all. Most commonly, the main street of a main and secondary intersection has no loops. The controller is programmed to always assume there is traffic on the main street, and return green to that movement. Right turn only lanes often do not have detection loops.