For vehicle detection, loops are coils of wire buried in the pavement that operate on the same principle as a metal detector. These loops are very simply constructed, they will can be square, rectangular, diamond shaped, or round. Sizes range from as small as four foot square to a rectangular size of six feet wide by fifty feet long.

To install a loop, you cut a groove in the pavement at least two inches deep and a half inch wide, forming a loop. Round is theoretically the best, however, rectangular and diamond shapes are the most common. On diamond and rectangular shaped loops, the corners are cut at a 45ø angle because copper wire becomes weakened by sharp corners. The typical size of circular loops are six feet in diameter. You will also need to cut a groove from the loop leading off the road, to bring in the feeder wires.

Using a good grade underground stranded wire of 12 AWG, such as, IMSA 51-5. Begin off the road where you plan to install the wire termination block, and string the wire up the feeder groove to the loop groove(s). String the wire into the loop grooves going around typically four times. Then string the wire back out the feeder groove to the starting location. Then with somebody holding the wires down where they enter the loop grooves, twist the feeder wire pair together tightly. This can be done most easily by jamming both wires into a portable drill chuck and spinning them up. They can also be twisted up by hand, its just slower and more work. The direction of the twist is not important, and neither is the direction of the wire wraps in the loop. Also, the wire will shorten up as you twist it, so allow for this when initially stringing the wire.

You can protect the feeder wire with an appropriate plastic pipe before burying it if you wish.

There are a few things to keep in mind when deciding where to install your loop if it is to work properly.

First, your loop will be seriously degraded and may not work at all if there is buried rebar under the pavement where you are installing your loop. It is wise to run a metal detector over the area first if you are not certain.

If you are installing over an old loop that is no longer functioning, you must remove all of the old loop wire, otherwise, if it has a short (a common failure mode), it will make your new loop not operative.

Look around. Loops will not work in the near vicinity of Power Sub Stations, be at least a block away. Also, loops may not work reliably near Radio or TV transmitters, you should be several blocks from the nearest one. Also, metal bridges are a no-no, and on top of metal culverts may degrade performance.