Installing the Loop

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 45 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 non-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 commercial radio or TV transmitters, you should be several blocks from the nearest one.

If you are installing multiple loops, there are a couple of extra things to watch out for. If the loops are all being controlled by the same piece of equipment, then spacing between loops should not be a problem. Modern loop detectors scan the loops one at a time, to prevent interference between loops. However, if some of the loops are to be controlled by separate pieces of equipment, you will need to space the loops further apart, a good rule of thumb would be five times the diameter of the largest loop.

Interfacing to the Loop

Loops can be used with counters in two main configurations: Permanent and Portable. We are only interested in portable counters in this article.

In permanent installations, such as signal cabinets with built-in controller electronics, portable counters, with contact closure inputs, can be connected to the existing loop detector outputs. This gives you a very cost effective way to add counting to existing signal cabinets, particularly older units with no counting, recording or reporting abilities. Portable counters draw very little power and can sometimes be supplied directly from the cabinet 12-24 Vdc supplies, saving on batteries.

In permanent installations, such as signal cabinets with no installed equipment, portable counters with built-in loop detectors can be connected to the permanent loops terminating inside the cabinet.

In portable loop installations, the loop is taped to the roadway surface and connected to a portable counter with built-in loop detectors. This approach is most common around construction sites.