Counting Cars

Go to my Home page.
Go to my Joy of High Tech page.

The Joy of High Tech

Rodford Edmiston

This being a collection of random thoughts on bits and pieces of technical information which should interest the technically oriented reader.

More Than You Ever Wanted to Know About Counting Cars

I am sure that you have all seen those black tubes stretched across roads, and probably know that these are for counting cars. This is the oldest method of automatic traffic counting. The tube is hollow and sealed on one end, with the other leading to a box off the side of the road. Every time the tube is compressed by a wheel, the pulse of air generated is registered by a pressure switch.

Now, this counts axles, not vehicles. To get the actual vehicle count you must use a correction factor. To get the correction factor, you must know the average number of axles on the vehicles which have crossed the tube. Tables of averaged ratios for different road types are available, built up over a period of decades from thousands of manual counts at hundreds of locations. In the past, all of this data was gathered by hand, most commonly by people sitting off the side of the road with clicker boards. That is still the most reliable method for collecting detailed information on what types of vehicles use a road, but over the past couple of decades automatic devices for performing this chore have become available.

Besides counting compressions rather than vehicles, tubes also have other problems. For instance, when a single tube is stretched across more than one lane, there is no way to tell which lane an activation is in. To get counts for single lanes in multi-lane situations several tubes must be used. One stretches all the way across the road; the next one lane less, and so on. Individual lane counts are acquired by subtracting the count made by a short tube from the next-longest tube. Tubes also tend to come loose in heavy traffic, have trouble registering signals in rapid sequence, and are prone to vandalism. Finally, if the vehicles don't cross the tube in a fairly straight alignment, there will be two counts for each axle. However, the pneumatic tubes are cheap and easy to install, so they still see a great deal of use. They maintain a place in traffic counting, when placed properly and when the data they produce is processed with an awareness of their limitations.

There are other axle detectors on the market. One of the best is the piezoelectric sensor; when a piezoelectric substance is squeezed it produces a voltage pulse, generating a signal. These can be in a tube form - similar to the pneumatic tube and with some of the same disadvantages - or a bar in the pavement. This last is bedded in a groove, held in place by epoxy. While the piezoelectric devices are accurate, cycle quickly and require little maintenance, they are less wear resistant than tubes and more expensive. The permanently embedded version also cannot be moved from place to place, as tubes can.

Tape switches are cheap, easy to install (they are glued to the pavement) and cycle more quickly than pneumatic tubes, so they work well on high speed/high traffic roads. They are also throwaway items; once the count at a station is completed the tape strip in simply left in place, to be gradually worn away or scooped up by a snow plow. A tape switch can be placed across a single lane, the wires leading to the counter box taped down but not registering cars which pass over them. This allows individual lane counts without the complication of making subtractions. However, they are not as rugged as tubes or piezoelectric strips, and can fail during a count.

Another method for counting cars is to use induction coils. These are usually grooves in the pavement forming rectangles with cuts across the corners, and another groove leading to the edge of the road. This is an induction loop traffic sensor, which are also commonly used at traffic signals. An induction loop detects the presence of a vehicle by the disturbance it makes in the magnetic field produced by the current in the wires of the loop. This, by itself, won't determine what type of vehicle crosses the loop, but it does count vehicles instead of axles. The loops are cheap, but installation is expensive, requiring equipment and operators to cut the grooves, lay the loop and inject epoxy sealant. They may be taped down for a short term count, but this is uncommon. The equipment used with them to actually keep record of the number of vehicles is moderately more expensive than the simpler switch counters, in part because the device must supply a continuous electric current through the loop.

Classifying vehicles by number of axles gives you a pretty good idea of their type. If a single vehicle has four or more axles, the odds are it is a tractor and semitrailor. To perform this feat automatically, you need both a method which detects passage of a vehicle and something which counts how many axles it has. One solution is to combine a tube or other axle detector with one or more loops. You can also determine speed with two or more axle or vehicle presence detectors, simply by measuring the time it takes for the vehicle to travel between them.

In addition to counting and classifying vehicles, the technology is now available to actually weigh vehicles at highway speeds. There are two basic methodologies for doing this, capacitance and piezoelectric. Both of these devices can measure weight to within about ten percent under typical use conditions. Some models are accurate to within five percent. The capacitance method measures the change in the capacitance between two copper plates separated by a sheet of insulator, and comes in the form of a mat glued to the road. The greater the weight of the vehicle, the closer together the plates are squeezed and the greater the change in capacitance. Piezoelectric weigh-in-motion devices are similar to the piezoelectric axle sensors, except that the magnitude of the pulse is measured, not just it's occurrence. With a properly calibrated system the weight of each axle can be measured accurately enough in this way for most requirements without the need to bring trucks into weigh stations.

A recently developed technology uses low-power infrared lasers (or some other tightly focused light beam) on the shoulder aimed at reflectors in the lane. This counts one axle for each interruption in the light. Unfortunately, this method is only practical when there is little dust, water or snow on the pavement, since those can block the beams.

Given all this technology, traffic planners can obtain detailed information on the numbers and types of vehicles using a section of road. This allows them to adjust maintenance schedules and lets enforcement authorities know where and when to monitor for overweight vehicles without carrying out expensive manual operations on every road. So the next time you see someone placing traffic counters, or sitting by the road taking a manual inventory, smile and wave. They're saving you money and providing better highways.