Counting Cars (those rubber strips)

I’ve been seeing more and more of these popping up on roads, those thin little cylindrical strips they lay across the road attached to a small box. What exactly are they for? Is it simply to count how many cars cross this point? I assume they do at least that. Do they record information like what time there is heavy traffic. Can they distinguish between cars, motorbikes, trucks, etc? Do they record what speed you’re travelling? Why do they sometimes have two strips about a foot apart?

I’m hoping it means they’re planning on adding an extra lane or something. For the record, I live in Australia if that makes any difference.

I’m not a traffic engineer, and I don’t have the user manual for the traffic recorder box, but I suspect the distance is carefully measured so that the time difference data can be converted to speed. I’ve also seen boxes where the first strip extends into one lane and the second extends across two lanes. In that case you could take the data from strip A and subtract the pulses that strips A and B had in common to get only the data for the B lane. Since a small error in distance between the strips would give bad results for speed, I’m leaning towards the latter explanation.

The proliferation of the boxes probably has to do with some component in the box now being available at a much lower cost or at a much higher reliability. It could also be caused by one state (a big one like California) placing a large order for them, financing a drop in price and also providing a wealth of data for other traffic engineers to look at. An electrical or computer engineer could tell us about trends in the prices of microcontrollers and pressure switches.

They’re nothing to do with speed. They are used to count vehicles for future road infrastructure projects. The second strip, I believe, is to help the system work out the number of vehicles without being fooled by the multiple axles of large trucks and the like.

I just figured it was a backup to ensure a good count. I don’t see how a second strip could resolve multiple axle vehicles.

When I was about thirteen I read one of those little blurbs in the Reader’s Digest about somebody following two youths in a tiny sports car. When they came across a counting strip, they stopped halfway across and lifted the rear axle over, having a good yuk thinking about the traffic engineers puzzling over half a car in their stats. “Numbskulls,” I thought after a moment’s reflection. "There are trucks with three and trucks with five axles, not to mention trailer and semi-trailer combinations. The engineers won’t think a blinkin’ thing about an odd number of axles in the count.

The first strip sends the “melody” of thumps back to the system. The second does the same. It then resolves the time interval between the first and second thumps, matches that to the same thing on the other strip, and ignores the rest. At least, that’s my WAG. I’m sure it’d be pretty easy to get the software to do that.

AS a kid, I remember these things were always single strip. These days, they are usually (but not always) double. It just occurred to me whilst typing up this post that this change coincides with the likelihood of the little grey roadside box being computerised. I’m sure the older ones would have been mechanical, and the guys reading it would have had to apply some sort of industry-developed formula to estimate the number of vehicles with three or more axles.

I didn’t know it was just to count the cars, I thought it was to count speed.

Anyway, I’m gonna WAG off your WAG if I can…

I think that the spacing of the strips is the key point. I think that if both strips are thumped at the same time, it would invalidate the count as a big rig. I mean if its not a BIG truck, the axles shouldnt be that close == both strips down at once.

Here’s how the pneumatic hose types work:

From here. This site has some articles about other kinds of vehicle counters, as well.

It matters less how many vechicles hit the strip, and matters more how many axles hit the strip. Civil engineers use what’s called an ESAL, or estimated single axle load. Knowing how many axles pass, combined with the ESAL, it can be determined the robustness necessary for the next maintenance project.

NinetyWt’s source is correct. The older counters were just a tube connected to a box that counted puffs of air. You take the total and divde by two. Yeah, it won’t be exactly right but generally it won’t make a huge difference. We still have a couple of the old tube counters around the office that I will occasionally use. They’re super easy to set up and operate.

If I need a more precise count that has speed and classifications, I’ll hire a company with the newer equipment to go do the counts for me. I’ll get spreadsheet back that will have breakdowns by #s of axles and speeds. Used to be, to get that data, you’d have to station someone along the road to count trucks (or video tape the road and then count the trucks, extremely tedious) but the newer tube counters are a lot easier (and cheaper).

For turn counts (information on which direction people head at an intersection), we generally still hire someone to go out and count. You could blanket the intersection with tubes and work it out mathematically but it’s usually easier to just hire a guy.

Depends on why you’re counting. Most roads are designed for a certain Average Daily Traffic (ADT). When you start exceeding that, you can have breakdowns in the road network as intersections are overloaded and wait times become excessive.

The last classification count I did was at the site of a future school. We had concerns from the neighborhoods that “over a hundred” tanker trucks from a near by tank farm were speeding down the hill adjacent to the school site. With a classification count, I could show the county that 1) yes, a majority of vehicles were speeding and 2) while there was significant multi-axle truck traffic, it was only around 30 and not in the hundreds.