Traffic counters

You know those black hose-type things they stretch across streets to read traffic levels? Well, recently I’ve seen numerous places where two of them are across the road a few feet from one another, and I’m at a loss as to why this might be. Why two? What information can you get from two that you can’t get from one? I have a few theories, but I’m not confident about any of them.

  1. Multiple samples to increase accuracy. I have a hard time believing this one. Can the data these things put out be sufficiently inaccurate as to matter for the purpose it is used? I doubt it.

  2. Measure speed of traffic by measuring the time between the first strip being hit and the second. There are some technical difficulties with this. The timer would have to be really sensitive. With the strips 1m apart (I’ve seen some closer than that, and some further apart, but the calculation is easiest at 1m :)), you’d hit the second strip .07 seconds after the first travelling at only 50kph. Plus you have to keep track somehow of which hit on strip 1 corresponds with which hit on strip 2, as the distance between wheels on multi-axle vehicles and trailers might be small enough that trailing wheel hits strip 1 before leading wheel hits strip 1. You could solve this by moving the strips closer together, but this greatly increases the difficulty of getting sufficiently accurate timing data to generate meaningful data. Anyways, wouldn’t setting up a radar gun be simpler?

  3. One strip measures all traffic, second strip is only triggered by heavy traffic (large trucks, etc). This is my preferred theory, since the data would be quite helpful, as the heavy traffic stresses the road disproportionately to its volume. I don’t know if those strips can be set to disregard traffic below a certain weight, though. I’m also not sure if fast, light traffic would hit the strip just as hard as slower, heavier traffic, making it hard to distinguish between them. Moreover, the first place I saw the double strip thing was on residential streets in school zones, where there is hardly any heavy traffic, and you’d think the powers that be would be more interested in speed than weight.

So much for my speculating. Anyone know the Straight Dope on this?

Direction. Whichever one gets hit first tells from which direction the vehicle was approaching.

Can’t be.

In the school zones, the strips extended only to the middle of the street. I think we can reasonably rule out the likelihood of city planners wanting to know how many people were driving on the wrong side of the road in school zones.

The other place I’ve seen this most frequently is on a divided freeway. I think we can also reasonably rule out the likelihood of city planners wanting to know how many people are going southbound in the northbound lanes.

Direction is much easier to determine just by limiting your strip to one side of the road, since you don’t have to control for the pesky issue of which trigger of the second strip corresponds to which trigger of the first strip.

Here’s a munufacturere’s site: