Black cords across the road

Why do you sometimes see things that look like long black bungee cords stretched across the road? The only possible use for them that I can come up with is that maybe they’re somehow part of something that counts how many cars pass over them.

For bonus points, why (at least in New York) do I see signs on highways that say “Road Service By Permit Only”? Somehow I don’t think there’s a huge problem with unauthorized road construction. (“Hey, Joe, you wanna go over to the Bronx River Parkway and try out my new steamroller?”)

You got it. Traffic counters.

The pneumatic cords count axles (portions of vehicles) and sometimes speed. Usually used for traffic pattern analysis.

Just a WAG but I’d say that these signs are probably referring to maintenance, not construction. So stuff like picking up litter, which is probably supposed to be done by a unionized worker who doesn’t want some volunteer group taking his work away. But I could be totally wrong.

You sometimes see the tubes laid out in pairs so one goes all the way across and the other only goes to the centre line. The short tube counts that lane only while the long one counts both, so the difference allows a single setup to count the lanes individually.

How could they measure speed? Not all cars have the same wheelbase.


Got it, they use more than one cord.

Or the counter has an accurate timer that can calculate a car’s speed from the pause between the first hit (the front axle) and the second hit (the rear axle). Ignoring a few anomalous vehicles like motorcycles, large trucks and limousines, and selecting an nice (heh) middle-of-the-road value for the axle-to-axle length of an average car, the speed of traffic at various points in the day can be computed with reasonable accuracy.


Motorcycles have 2 axles, same as a car. Multi-axled vehicles don’t usually go down the type of roads theses types of surveys work best in. That, and the software can make some pretty good guesses.

A twin strip system referred to as ESP is used in PA. Time from front axle crossing strip 1 to strip 2 is an accurate time/distance measurement.

I saw one of those suckers snap one time. Every time a car went over it, it jumped around like an angry, wild, rabid cobra on speed.

Yes, but the axle-to-axle distance is much shorter than a typical car, just as the distance is greater on a limousine. Statistical software will allow the analyst to eliminate outlying observations that might bias the results.

Yes, a 1/2 second between axles followed by a 2 minute lull does not mean that what was measured was the rear axle of one car followed by the first wheel of an insanely long wheel based car. That’s what I was talking about by saying that the software can make some pretty good guesses.

Besides the data always starts out with 1 vehicles worth of data (a good thing to base counts on), otherwise they’d have to toss it under a moving car. I don’t think I explained that very well.

They track both traffic patterns and types of traffic. If the DOT wants to figure out the best way to spend money on roads, they want to know where improvements are needed first, and the best way to repair them. The strips count axels and, in most cases, weight of axels. Think of an 18-wheeler. You have 5 axels crossing in a time that tells the software what it is. Much different from a motorcycle crossing.

For the most part, you won’t find these on an Interstate highway, as they’re designed for heavy traffic. You also probably won’t find them on a quiet residential street. For the most part they’re on intermediate roads to find if the construction of the road is sufficient for traffic. Or they’re using it as a sample.

Or they’re just spending money to justify a funding increase for the next year. :slight_smile:

If the road has more than one traffic lane, the reference count is going to get out of whack in a hurry. Two cars at once could combine to appear as two, three, or four axles. Even with a single lane, a vehicle with an odd number of axles could screw the count up, especially in heavy traffic. Of course, that’s where the clever software of your first point comes in.
(I see that I’m not the only one who thinks about these details whenever I drive over a counter hose :))

Could this refer to breakdown recovery? You have to have a permit to stop on a motorway to recover a broken down vehicle?

Alternatively, there has been a problem here in Ireland with telecoms companies etc. opening roads to fix faults or lay cables without proper authorisation. Could that be the case there?

There’s a road that I take on the way to work every day. There is one T intersection that goes to a housing development. For a couple of weeks there was a pair of “cords” at some distance from both sides of the intersection. This is not a road that sees much traffic, BTW.

So just for giggles, I’d do the speed limit or less when I rode (or drove) over one paid of detectors. Then I’d speed up to my normal speed on the road (it’s posted at 35 mph, but most people do 50-60 mph) until I got to the other pair of detectors. Then I’d slow down to 30-35 mph. I imagined someone would be looking at the data: “Okay, we have a vehicle doing the speed limit on both pairs of detectors; but the interval between the events is wrong. It’s too short! What’s going on here?” Okay, one vehicle doing this twice a day isn’t going to make a lot of difference even on a lightly travelled road. But I wondered if anyone would notice. :smiley:

If you want to see a huge database, then get into axle weight in motion data. Those dbs get just huge. Basically it’s useful to know the distribution of axle weights and spacings that are actually on the road. There are lots of highway research projects that use this information. Most often, the cords you see are not collecting the weights, they are simple counters. Weight in motion detectors are much less common. Traffic counts are essential data items. Many times you’ll see them when projects are in the design phase, they often determine whether an intersection for example would benefit from a stop light or a simple stop sign.

So, how do these cords actually work? Are they pneumatic, such that the wheels squash the tube flat and push a puff of air into a detector at the open end?