What are the black strip on the road?

October 25, 2005 2:40 PM

What are those black strips that stretch across the road and into a small gray box on the side of the road (and generally chained to a pole)?

At many different places along the road I'll see one, sometimes two, black strips/wires that stretch across the street and into a gray box. Possibly some sort of traffic analyzer?

posted by JPigford to travel & transportation (16 comments total)

I believe they record traffic numbers and patterns.
posted by wsg at 2:42 PM on October 25, 2005

The count the number of cars that drive over them to measure traffic flow
posted by tristeza at 2:42 PM on October 25, 2005

Yes, counters.
posted by fire&wings at 2:48 PM on October 25, 2005

Depending on where they are and how they are set up, they can also measure speed. So if you see two or three right after each other, they're measuring speed.

And of course, the diamond/rectangle sensors at intersections are there to trigger the traffic light. Some intersections also have the mid-road sensors leading up to a light to help with the timings as well.
posted by skynxnex at 2:50 PM on October 25, 2005

They're portable axle sensors used by municipalities to measure traffic load and average speed (some states have "anti-speedtrap" laws that require cities to set speed limits relative to measured speeds). The "strips" themselves are typically pneumatic tubes.
posted by RichardP at 2:57 PM on October 25, 2005

Odd bit of history, Bill Gates first company, Traf-O-Data was designed around those. It was a failure and from there, Microsoft was started.
posted by KirTakat at 3:55 PM on October 25, 2005

I bet it over-reported all the numbers by a factor of x2.
posted by scarabic at 4:25 PM on October 25, 2005

So if you see two or three right after each other, they're measuring speed.

Not necessarily true, is it? If I had to measure traffic patterns, I would use two, no matter if I cared about speed or not. If we only had one strip (A), all we would see was...

AAAAAAAAAAAAA

...and all we would know is that 13 cars drove over it (and maybe what times they did). If we had two strips (A and B), we would see....

ABABABABABABABBABABABA

...and we would know that 7 cars drove in one direction, and 4 cars drove in the other direction (and perhaps the times involved and such). I would have to assume that this would be much more valuable information.
posted by NotMyselfRightNow at 6:06 PM on October 25, 2005

(Sorry... meant to italicize that first sentence above...)
posted by NotMyselfRightNow at 6:15 PM on October 25, 2005

Also, multiple axle vehicles, especially trailers, could throw out speed checks by this method, I imagine?
posted by wackybrit at 6:29 PM on October 25, 2005

Right. Unless you were measuring unicycle traffic, you would need to find a way to compensate for this.
posted by NotMyselfRightNow at 6:39 PM on October 25, 2005

NotMyselfRightNow: They usually only go to the middle of the road, so direction isn't an issue.

wackybrit: On the ones with two (or more) strips, they are much closer together than the width of a truck wheel, so 18-wheelers don't screw up speed measurements. If they go across two lanes though, two vehicles going over it at the same time would screw it up.
posted by krisjohn at 7:27 PM on October 25, 2005

Single tubes (strips) across the pavement usually are completing an aggregate vehicle count. When two tubes are placed, they are set at a specific distance. The counting devices (small gray box on the side of the road [and generally chained to a pole]) are sophisticated enough to classify the vehicle type based on the time between the pulses.

Occasionally, there will be one tube set across two lanes (bidirectional) and an additional tube set across only one of the lanes. This provides data for the entire roadway as well as data for each direction.

The chances that two vehicles crossing the tubes at the same time are rare and usually removed by averaging several days worth of data.

Speed data is usually collected manually through floating car study or with speed (radar) detectors. Speed is also measured through more permanent installations on high-speed facilities through pairs of inductive loops in each lane of the roadway. This is done by measuring the time the vehicle disturbs the magnetic field. All that data is transmitted, along with video feeds to a central operations center where it is then provided to your local newscaster and "eye-in-the-sky" reporter, and even Yahoo! Traffic.

One very cool application is for the Marquette Interchange construction in Milwaukee. MChange.org is the source for users for traffic updates to find real-time data and even map their way home or into downtown. Be sure to check out the Traffic Information Page. Disclaimer: I work for one of the companies that created the website, and even assisted with the traffic management of that project.
posted by RobbyB at 8:26 PM on October 25, 2005

I don't think you can measure speed with those things...the distance between the axle is a wild variable that I don't think can be factored for. Motorcycles and large trucks will skew your numbers wildly. You can throw out the skewed data I suppose, but then your survey is biased (only cars get measured) and even then your odd suicidal outiler is going to be omitted, which fucks your data in all sorts of ways.
posted by baphomet at 9:41 PM on October 25, 2005

I don't think you can measure speed with those things...the distance between the axle is a wild variable that I don't think can be factored for.

As NotMyselfRightNow mentioned, using two pneumatic tubes eliminates any need to estimate a vehicle's wheelbase (distance between front and rear axles) when calculating a vehicle's velocity since you can measure how long it takes an axle to travel the known distance between the two sensors. Sometimes you'll see two tubes on the street but some sensor models look like a strip which actually contains two or more tubes a few inches apart.

It is, however, possible to calculate a vehicle's velocity with just a single tube. In addition to knowing when an axle encounters the pneumatic tube, the sensor can also measure the width of the pressure pulse. Because the tube itself has a known diameter this allows an estimate of vehicle velocity independent of any need to estimate the vehicle's wheelbase (although such estimates might still play a part in the final calculation). However, this is a fairly crude measure of velocity, multiple tube sensors or the mechanisms mentioned by RobbyB are usually preferred.
posted by RichardP at 1:16 AM on October 26, 2005

The software that analyzes the data collected by the counters is very accurate. A traffic engineer can usually get detailed by breaking the info down to individual vehicles if need be. It is easy to determine bad data and to throw it out. Obviously if there is a speed listed above 200mph, it can be thrown out of the set. Typically, general data on the roadway volumes and speeds are already known so that they can be used for comparison.

Speeds are more accurately measured by road tubes than by radar believe it or not. You don't need to know the wheelbase because the tubes are a set distance apart.

The creators of these programs have a good handle on all types of traffic and the problems associated with counting. Everything is timed by an accurate timer within the counter and the software is based on an incredible amount of historical data. A lot of times engineers will compare the data with a peak hour manual count. Otherwise the typical high volume of roads tends to average out minor errors. About the only problems I've ever encountered when doing traffic studies is when a car parks on a tube or when a street sweeper comes by and rips up the tube. We always notify the sanitation people but occasionally it happens that they forget.
posted by JJ86 at 6:35 AM on October 26, 2005