Why You Sometimes See Black Tubes Stretched Across the Road

If you spend enough time driving down the right route, you may notice them: the skinny black tubes that seem to appear on stretches of road at random. But the scaled-down speed bumps are easy to miss. Unlike other features on the highway, these additions are meant to be used by the government, not drivers.

According to Jalopnik, those mysterious rubber cords are officially known as pneumatic road tubes. The technology they use is simple. Every time a vehicle’s tires hit the tube, it sends a burst of air that triggers a switch, which then produces an electrical signal that’s recorded by a counter device. Some tubes are installed temporarily, usually for about a day, and others are permanent. Rechargeable batteries powered by something like lead acid or gel keep the rig running.

Though the setup is simple, the information it records can tell federal agencies a lot about traffic patterns. One pneumatic tube can track the number of cars driving over a road in any given span of time. By measuring the time that passes between air bursts, officials can determine which time of day has the most traffic congestion. Two pneumatic tubes installed slightly apart from each other paint an even broader picture. Using this method, government agencies can gauge the class, speed, and direction of each vehicle that passes through.

Based on the data, municipalities can check which road signs and speed limits are or aren't working, and decide how much money to allot to their transportation budgets accordingly.

For a closer look at how these tubes are installed, check out the video below.

Now that you know all you need to know about pneumatic road tubes, find out how often you should get your oil changed and what all the symbols on your car’s dashboard mean.

A version of this story ran in 2018; it has been updated for 2023.