What Are Those Black Cables On The Road For?

You've undoubtedly seen them before — black cables placed in a straight line across the road. They look similar to electrical wires, which might be concerning at first. No one wants to drive over a potentially live electrical cable, after all. However, there's nothing to worry about — these are pneumatic road tubes (sometimes just called road tubes), and they serve a specific purpose.

Road tubes are used to monitor traffic, working as a traffic ticker to determine how often the monitored road is used. The way they count traffic is clever, using air pressure when a car drives over it to collect data. When a tire rolls over the cable, the compression sends an air pressure pulse to an air switch, which sends a signal to count the vehicle.

This traffic data is used mainly for city planning and design, helping analysts decide how to maintain current roads and build new ones in the future.

Pneumatic road tubes can log various data, particularly if there are two on the road near one another. In addition to counting vehicles as they go by, the timing and frequency of the logs can help determine traffic flow.

For instance, if cars frequently go over the wire, but the timing of the logs is stilted, that can indicate that traffic lights are interrupting the flow of traffic or that there's too much congestion on the road.

Analysts can gather even more detailed information if two of these cables are placed close together. Two pneumatic road tubes allow the logging of a vehicle's speed and the direction of traffic. This data can be used to determine whether speed limits should be adjusted. Thankfully, the speed limit data is only used for data information and collection, not to find speeders and issue tickets.

While you may often see these black cables on the road, road tubes are far from the only method to collect traffic data. Road tubes also have a number of disadvantages, such as less accurate counts due to limitations of the technology, improper installation, or the tubes wearing down quickly. Pneumatic road tubes can also run into issues if the weather is poor.

Essentially, cities use pneumatic road tubes due to their low cost. Temporary road tubes are easily found for a couple hundred dollars, so most cities can install them on multiple roads and replace them whenever they wear out. Installation is also a lot easier to set up than other options like a video traffic counter, which is important since most road tubes are only set down for a day or so. For short-term traffic counting, a road tube will be the easiest for cities to use, so despite the other options, you'll be seeing them on the road for a good while yet.