I’ve been thinking about tubes. Specifically, the weird black tubes that you often find stretched across a roadway. What are those tubes for? What are they tracking? How do they work?
The official name for these ominous road snakes is just “pneumatic road tube,” and if you guessed that they probably just take a traffic count, you are mostly correct. But they also do so much more!
Here’s a good description of the actual operation of the tube setup from the U.S. Department of Transportation:
Pneumatic road tube sensors send a burst of air pressure along a rubber tube when a vehicle’s tires pass over the tube. The pressure pulse closes an air switch, producing an electrical signal that is transmitted to a counter or analysis software. The pneumatic road tube sensor is portable, using lead-acid, gel, or other rechargeable batteries as a power source.
Pneumatic road tubes can be set up to be either permanent or temporary, and temporary configurations are usually only installed for about a day. Permanent installations are linked up to a counter device that’s kept in a permanent roadside lock box.
For the more-common temporary setup, the tubes are a stretched across the road perpendicular to traffic until there’s very little slack. On one end, they’re held in place in the road using a concrete nail and the tube is pinched shut. On the other end, the tube is fed into a counter device that is chained to a street sign or post to make sure it doesn’t go anywhere.
You can see how it works in this video from the City of Bloomington, Minnesota’s YouTube page:
Technicians target road locations that have little to no curve and with as few chances of interference as possible, like you might get from on-ramps, parking lots or driveways.
A single pneumatic road tube is most commonly used to simply count the number of cars on the road, as well as time the gaps between individual vehicles.
If two pneumatic road tubes are set up spaced slightly apart, the counter can track the number of axles a vehicle has to better determine each individual vehicle’s class, the direction of traffic and the speed at which vehicles are moving.
If the city receives complaints about speeding or motorists cutting through side roads, the tubes can be installed to investigate the claims and determine if any changes to road signage or lane markings need to be made. The technician in the video above also mentions that accurate traffic tracking and management is vital, as the data collected factors into the city’s transportation budgets and helps to fund fixes.
Fortunately, it doesn’t seem like these traffic counters are ever setup as speed traps. Instead, the speed tracking is used for information gathering purposes which can then help determine if changes to the speed limit, or perhaps changes in speed enforcement, need to be made.
If you want to learn more about pneumatic road tubes and traffic counting, feel free to watch this 18-minute instructional video from the Pennsylvania Department of Transportation.