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.”…
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.
A relatively simple device to figure out how many vehicles are on the road. These traffic counts are then very helpful for additional decisions like traffic control, widening or adding to roads, and even thinking about new roadways.