From a tire impact to a count
A temporary road tube is a hollow rubber hose stretched across a lane and connected to a roadside counter. When a tire rolls over it, the tube briefly flattens and pushes a small burst of air toward a pressure-sensitive switch. The switch turns that pulse into an electrical signal, which the counter records.
The important detail is that the tube senses axles, not cars. A two-axle car typically sends two pulses; a truck with several axles sends more. The counter must group those pulses into vehicles, using their timing and, in more capable setups, signals from a second tube placed a measured distance away. Two tubes can also help estimate speed from how long a vehicle takes to travel between them.
Why count axles at all?
Axle patterns carry information that a simple vehicle tally misses. A traffic survey can use them to distinguish ordinary cars from larger vehicles, or to estimate how heavily a road is being used. The exact classification depends on the counter and its setup; a single pressure tube does not identify a vehicle as precisely as a camera or a sensor that measures its shape.
The method is practical because the equipment can be laid across a road without building a permanent installation. But it has a tradeoff: the data come from brief physical impacts, so a tube must be positioned and maintained well enough to register them reliably. What looks like a rubber strip on pavement is really a simple chain of measurement—tire, air pulse, switch, then a record of traffic passing one axle at a time.
