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.
The road tube is installed perpendicular to the traffic flow direction and is commonly used for short-term traffic counting, vehicle classification by axle count and spacing, planning, and research studies. Some models gather data to calculate vehicle gaps, intersection stop delay, stop sign delay, saturation flow rate, spot speed as a function of vehicle class, and travel time when the counter is utilized in conjunction with a vehicle transmission sensor (JAMAR Technologies).
Advantages of road tube sensors are quick installation for permanent and temporary recording of data and low power usage. Road tube sensors are usually low cost and simple to maintain. Sensor manufacturers often supply software packages to assist with data analysis.Disadvantages include inaccurate axle counting when truck and bus volumes are high, temperature sensitivity of the air switch, and cut tubes from vandalism and truck tire wear.