Historically this has been a popular method of vehicle sensing. It has several disadvantages though, amongst them accuracy limitations and the possiblity of two vehicles being counted as one. (See also CCTV vehicle counting.)
In pneumatic tube vehicle counting, one or more rubber hoses are stretched across the road and connected at one end to a data logger. The other end of the tube is sealed. When a pair of wheels hits the tube, air pressure in the squashed tube activates the data logger which records the time of the event.
A pair of tubes can be stretched across several lanes of traffic. The data logger can establish vehicle direction by recording which tube is crossed first. This has the drawback that if two vehicles cross the tubes at the same time then the direction can't be accurately determined. Should two cars be very close together when they cross the tubes, the system may see them as one multiaxle vehicle.
Vendors claim an accuracy of 99%. Studies1 show though, that the absolute error of a typical 15-minute count averaged closer to ten percent. This suggests that the level of inaccuracy is being masked by the positive and negative counting errors cancelling each other out.
The counts need to be physically downloaded onto a computer from the loggers at the roadside. At least one road tube is needed for each direction on every road or junction at which you want to count. Installation requires working within the traffic lane.
1Accuracy of Pneumatic Road Tube Counters. McGowen and Sanderson.
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