Accuracy of pneumatic road tube counters

Pneumatic road tube counters are a tool that is commonly used to conduct traffic counts on streets and roads. Many professionals have high confidence in the accuracy of road tube counts, and vendors of pneumatic road tubes often claim accuracy rates in the neighborhood of 99 percent. Several studies have been conducted in Montana intended to test the accuracy of road tube count data. These studies have compared road tube data to hand counts and other traffic-counting technologies, and compared results among multiple road tubes set up in series at a single location. The studies found that though the average error in a daily traffic count might be near zero, the absolute error of a typical 15-minute count averaged closer to ten percent. These results suggest that the level of inaccuracy is being masked by the positive and negative counting errors canceling each other out. Errors in speed and classification were much greater. These results raise questions about the reliability of pneumatic road tube counters in accurately measuring traffic volumes. This report compiles the results of these studies and provides a framework for measuring and reporting error.