Comparison of Portable Speed Measurement Devices

The accuracy and precision of five common portable speed measurement systems were evaluated in a controlled field evaluation. The following systems were evaluated: traffic classifier with pneumatic tubes, traffic classifier with piezoelectric sensors, tape switches, radar, and lidar (i.e., police laser). A test vehicle with a calibrated Nitestar distancemeasuring instrument (DMI) made 100 passes through the test site at two speed levels (50 passes at 55 mph, 50 passes at 35 mph), and speed was recorded by each device for each pass. DMI speed was deemed the "true" speed for each pass, and deviations from the DMI speed for a given device were considered errors. Paired t-tests were performed on the speed data measured by each device versus the DMI. The following conclusions were drawn: (a) All devices performed equally well for the 35-mph trials. (b) Lidar and radar were the most accurate and precise devices for the 55-mph trials. (c) For all devices, any errors that occurred for a single speed measurement were relatively small (less than ±1.5 mph). (d) With the exception of radar, all devices were slightly less accurate and less precise at higher speeds. (e) There was little difference in performance between on-pavement devices (i.e., tubes, piezoelectric sensors, and tape switches). (f) Inaccuracies observed in on-pavement equipment likely were caused by slight measurement errors during placement of the sensors or movement of the sensors resulting from repeated tire hits. Because all devices were relatively accurate, the researchers recommended that portable speed measurement equipment be selected to suit the characteristics of a given data collection situation.