Verification methods and antenna horizontal beamwidth of across-the-road radar for traffic speed enforcement in China

Across-The-Road (ATR) radar is the most common type of traffic speed measurement devices used in China for traffic law enforcement. Its speed measurement accuracy is highly concerned by drivers, lawyers, policemen and metrologists as well. Among those influence factors, antenna horizontal beamwidth is an important one, which has not been taken into account in the current verification regulation of ATR radar in China. In this paper, the current verification methods of ATR radar in China are firstly introduced, and the reason for the field measurement error of ATR radar is analyzed. The speed measurement error of ATR radar related to antenna horizontal beamwidth is investigated in details, and theoretically calculated in various combinations of installation angles and horizontal beamwidths. Field tests with three equal-distanced piezo sensors each lane as triggering reference have also been performed on two ATR radar samples with horizontal beamwidth 5° and 10° respectively. The results of theoretical analysis and field tests verify that antenna horizontal beamwidth smaller than 6° is a precondition for accurate and reliable field measurements, which has also been added to the newly published pattern approval regulation of ATR radar in China.

► We analyze the reason for the existence of on-site measurement error in ATR radar.
► We calculate the relative error of speed measurement in theory.
► Horizontal beamwidth should not exceed 6° to meet on-site measurement accuracy.
► We introduce the on-site speed measurement standard system in China.
► The field test results of two ATR radar samples validate the theoretical analysis.