An increasing number of new vehicles are being equipped with backup proximity sensors. These sensors detect the presence and proximity of objects in the pathway of the reversing vehicle and warn the driver through an audible signal. This report investigates the performance capabilities and potential safety effectiveness of these systems in reducing the risks to small children and other pedestrians from reversing vehicles. These sensor systems are primarily designed and marketed as parking aids. However, some are being promoted as safety systems with the potential to reduce or prevent collisions with pedestrians, especially small children. The performance capabilities of six commercial reversing aid systems were evaluated in laboratory tests. Four systems were fitted to the vehicles as standard equipment. Two systems were purchased from aftermarket companies and installed on the test vehicles. All six systems used ultrasonic sensor technology. Laboratory tests consisted of 3-dimensional mapping of the detection zones, the system response time, and the effects of dust / dirt on sensor performance. In terms of detection area performance, parking aid systems sacrificed detection distance and height in order to suppress false or nuisance alarms. The durability and reaction time results revealed there were no substantial performance differences between the systems. The safety benefits of these devices were then estimated based on these test results.