A traffic signal that is dark or flashing contributes to traffic congestion and jeopardizes safety for vehicles and pedestrians. A potential solution identified by the Illinois Department of Transportation (DOT) was to have traffic signals operate on uninterruptible power supplies (UPSs)—or battery backups. As of 2000, however, only seven states had installed UPSs at traffic signals. Before the potential benefits of UPSs could be realized, the technology had to be evaluated to determine how long UPSs would operate under full load at varying temperatures, and how the switchover from line power to UPS and vice versa would function at specified voltages. Research addressed most of the initial concerns of the DOT and also resulted in several recommendations to improve the performance of UPSs in different situations. As a result of the research, more than 400 traffic signals have been installed or retrofitted with UPSs in Illinois.