In the United States, nearly 350,000 parked or disabled motor vehicle crashes occur annually; these crashes cause the deaths of an average of 507 occupants and nonmotorists and injury to more than 30,000 people. Among safety measures aimed at reducing the frequency and severity of this type of crash, the deployment of flares is widely recommended and publicly accepted. The research presented supplements the literature on highway safety flare use by addressing the relative nighttime effectiveness of the number of flares, placement and spacing of flares, and use of flares with and without the additional activation of a police vehicle's light bar. The behavior of almost 4,000 passing vehicles was observed unobtrusively through the use of roadway sensors that counted the number and type of vehicles passing the study area and measured passing vehicle speed, lane distribution, and lateral separation from the roadway edge. Flare use had a significant and positive impact on all the dependent variables.