The police siren is used primarily to alert other motorists and pedestrians that an officer is responding to an emergency call, and therefore, all within earshot are expected to yield to the police siren so that the officer has a clear route to the area in which she is needed. However, new concerns have changed the way departments designate when, under what circumstances, and who should be sounding the police siren in the event of an emergency.
The patrol car smash-ups that often kill or seriously injure police officers and innocent bystanders may be prevented by a policy adopted by law enforcement agencies that generally allow only one responding squad car to use lights and a police siren at a given time. The "washout" factor, that is a phenomenon during which competing lights and more than one police siren cancel each other out, leaving officers potentially oblivious to other patrol cars and emergency vehicles arriving at the same scene is believed to be the cause of many car wrecks that claim the life of officers, or land them in serious or critical condition. Law enforcement agencies that have had fatalities have adopted policies limiting who can respond with lights and a police siren to eliminate the danger from "washout" and limit the risk brought by many patrol cars rushing to calls. Depending upon the nature of the call, there's always the potential for there being a collision and the idea of designating a single police siren limits the risk.
New recruits often undergo a 40-hour course that nearly doubles the former state minimums to make sure they are attuned to the sound of rival emergency vehicles and the competing police siren. Recruits learn strategies like "lane by lane" clearance, as well as strategies for seriously reducing the risk of more than police siren hampering emergency responders.