<Li> White is rarely used as the only color on a lightbar, though some states require flashing white beacons on the rear of school buses as a rear - end collision prevention measure . </Li> <Li> Historically, white was used in combination with red bulb lenses in the Federal signal company Beacon Ray, introduced in the late 1940s and the later 1960s and 1970s model PA lights equipped with Clear domes on through the 1980s, when revolving bulb and reflector type emergency lighting including the later 1970s Twinsonic and aerodynamic light bars were supplanted with newer strobe types . The clear dome extended Hi base 4 - Bulb Beacon Ray with colored lenses over the bulbs were used extensively in the 1960s by the NYPD with two red bulbs alternated with a white bulb and an amber bulb canted upwards at a 45 - degree angle so the amber flash would reflect off upper - floor building windows for added traffic attention / visibility . This clear dome / colored bulb lens combination also found extensive use across the US as fire apparatus and ambulance lighting . </Li> <Li> Certain railroad - related machines, like fueling tankers, track maintenance and switching engines, may also use a flashing white light . Certain government vehicles, such as rural mail delivery vehicles, use a flashing white beacon in some states . </Li> <Ul> <Li> Green emergency lighting since 9 / 11 is now used by Homeland Security Agencies and government / private security agencies protecting high risk government and critical infrastructure . </Li> </Ul>

Why do fire trucks have a green light