subject: Emergency Vehicle Lighting - Revolving Lights, Police Siren, Ambulance Siren [print this page] Emergency vehicle lighting refers to any of several visual warning devices, which may be known as light bars or beacons, fitted to a vehicle and used when the driver wishes to convey to other road users the urgency of their journey, to provide additional warning of a hazard when stationary, or in the case of law enforcement as a means of signalling the driver to stop for interaction with an officer. These are additional to any standard lighting on the car such as hazard lights and are often used along with a siren (or occasionally sirens) in order to maximize their effectiveness. In many jurisdictions, the use of these lights may afford the user specific legal powers, and may place requirements on other road users to behave differently, such as compelling them to pull to the side of the road and yield right of way so the emergency vehicle may proceed through unimpeded.
Police cars sit at the base of the I-35W bridge collapse. The lightbars mounted on the cars are LED-based. The illuminated back-up lamps seen in the two cars in the foreground may be flashing as emergency lights, rather than burning steadily to indicate that the cars are in reverse gear.
LED-based lighting is becoming very popular in the emergency services for several reasons. Light-emitting diodes are small, completely solid state, very power-efficient, long-lasting (as they have no filaments to burn out) and can be seen very easily even at great distances.
Whether as lightbars or single beacons, LED-based lights typically use a clear, colorless dome because the light color is an intrinsic property of the LEDs themselves. LED-based lightbars can be made very thin, reducing wind resistance by around 8-10 percent, or made very flat and used in novel applications, for example to flip up under a sun visor.
LED lights are often used in a mode similar to conventional strobe lights, however they can be programmed with a wider variety of flash patterns because of their ability to be switched directly by electronics, as opposed to discharging a capacitor through a gas-filled tube.
LED lights produce relatively little heat when in use. In colder inclement climates, this has resulted in LED emergency vehicle warning lights (as well as traffic lights) being obscured by the buildup of frost or snow, raising safety concerns. Solutions are being researched to provide a heat source, as necessary in certain weather conditions, to keep LED lights clear of snow and frost.
