of a lightbar, on a car's package shelf (shining out the rear window) or on some other conspicuous location on the rear of a vehicle. Information Matrix Signs - These special active visual warnings are used to convey words to vehicles approaching from behind the emergency vehicle and often carry messages such as "Police. Stop" or other relevant message. Some systems allow only preprogrammed messages, where others can be fully customised. Many governments list specific requirements for emergency vehicle lighting. These requirements may address the colour, location and intensity/visibility of the lights, and whether they should flash or burn steadily. Laws also may regulate what vehicles may display these lights, and under what circumstances they may do so. The warning lights may be of several types, which includes: Incandescent - These are 'traditional' light bulbs, and may be found in the 'beacon' type lights, and will also be found where the vehicle's own lighting is used (such as wig-wag). Bulbs may be Halogen or Xenon type. They may use a rotating mirror to make them flash, or are simply turned on and off. Strobe lights - These lights give a short flash, which has high brightness compared to an incandescent light. They are usually made to work in multiples (such as three rapid flashes consecutively) and are often used in patterns between sets, which increases their effectiveness. LED lights - These are more recent developments, using the 'Super Bright' LED technology, and are entering wide use as they are easy to fit (being only a few millimetres wide, rather than several centimetres for most strobes and incandescent lights). This makes them particularly suitable for covert use. Though currently much more expensive than other types of lamps, there are substantial benefits. They draw much less electric current than other types of lamps which is a valuable reduction in electrical load on these vehicles' over-stressed electrical systems. They are much more resistant to vibration and have much longer service lives. They also present very saturated colors and so enhance visibility during daylight hours. Audible warning devices When an emergency vehicle is responding, it often uses audio warning devices in addition to the visual warnings provided by its warning lights. Audio warning devices are turned off once the vehicle is on-scene. Such devices include: Sirens - These can be fully electronic, electric, or manual, but are all designed to create changing sound patterns.