A street light is a light installed at the edge of a roadway to illuminate the side of the road. The history of lighting streets and roadways is ancient, with evidence that many communities in the Ancient World encouraged citizens to keep outside lights burning through the course of the evening for navigation and safety. The earliest formal street lights operated as a municipal service were gas lights, which were handlit by a crew of people every evening. Eventually, gas lighting was replaced with electric lighting, and street lights today are usually fully automated.
The purpose of a street light is not to illuminate the roadway; at night time, drivers should be using headlights, which will illuminate the road for them. Street lights light up the side of the road, alerting drivers to potential hazards and situations which they need to be aware of. They can also be used to signal the presence of an intersection, so that a driver can be prepared for cross traffic or get ready to turn onto a side street.
Street lights are also a safety feature. Well lit areas tend to be less dangerous after dark because it is harder to commit a crime when the scene is fully illuminated. Street lights which keep parking lots and streets lit up at night make it safer for citizens to be out after dark. They can also be useful for navigation for people walking, allowing people to clearly see the sidewalk and surrounding area.
Street lights are sometimes criticized as a source of light pollution. A number of manufacturers have responded to this with designs which are intended to reduce light leakage. The poles on which street lights are mounted can also be a hazard, as people may hit them in cars, and the poles can also fall in storms or in freak incidents as a result of metal fatigue. A street light can also develop electrical problems which cause the entire pole to become electrified, although this is rare.
Many street lights today operate with the use of solar sensors which activate the light when it gets dark. Some are also solar powered, allowing them to operate on standalone poles without the need for an energy source. Some cities may use timers for their street lights, primarily in the case of older systems which have not been upgraded.
Municipal authorities usually appreciate being made aware of street light outages. If someone notices that a street light is out, he or she can call the city department or the electric company to report the outage.