Variable speed limit

Road speed limits are used in most countries to set the maximum (or minimum in some cases) speed at which road vehicles may legally travel on particular stretches of road. Speed limits may be variable and in some places speeds are unlimited (e.g. on some Autobahn sections in Germany). Speed limits are normally indicated on a traffic sign. Speed limits are commonly set by the legislative bodies of nations or provincial governments and enforced by national or regional police and/or judicial bodies.

The first maximum speed limit was the 10 mph (16 km/h) limit introduced in the United Kingdom in 1861. The highest posted speed limit in the world is 140 km/h (87 mph), which applies to some roads in Poland and Bulgaria; similarly Texas posts 85 mph (137 km/h) on one 40-mile (64 km) long toll road. However, some roads have no speed limit for certain classes of vehicles. Best known are Germany's less congested Autobahns, where automobile drivers have no mandated maximum speed. Measurements from the German state of Brandenburg in 2006 showed average speeds of 142 km/h (88 mph) on a 6-lane section of autobahn in free-flowing conditions.Rural roads on the Isle of Man and the Indian states of Andhra Pradesh,Maharashtra, and Telangana, also lack speed limits.

Speed limits are usually set to attempt to cap road traffic speed; there are several reasons for wanting to do this. It is often done with an intention to improve road traffic safety and reduce the number of road traffic casualties from traffic collisions. In their World report on road traffic injury prevention report, the World Health Organization (WHO) identify speed control as one of various interventions likely to contribute to a reduction in road casualties. (The WHO estimated that some 1.2 million people were killed and 50 million injured on the roads around the world in 2004.) Speed limits may also be set in an attempt to reduce the environmental impact of road traffic (vehicle noise, vibration, emissions) and to satisfy local community wishes for streets usable by people out of cars. Some cities have reduced limits to as little as 30 km/h (19 mph) for both safety and efficiency reasons. However, It has also been shown that in some circumstances changing a speed limit has little effect on the average speed of cars.

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Wikipedia