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Speed limit

A speed limit is the maximum speed of travel permitted by a vehicle on a road (usually, although speed limits are applied elsewhere on different modes of transport e.g. on some stretches of railroad, on boats in harbours, etc) by law.

Speed limits vary by type of road. Residential streets, with primarily an access function, typically have much lower maximum speeds than intercity roads, with primarily a movement function.

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Speed limits are usually marked with a speed limit sign.

International speed limit sign (in kilometres per hour)

North American speed limit sign

Note: In Canada in kilometres per hour and in United States in miles per hour (often units are given, especially in border areas)

Design Speed

Speed limits are generally peripherally related to the design speed[?] of the road, which is "a selected speed used to determine the various geometric design features of the roadway." according to the 2001 AASHTO[?] Green Book, the highway design manual. It has been changed from previous versions which considered it the "maximum safe speed that can be maintained over a specific section of highway when conditions are so favorable that the design features of the highway govern."

85th Percentile Rule

Traffic engineers are taught the 85th Percentile Rule, which claims that maximum speed limits should be set at a speed at and above what 85% vehicles are driving. (Thus 15% of vehicles are speeding). This rule has been used for many years, yet no scientific evidence has been produced that this particular rule is safer than any other.

Speed Limits on United States Interstate Highways

On interstate highways in the United States speed limits range from 55 mph to 75 mph (about 88 km/h to 121 km/h). Before the oil embargo crisis in the 1970s, some states posted no speed limit on the interstate highways. In 1974, Congress imposed a nationwide 55 mph (88 km/h) speed limit by threatening to withhold highway funds from states that did not adopt this limit. This limit was unpopular, especially in Western states. In 1987 states were permitted to raise speed limits to 65 mph (104 km/h) on rural interstate highways. The federal restriction on speed limit was lifted in 1995, leaving speed setting to the states. All states except Montana imposed numerical speed limits (Montana had a "reasonable and prudent" speed limit), many higher than 65 mph.

In addition to the legally defined maximum speed, there is often also a minimum speed. Vehicles are expected to travel above 45 mph (about 72 km/h) under normal conditions.


The question of speed limits and safety is also an important one. It is argued that lower speeds save lives. Vehicles crashing at slow speeds rarely cause deaths. However, the evidence from raising speed limits in the 1980s and 1990s found mixed empirical evidence. While there were more fatalities on the interstate roads immediately affected, overall roadway death rates went down. This is because high speed drivers switched from even more dangerous non-interstate facilities to interstates, now that the risk of being caught for speeding was diminished. Thus fatal accidents on non-interstates were reduced. Others argue that it is speed variance that kills, and accidents are caused by vehicles traveling at very different speeds. (Vehicles traveling the same speed in the same lane will not crash).

See also: hierarchy of road[?], interstate highway, Speed trap, Road-rule enforcement camera

An axiom of Einstein's relativity theories states that the Speed limit of the Universe is Light speed, 2.997 924 58 108 metres per second (or the distance light travels).

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