Speed limit

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

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

Signage

Speed limits are usually marked with a speed limit sign.

''International speed limit sign

United States speed limit sign

Note: In Canada in kilometres per hour (and with the term "maximum" in front) and in United States in miles per hour (Oregon has dropped the word "limit" from speed limit signs, and in California, most signs use "Maximum Speed" instead of "Speed Limit")
(In Canada units are given, and in the US at the first few signs upon entering the country)

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 mi/h to 75 mi/h (about 88 km/h to 121 km/h). Before the 1973 energy crisis, some states posted no speed limit on the interstate highways. In 1974, Congress imposed a nationwide 55 mi/h (88 km/h) speed limit by threatening to withhold highway funds from states that did not adopt this limit. It was estimated that a speed of 55 mi/h used 17% less fuel per mile than a speed of 75 mi/h. This limit was unpopular, especially in Western states. In 1987 states were permitted to raise speed limits to 65 mi/h (104 km/h) on rural interstate highways. The federal restriction on speed limit was lifted on November 28, 1995, leaving speed setting to the states. All states except Montana imposed numerical speed limits (Montana had a "reasonable and prudent" speed limit before and after, until 2000 when Interstates were generally posted at 75 mi/h), many higher than 65 mi/h. However, no Interstate Highway or express way is signed for over 75 mi/h, and within Major City Limits, few are over 65 mi/h. South Carolina raised the speed limit to 70 mi/h in 1999, but the 65 mi/h speed limit is used on certain Interstate highways.

In addition to the legally defined maximum speed, there is often also a minimum speed on certain roads. Vehicles are expected to travel above 40 mi/h (about 72 km/h) under normal conditions. However, most states do not state this nor enforce it.

Safety

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).

However, unrealistically (or unreasonably) low speed limits, especially interstate, lead to disrespect for the law, contempt of law enforcement, and high difference between faster and slower drivers. They will make violators out of citizens who would be otherwise law-abiding. If the posted speed limit is unrealistically low, most vehicles, according to the 85th percentile rule, will ignore the sides while only a few will abide by the posted limits.

Roads without speed limits

There still remain a few public roads where speed limits do not apply. The most famous of these are German intercity Autobahns. The Northern Territory, Australia also has no blanket speed limits outside major towns. Traffic levels on the Territory's roads are extremely light.