A road speed limit is the maximum speed allowed by law for road vehicles. Speed limits are commonly set and enforced by the legislative bodies of nations or provincial governments, such as countries within the world.

In addition to setting an explicit maximum speed limit, most governments also enforce speed limits that are related to driving conditions; for example, requiring drivers to adjust their speed when driving in fog or heavy rain. California Vehicle Code section 22350 is typical; it states that "No person shall drive a vehicle upon a highway at a speed greater than is reasonable... and in no event at a speed which endangers the safety of persons or property." This "basic rule", or similar legal language, applies even where no maximum speed limit is in place (such as formerly in the U.S. state of Montana). Some roads also have "minimum speed limits", where slow speeds are considered to impede traffic flow or be dangerous.

The first speed limit was the 10mph (16.1km/h) limit introduced by the Locomotive Act of 1861 (or "Red Flag Act") in the United Kingdom (automobiles were in those days termed “light locomotives”). In 1865, the revised Locomotive Act reduced the speed limit to in the country and in towns. The 1865 Act required a man with a red flag or lantern to walk ahead of each vehicle, enforce a walking pace, and warn horse riders and horse drawn traffic of the approach of a self-propelled machine. The replacement of the "Red Flag Act" by the Locomotive Act of 1896, and the increase of the speed limit to has been commemorated each year since 1927 by the London to Brighton Veteran Car Run.

The Isle of Man and the Indian state of Uttar Pradesh are the only places in the world that do not have a general speed limit. In Germany, 57% of the autobahn system remains free from speed limits.Fact: date=September 2008 The highest posted speed limit in the world is 160 km/h (99 mph), which was experimentally applied during 2007 on selected test stretches in Austria and the United Arab Emirates.

Factors in setting speed limits

Traffic engineers observe that the majority of drivers drive in a safe and reasonable manner, as demonstrated by consistently favorable driving records. A report from the British Columbia Ministry of Transportation includes in its summary the finding that the incidence of crashes depends more on variations in speed between vehicles than on absolute speed, and that the likelihood of a crash happening is significantly higher if vehicles are traveling at speeds slower or faster than the mean speed of traffic.

Speed limits are most frequently set through statutes. Speed limits can usually be lowered, or sometimes raised, from the legislated speed limit through a process called speed zoning. Common factors influencing speed zoning are: