page_title: Speed limit
section_title: Road traffic safety
hierarchical_section_title: Speed limit / Justification / Road traffic safety
caption_attribution_description: One-lane chicane (likely located in Seattle, Washington)
context_page_description: Road speed limits are used in most countries to set the legal maximum or minimum speed at which road vehicles may travel on a given stretch of road. Speed limits are generally indicated on a traffic sign reflecting the maximum or minimum permitted expressed as kilometres per hour and/or miles per hour. Speed limits are commonly set by the legislative bodies of national or provincial governments and enforced by national or regional police and judicial authorities. Speed limits may also be variable, or in some places unlimited, such as on most of the Autobahn in Germany.
  The first numeric speed limit for automobiles was the 10 mph limit introduced in the United Kingdom in 1861.
  The highest posted speed limit in the world is 160 km/h, which applies to two motorways in the UAE. Although speed limit and safety distance are poorly enforced in this country specifically on the Abu Dhabi to Dubai motorway resulting in dangerous traffic according to a French government travel advisory, while "drivers often drive at high speeds. Unsafe driving practices are common, especially on inter-city highways.
context_section_description: According to a 2004 report from the World Health Organization, 22% of all injury mortality worldwide was from road traffic injuries in 2002, and without "increased efforts and new initiatives" casualty rates would increase by 65% between 2000 and 2020. The report identified that the speed of vehicles was "at the core of the problem", and recommended that speed limits be set appropriately for the road function and design, along with the implementation of physical measures related to the road and the vehicle, and increased effective enforcement by the police.  Road incidents are said to be the leading cause of deaths among children 10–19 years of age (260,000 children die a year; 10 million are injured).
  Maximum speed limits place an upper limit on speed choice and, if obeyed, can reduce the differences in vehicle speeds by drivers using the same road at the same time.  Traffic engineers observe that the likelihood of a crash happening is significantly higher if vehicles are traveling at speeds faster or slower than the mean speed of traffic; when severity is taken into account, the risk is lowest for those traveling at or below the median speed and "increases exponentially for motorists travelling much faster".
  It is desirable to attempt to reduce the speed of road vehicles in some circumstances because the kinetic energy involved in a motor vehicle collision is proportional to the square of the speed at impact. The probability of a fatality is, for typical collision speeds, empirically correlated to the fourth power of the speed difference (depending on the type of collision, not necessarily the same as travel speed) at impact, rising much faster than kinetic energy.
  Kinetic energy
  Braking distance during danger
  Typically motorways have higher speed limits than conventional roads because motorways have features which decrease the likelihood of collisions and the severity of impacts. For example, motorways separate opposing traffic and crossing traffic, employ traffic barriers, and prohibit the most vulnerable users such as pedestrians and bicyclists. Germany's crash experience illustrates the relative effectiveness of these strategies on crash severity: on autobahns 22 people died per 1,000 injury crashes, a lower rate than the 29 deaths per 1,000 injury accidents on conventional rural roads. However, the rural risk is five times higher than on urban roads; speeds are higher on rural roads and autobahns than urban roads, increasing the severity potential of a crash. The net effect of speed, crash probability, and impact mitigation strategies may be measured by the rate of deaths per billion-travel-kilometers: the autobahn fatality rate is 2 deaths per billion-travel-kilometers, lower than either the 8.7 rates on rural roads or the 5.3 rate in urban areas. The overall national fatality rate was 5.6, slightly higher than urban rate and more than twice that of autobahns.
  The 2009 technical report An Analysis of Speeding-Related Crashes:Definitions and the Effects of Road Environments by the National Highway Traffic Safety Administration showed that about 55% of all speeding-related crashes when fatal listed "exceeding posted speed limits" among their crash factors, and 45% had "driving too fast for conditions" among their crash factors. However, the authors of the report did not attempt to determine whether the factors were a crash cause, contributor, or an unrelated factor. Furthermore, separate research finds that only 1.6% of crashes are caused by drivers that exceed the posted speed limit. Finally, exceeding the posted limit may not be a remarkable factor in the crash analysis as there are roadways where virtually all motorists are in technical violation of the law.
  The speed limit will also take note of the speed at which the road was designed to be driven (the design speed), which is defined in the US as "a selected speed used to determine the various geometric design features of the roadway".  However, traffic engineers recognize that "operating speeds and even posted speed limits can be higher than design speeds wit
caption: Traffic calming can be effective on lower speed roads
exif: {"Image Artist": "Richard Drdul", "Image Copyright": "\u00a9 2006 Richard Drdul"}