thumb|A road sign shows maximum and minimum speed limit for different types of vehicle on expressway in [[China. Speed limit signs are shown in kilometres per hour.]]
Speed limits on road traffic, as used in most countries, set the legal maximum speed at which vehicles may travel on a given stretch of road. Speed limits are generally indicated on a traffic sign reflecting the maximum permitted speed, expressed as kilometres per hour (km/h) or miles per hour (mph) or both. 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 nonexistent, such as on most of the Autobahnen in Germany.
The first numeric speed limit for mechanically propelled road vehicles was the limit introduced in the United Kingdom in 1861.
Speed limits are frequently introduced for health reasons with a goal to improve road traffic safety and reduce the number of casualties from traffic collisions. There is a relationship between speed and health impacts, whereby it has been posited that a 5% increase in speed is linked to a 10% increase in injury, and a 20% increase in deaths. Speed limits tend to be stricter in areas with more pedestrians, in particular those frequented by children. As of 2021, the WHO estimates that approximately 1.3 million people die of road traffic crashes each year.
Authorities may also set speed limits to reduce the environmental impact of road traffic (vehicle noise, vibration, emissions, wildlife) or to enhance the safety of pedestrians, cyclists, and other road-users. For example, a draft proposal from Germany's National Platform on the Future of Mobility task force recommended a blanket 130 km/h (81 mph) speed limit across the Autobahnen to curb fuel consumption and carbon emissions. Some cities have reduced limits to as little as for both safety and efficiency reasons. However, some research indicates that changes in the speed limit may not always alter average vehicle speed.
Lower speed limits could reduce the use of over-engineered vehicles.
History
thumb|Historic [[New Hampshire speed limit sign]]
In Western cultures, speed limits predate the use of motorized vehicles. In 1652, the American colony of New Amsterdam passed a law stating, "No wagons, carts or sleighs shall be run, rode or driven at a gallop". The punishment for breaking the law was "two pounds Flemish", the equivalent of US$50 in 2019. The 1832 Stage Carriage Act introduced the offense of endangering the safety of a passenger or person by "furious driving" in the United Kingdom (UK). In 1872, then-President of the United States Ulysses S. Grant was arrested for speeding in his horse-drawn carriage in Washington, D.C.
A series of Locomotive Acts (in 1861, 1865 and 1878) created the first numeric speed limits for mechanically propelled vehicles in the UK; the 1861 Act introduced a UK speed limit of on open roads in town, which was reduced to in towns and in rural areas by the 1865 "Red Flag Act". The Locomotives on Highways Act 1896, which raised the speed limit to is celebrated by the annual London to Brighton Veteran Car Run.
On 28 January 1896, the first person to be convicted of speeding is believed to be Walter Arnold of East Peckham, Kent, UK, who was fined 1 shilling plus costs for speeding at .
thumb|right|upright=1.3|In 1920 a pastor was fined US$10 () for "overspeeding his machine ... at a rate greater than ... in what appeared to be a reckless manner."
In 1901, Connecticut was the first state in the United States to impose a numerical speed limit for motor vehicles, setting the maximum legal speed to in cities and on rural roads. Speed limits then propagated across the United States; by 1930 all but 12 states had established numerical limits.
In Australia, during the early 20th century, there were people reported for "furious driving" offenses. One conviction in 1905 cited a vehicle furiously driving when passing a tram traveling at half that speed.
In May 1934, the Nazi-era "Road Traffic Act" imposed the first nationwide speed limit in Germany.
In the 1960s, in continental Europe, some speed limits were established based on the V85 speed, (so that 85% of drivers respect this speed).
In 2010, Sweden defined the Vision Zero program,
Reasonable speed
Most legal systems expect drivers to drive at a safe speed for the conditions at hand, regardless of posted limits.
In the United Kingdom, and elsewhere in common law, this is known as the reasonable man requirement.
The German Highway Code (Straßenverkehrs-Ordnung) section on speed begins with the statement (translated to English):
<blockquote>Any person driving a vehicle may only drive so fast that the car is under control. Speeds must be adapted to the road, traffic, visibility and weather conditions as well as the personal skills and characteristics of the vehicle and load.</blockquote>
In France, the law clarifies that even if the speed is limited by law and by local authority, the driver assumes the responsibility to control a vehicle's speed, and to reduce that speed in various circumstances (such as when overtaking a pedestrian or bicycle, individually or in a group; when overtaking a stopped convoy; when passing a transportation vehicle loading or unloading people or children; when the road does not appear clear, or risky; when visibility is low, etc.). If drivers do not control their speed, or do not reduce it in such cases, they can be penalized. Other qualifying conditions include driving through fog, heavy rain, ice, snow, gravel, or when drivers encounter sharp corners, a blinding glare, darkness, crossing traffic, or when there is an obstructed view of orthogonal traffic—such as by road curvature, parked cars, vegetation, or snow banks—thus limiting the Assured Clear Distance Ahead (ACDA).
In the United States, this requirement is referred to as the basic rule, as outlined by US federal government law (49 CFR 392.14), which applies in all states as permitted under the commerce clause and Due Process Clause. The basic speed law is almost always supplemented by specific maximum or minimum limits but applies regardless. In California, for instance, Vehicle Code section 22350 states that "No person shall drive a vehicle upon a highway at speed greater than is reasonable... and in no event at a speed which endangers the safety of persons or property". The reasonable speed may be different than the posted speed limit. Basic rule speed laws are statutory reinforcements of the centuries-old common law negligence doctrine as specifically applied to vehicular speed. Citations for violations of the basic speed law without a crash have sometimes been ruled unfairly vague or arbitrary, hence a violation of the Due Process Clause, at least in the State of Montana. Even within states, differing jurisdictions (counties and cities) choose to prosecute similar cases with differing approaches.
Excessive speed
Consequential results of basic law violations are often categorized as excessive speed crashes; for example, the leading cause of crashes on German autobahns in 2012 fell into that category: 6,587 so-called "speed related" crashes claimed the lives of 179 people, which represented almost half (46.3%) of 387 autobahn fatalities in 2012. However, "excessive speed" does not necessarily mean the speed limit was exceeded, rather that police determined at least one party traveled too fast for existing conditions. Examples of conditions where drivers may find themselves driving too fast include wet roadways (due to rain, snow, or ice), reduced visibility (due to fog or "white out" snow), uneven roads, construction zones, curves, intersections, gravel roads, and heavy traffic. Per distance traveled, consequences of inappropriate speed are more frequent on lower speed, lower quality roads; in the United States, for example, the "speeding fatality rate for local roads is three times that for Interstates".
For speed management, a distinction can exist between excess speed, which consists of driving in excess of the speed limit, and inappropriate speed, which consists of going too fast for the conditions.
Maximum speed limits
thumb|Maximum speed limits by country in Europe in kilometers per hour
Most countries have a legally assigned numerical maximum speed limit which applies on all roads when no other speed limit indications are present; lower speed limits are often shown on a sign at the start of the restricted section, although the presence of streetlights or the physical arrangement of the road may sometimes also be used instead. A posted speed limit may only apply to that road or to all roads beyond the sign that defines them depending on local laws.
The speed limit is commonly set at or below the 85th percentile speed (the operating speed which no more than 15% of traffic exceeds), and in the US is frequently set below that speed. Thus, if the 85th percentile operating speed as measured by a "Traffic and Engineering Survey" exceeds the design speed, legal protection is given to motorists traveling at such speeds (design speed is "based on conservative assumptions about the driver, the vehicle, and roadway characteristics"). The theory behind the 85th percentile rules is that, as a policy, most citizens should be deemed reasonable and prudent, and limits must be practical to enforce. However, there are some circumstances where motorists do not tend to process all the risks involved, and as a mass, choose a poor 85th percentile speed. This rule, in practice, is a process for "voting the speed limit" by driving, in contrast to delegating the speed limit to an engineering expert.
The maximum speed permitted by statute, as posted, is normally based on ideal driving conditions and the basic speed rule always applies. Violation of the statute generally raises a rebuttable presumption of negligence.
On international European roads, speed should be taken into account during the design stage.
{| class="wikitable"
|+Maximum speed recommended by international European road design standards:
!Road classification
!60 km/h
!80 km/h (50 mph)
!100 km/h (60 mph)
!120 km/h (75 mph)
!140 km/h (85 mph)
|-
|Motorway || x || 80 || 100 || 120 || 140
|-
|Express road || 60 || 80 || 100 || 120 || x
|-
|Road || 60 || 80 || 100 || x || x
|}
Minimum speed limits
Some roads also have minimum speed limits, usually where slow speeds can impede traffic flow or be dangerous. The use of minimum speed limits is not as common as maximum speed limits, since the risks of speed are less common at lower speeds.
Middle speed limits
Traffic rules limiting only middle speeds are rare. One such example exists on the ice roads in Estonia, where it is advised to avoid driving at the speed of as the vehicle may create resonance that may in turn induce the breaking of ice. This means that two sets of speeds are allowed: under and between .
Variable speed limits
thumb|upright|Example variable speed limit sign in the United States, in mph
thumbnail|upright|Digital speed limit sign for variable speed limits
In Germany, the first known experiments with variable speed limit signs took place in 1965 on a stretch of German motorway, the A8 between Munich and the border city of Salzburg, Austria. Mechanically variable message signs could display speeds of 60, 80 and 100 km/h, as well as text indicating a "danger zone" or "accident". Personnel monitored traffic using video technology and manually controlled the signage. Beginning in the 1970s, additional advanced traffic control systems were put into service. Modern motorway control systems can work without human intervention using various types of sensors to measure traffic flow and weather conditions. In 2009, of German motorways were equipped with such systems.
In the United States, heavily traveled portions of the New Jersey Turnpike began using variable speed limit signs in combination with variable message signs in the late 1960s. Officials can adjust the speed limit according to weather, traffic conditions, and construction. More typically, variable speed limits are used on remote stretches of highway in the United States in areas with extreme changes in driving conditions. For example, variable limits were introduced in October 2010 on a stretch of Interstate 80 in Wyoming, replacing the winter season speed reduction from that had been in place since 2008. This Variable Speed Limit system has been proven effective in terms of reducing crash frequency and road closures. Similarly, Interstate 90 at Snoqualmie Pass and other mountain passes in Washington State have variable speed limits as to slow traffic in severe winter weather. As a response to fog-induced chain-reaction collisions involving 99 vehicles in 1990, a variable speed limit system covering of Interstate 75 in Tennessee was implemented in fog-prone areas around the Hiwassee River. The Georgia Department of Transportation installed variable speed limits on part of Interstate 285 around Atlanta in 2014. These speeds can be as low as but are generally set to . In 2016, the Oregon Department of Transportation installed a variable speed zone on a stretch of Interstate 84 between Baker City and Ladd Canyon. The new electronic signs collect data regarding temperature, skid resistance, and average motorist speed to determine the most effective speed limit for the area before presenting the limit on the sign. This speed zone was scheduled to be activated November 2016. Ohio established variable speed limits on three highways in 2017, then in 2019 granted the authority to the Ohio Department of Transportation to establish variable limits on any of its highways.
In the United Kingdom, a variable speed limit was introduced on part of the M25 motorway in 1995, on the busiest section from junction 10 to 16. Initial results suggested savings in journey times, smoother-flowing traffic, and a decrease in the number of crashes; the scheme was made permanent in 1997. However, a 2004 National Audit Organisation report noted that the business case was unproved; conditions at the site of the Variable Speed Limits trial were not stable before or during the trial, and the study was deemed neither properly controlled nor reliable. Since December 2008 the upgraded section of the M1 between the M25 and Luton has had the capability for variable speed limits. In January 2010 temporary variable speed cameras on the M1 between J25 and J28 were made permanent.
New Zealand introduced variable speed limits in February 2001. The first installation was on the Ngauranga Gorge section of the dual carriageway on State Highway 1, characterized by steep terrain, numerous bends, high traffic volumes, and a higher than average accident rate. The speed limit is normally .
Austria undertook a short-term experiment in 2006, with a variable limit configuration that could increase statutory limits under the most favorable conditions, as well as reduce them. In June 2006, a stretch of motorway was configured with variable speed limits that could increase the general Austrian motorway limit of . Then Austrian Transport Minister Hubert Gorbach called the experiment "a milestone in European transport policy-despite all predictions to the contrary"; however, the experiment was discontinued.
Roads without speed limits
Just over half of the German autobahns have only an advisory speed limit (a Richtgeschwindigkeit), 15% have temporary speed limits due to weather or traffic conditions, and 33% have permanent speed limits, according to 2008 estimates. The advisory speed limit applies to any road in Germany outside of towns which is either a dual carriageway or features at least two lanes per direction, regardless of its classification (e.g. Autobahn, Federal Highway, State Road, etc.), unless there is a speed limit posted, although it is less common for non-autobahn roads to be unrestricted. All other roads in Germany outside of towns, regardless of classification, do have a general speed limit of , which is usually reduced to at Allée-streets (roads bordered by trees or bushes on one or both sites). Travel speeds are not regularly monitored in Germany; however, a 2008 report noted that on the autobahn in Niemegk (between Leipzig and Berlin) "significantly more than 60% of road users exceed [and] more than 30% of motorists exceed ". Measurements from the state of Brandenburg in 2006 showed average speeds of on a 6-lane section of autobahn in free-flowing conditions.
Prior to German reunification in 1990, accident reduction programs in eastern German states were primarily focused on restrictive traffic regulation. Within two years of reunification, the availability of high-powered vehicles and a 54% increase in motorized traffic led to a doubling of annual traffic deaths, despite "interim arrangements [which] involved the continuation of the speed limit of on autobahns and of outside cities". An extensive program of the four Es (enforcement, education, engineering, and emergency response) brought the number of traffic deaths back to pre-unification levels after a decade of effort, while traffic regulations were conformed to western standards (e.g., freeway advisory limit, on other rural roads).
Many rural roads on the Isle of Man have no speed limits; Measured travel speeds on the island are relatively low.
The Indian states of Andhra Pradesh, Maharashtra, and Telangana also do not have speed limits by default.
Roads formerly without speed limits
Many roads without a maximum limit became permanently limited following the 1973 oil crisis. For example, Switzerland and Austria had no maximum restriction prior to 1973 on motorways and rural roads, but imposed a temporary maximum limit in response to higher fuel prices; the limit on motorways was increased to later in 1974.
thumb|Typical speed limit sign that one would see at the Montana state line from December 1995 to June 1999
Montana and Nevada were the last remaining U.S. states relying exclusively on the basic rule, without a specific, numeric rural speed limit before the National Maximum Speed Law of 1974. After the repeal of federal speed mandates in December 1995, Montana was the only state to revert to the basic rule for daylight rural speed regulation. The Montana Supreme Court ruled that the basic rule was too vague to allow citation, prosecution, and conviction of a driver; concluding enforcement was a violation of the due process requirement of the Montana Constitution. In response, Montana's legislature imposed a limit on rural freeways in 1999.
Australia's Northern Territory had no rural speed limit until 2007, and again from 2014 to 2016. Sections of the Stuart Highway had no limits as part of an open speed limit trial.
Method
Several methods exist to set up a speed limit:
- Engineering
- Harm minimization
- Economic optimization
- Expert system
For instance, the Injury Minimization (known as Safe System) method takes into account the crash types that are likely to occur, the impact forces that result, and the tolerance of the human body to withstand these forces to set speed limit. This method is used in countries such as the Netherlands and Sweden.
Operating speed
The Operating speed method sets the maximum speed at or around the 85th percentile speed, referred to as the 85th percentile rule. It refers to a speed where 85% of vehicles travel at or below. This reduces the need to enforce the speed limit, but also allows drivers to fail to select the appropriate travel speed, when they misjudge the risk their environment induces. This is one method used in the United States of America.
Critics of the guideline say that it is inappropriate to let drivers set the speed limit for a road via their own recorded speed.
Public safety advocates have advocated for the Federal Highway Administration to change their guidance on the usage of the 85th percentile rule in updates to Manual on Uniform Traffic Control Devices.
In Europe, between 2009 and 2012, 20% of European drivers have been fined for excessive speed.
The city of Munich has adopted self-explaining roads: roadway widths, intersection controls, and crossing types have been harmonized so that drivers assume the speed limit without a posted sign.
To be effective and abided by, the speed limits need to be perceived as credible; they should be reasonable regarding factors such as how well the driver can see ahead and to the sides on a particular road.|align=center
In the UK, in 2017, the average free flow speed for each vehicle type is correlated with the applicable speed limit for that
road type and for motorways and national speed limit single carriageway roads, the average free flow speed is below
the designated speed limit for each vehicle type, except motorcycles on motorways.
Relationship with crash frequency
A 1998 US Federal Highway Administration report cited a number of studies regarding the effects of reductions in speed limits and the observed changes in speeding, fatalities, injuries and property damage which followed. Some states increase penalties for more serious offenses, by designating as reckless driving, speeds greatly exceeding the maximum limit.
A 2018 OECD-ITF case study established a strong relationship between speed and crash frequency: when the mean speed decreases, the number of crashes and casualties decreases; to the contrary, when speed increases, the number of crashes and casualties increases. In no case was an increase in mean speed associated with a decrease in the number of crashes or casualties.