Speed limit

Road speed limits are used in most countries to set the maximum (or minimum in some cases) speed at which road vehicles may legally travel on particular stretches of road. Speed limits may be variable and in some places speeds are unlimited (e.g., on the Autobahn in Germany). Speed limits are normally indicated on a traffic sign. Speed limits are commonly set by the legislative bodies of nations or provincial governments and enforced by national or regional police and/or judicial bodies.

The first maximum speed limit was the 10 mph limit introduced in the United Kingdom in 1861. The highest posted speed limit in the world is 140 km/h, which applies to some roads in Poland and Bulgaria; similarly Texas posts 85 mph on one 40 mi long toll road. However, some roads have no speed limit for certain classes of vehicles. Best known are Germany's less congested Autobahns and areas of Australia's Stuart Highway, where automobile drivers have no mandated maximum speed. Measurements from the German state of Brandenburg in 2006 showed average speeds of 142 km/h on a 6-lane section of autobahn in free-flowing conditions. Rural roads on the Isle of Man and the Indian states of Andhra Pradesh, Maharashtra, and Telangana, also lack speed limits.

Speed limits are usually set to attempt to cap road traffic speed; there are several reasons for wanting to do this. It is often done with an intention to improve road traffic safety and reduce the number of road traffic casualties from traffic collisions. In their World report on road traffic injury prevention report, the World Health Organization (WHO) identify speed control as one of various interventions likely to contribute to a reduction in road casualties. (The WHO estimated that some 1.2 million people were killed and 50 million injured on the roads around the world in 2004.) Speed limits may also be set in an attempt to reduce the environmental impact of road traffic (vehicle noise, vibration, emissions) and to satisfy local community wishes for streets usable by people out of cars. Some cities have reduced limits to as little as 30 km/h for both safety and efficiency reasons.

In situations where the natural road speed is considered too high by governments, notably on urban areas where speed limits below 50 km/h are used then traffic calming is often also used. For some classes of vehicle, speed limiters may be mandated to enforce compliance.

Since their introduction, speed limits have been opposed by some motoring advocacy groups.

History


The United Kingdom Stage Carriage Act 1832 first introduced the offense of endangering the safety of a passenger or person by 'furious driving'. The first numeric speed limits were created in the UK by a series of Locomotive Acts (1861, 1865 and 1878); the 1861 Act introduced a UK speed limit of 10 mph on open roads in town, reduced to 2 mph in towns and 4 mph in rural areas by the 1865 'red flag act'. The Locomotives on Highways Act 1896, which raised the speed limit to 14 mph (being the estimated speed of a horse being driven 'furiously') is celebrated to this day by the annual London to Brighton Veteran Car Run.

The first person to be convicted of speeding is believed to be Walter Arnold of East Peckham, Kent, who on 28 January 1896 was fined for speeding at 8 mph. He was fined 1 shilling plus costs.

In Australia, during the early 20th century, there were people reported for "furious driving" offences. One conviction in 1905 cited furiously driving 20 mph when passing a tram traveling at half that speed.

Regulations


Most jurisdictions use the metric speed unit of kilometers per hour for speed limits, while some, primarily the United States and the United Kingdom, use speed limits given in miles per hour. Australia followed the United Kingdom system before changing to the metric system in the 1970s.

Basic rule
See also main article on the Basic Speed Law or Rule.

Reasonable speed
Drivers are required to drive at a safe speed for conditions. In the United States, this requirement is referred to as the basic rule, but more generally in Britain and elsewhere in common law as the reasonable man requirement. The German Highway Code (Straßenverkehrs-Ordnung) section on speed begins with a statement which may be rendered in English: "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."The US federal government has a similar law—49 CFR 392.14 —which applies in all states as permitted under by the commerce clause and due process clause.;  for example California Vehicle Code section 22350 which 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".

The basic speed law is almost always supplemented by specific maximum or minimum limits, but applies regardless. The reasonable speed may be different than the posted speed limit in conditions such as fog, heavy rain, ice, snow, gravel, sharp corners and blinding glare.

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 was 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 that the speed limit has been exceeded (if one even exists), rather that police determined at least one party traveled too fast for existing road or weather conditions. Examples of conditions where drivers may find themselves driving too fast include: wet roadways (rain, snow, or ice), reduced visibility (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"

Citations for violations of the basic speed law without a crash have sometimes been ruled unfairly vague or arbitrary, hence a violation the due process of law, at least in the State of Montana. Even within states, differing jurisdictions (counties and cities) choose to prosecute similar cases with differing approaches.

Maximum speed limits
Most public roads in most countries have a legally assigned numerical maximum speed limit which applies on all roads unless otherwise stated; 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. In the European Union, large signposts showing the national (default) speed limits of the respective country are usually erected immediately after border crossings, with a repeater sign some 200 to 500 m after the first sign. Some places provide an additional "speed zone ahead" ahead of the restriction and speed limit reminder signs may appear at regular intervals which may be painted on the road surface.

Signs are normally placed on both sides of the road and in some places there are small (less than 1/4 the size of the sign) rectangular orange reflector flags attached to both upper right corners of both signs. The speed limit sign marking the new speed zone may also have the orange flags; this practice can be observed in New York on highways where the speed limit varies such as New York State Route 17.

Signage in many countries, especially in Europe, conforms to the Vienna Convention on Road Signs and Signals using black text with a red circle on a white background.

In the United States, the signs are usually rectangular with the words "SPEED LIMIT" and the values in black on a white background. The U.S. Federal Highway Administration's Manual on Uniform Traffic Control Devices provides guidelines for the appearance of speed limit signs. In Alaska and California, speed limits are often labeled "MAXIMUM SPEED" instead. In Oregon, most speed limit signs just say "SPEED" and the number. Canada has similar signs bearing the legend "MAXIMUM" and in km/h instead of mph. "Maximum" is used instead of "Speed Limit" because it has similar meanings in English and French. Australian signs are rectangular but have a red circle like the Vienna Convention signs.

The speed limit is commonly set at or below the 85th percentile operating speed (being the speed which no more than 15% of traffic is exceeding)  and in the US is typically set 8 to 12 mph 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 driver, 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 substance is a process for voting the speed limit by driving; and 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.

Minimum speed limits
Some roads also have "minimum speed limits", where slow speeds can impede traffic flow or be dangerous.

Signs often use blue circles based on the obligatory signs of the Vienna Convention on Road Signs and Signals. A Japanese minimum speed sign has the same design as a maximum speed sign, but with a horizontal line below the number. In the United States of America, they are also identical to their respective maximum speed limit signs with the exception of the text "MINIMUM SPEED".

Variable speed limits


In 1965, the first known experiments with variable speed limit signs took place on a 30 km stretch of German motorway A8 between Munich and the border city of Salzburg, Austria. Mechanically variable message signs could display speeds of 60, 80 and 100 km/h,  "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, 1300 km of German motorways were equipped with such systems.

In the late 1960s, heavily traveled portions of the New Jersey Turnpike began using variable speed limit signs, in combination with variable message signs. 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 52 mi stretch of Interstate 80 in Wyoming, replacing the winter season speed reduction from 75 to 65 mph that had been in place since 2008. This Variable Speed Limit system has been proved to be effective in terms of reducing crash frequency and road closures. Similarly, Interstate 90 at Snoqualmie Pass and other mountain passes in Washington State variable speed limits are used 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 19 mi of Interstate 75 in Tennessee was implemented in fog-prone areas around the Hiwassee River.

A variable speed limit was introduced on part of Britain's M25 motorway in 1995 (on the busiest 14 mi section from junction 10 to 16). Initial results suggested savings in journey times, smoother-flowing traffic, as well as a fall in the number of crashes, and the scheme was made permanent in 1997. However a 2004 National Audit Organization 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 facility 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 dual carriageway on State Highway 1 with steep terrain, numerous bends, high traffic volumes, and higher than average accident rate. The speed limit is normally 80 km/h.

In 2006, Austria undertook a short-term experiment with a variable limit configuration that could increase statutory limits under the most favorable conditions, as well as reduce them. In June, a stretch of motorway was configured with variable speed limits that could increase the general Austrian motorway limit of 130 to 160 km/h. 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.

Oregon In 2016 ODOT has installed a variable speed zone on a 30 mile stretch of Interstate 84 between Baker City, Or 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 is scheduled to be activated November of 2016.

In 2014, the Georgia Department of Transportation installed variable speed limits on part of Interstate 285 around Atlanta. These speeds can be as low as 35 mph but are generally set to 65 mph.

Roads without speed limits
Just over half of the German autobahns have only an advisory speed limit (called in German Richtgeschwindigkeit), 15% have temporary speed limits due to weather or traffic conditions and 33% have permanent speed limits, according to 2008 estimates. Although many German federal highways do have general speed limits, there is no general speed limit on any road outside of towns which is either a dual carriageway and/or features at least two lanes per direction. 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 130 km/h. More than 30% of motorists exceed 150 km/h". Prior to German reunification in 1990, accident reduction programs in eastern German states were primarily focused on restrictive traffic regulation. Within two years after the opening, 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 100 km/h on autobahns and of 80 km/h 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., 130 km/h freeway advisory limit, 100 km/h on other rural roads, and 0.05 milligrams BAC).

In Australia's Northern Territory, from September 2015 a 336 km stretch of Stuart Highway between Barrow Creek and Alice Springs had speed limits removed for a 12-month trial. The Northern Territory had no blanket speed limits outside major towns until January 2007 when a general rural speed limit of 110 km/h was introduced although four major highways had higher 130 km/h zones. Speed limit advocates note that the per-capita fatality rate in 2006 was the highest in the OECD and twice the Australian average. In 2009, the opposition Country Liberal Party unsuccessfully sought the removal of the 130 km/h limits on three out of the four highways where it applied, arguing that total fatalities in the Northern Territory had increased significantly during the first two years of the speed limit. In argument against the motion, the government provided more detailed statistics than normally published, these statistics showed a reduction in fatalities along the highways where 130 km/h limits were introduced. In 2011 the opposition argued for a return to "open speed limits" . For the 2012 election the Country Liberals' transport policy promised an evidence based approach. After winning, de-restriction of Stuart Highway was proposed; a planned 12-month de-restriction was initiated on February 1, 2014. The trial on 200 km of Stuart Highway was expanded later in the year to another 72 km, and continued indefinitely in January 2015 during a review of the initial results since "in the first 11 months, there were no recorded fatalities."

The Isle of Man has no speed limit on many rural roads; a 2004 proposal to introduce a general speed limits 60 mph and of 70 mph on Mountain Road for safety reasons were not progressed following consultation. Measured travel speeds on the island are relatively low.

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 100 km/h maximum limit in quick response to higher fuel prices; the limit on motorways was increased to 130 km/h later in 1974.

Montana and Nevada were the last remaining U.S. states relying exclusively on the basic rule, without a specific, numeric rural speed limit prior to the National Maximum Speed Law of 1974. After repeal of Federal speed mandates in 1996, 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, in other words, enforcement was a violation of the due process requirement of the Montana Constitution; in response, Montana's legislature imposed a 75 mph limit on rural freeways in 1999.

Enforcement


Speed limit enforcement is the action taken by appropriately empowered authorities to check that road vehicles are complying with the speed limit. Methods used include roadside speed traps set up and operated by the police and automated roadside speed camera systems, which may incorporate the use of an automatic number plate recognition system.

Effectiveness
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: Annual surveys of speed on South Dakota Interstate roads show that from 2000 to 2011, the average speed rose from 71 to 74 mph; South Dakota increased its maximum speed limit from 65 to 75 mph (120 km/h) in 1996.

The Synthesis of Safety Research Related to Speed and Speed Limits report sponsored by the Federal Highway Administration published in 1998 found that changing speed limits on low and moderate speed roads appeared to have no significant effect on traffic speed or the number of crashes, whilst on high-speed roads such as freeways, increased speed limits generally resulted in higher traffic speeds and more crashes. It states that limited evidence suggests that speed limits have a positive effect on a system wide basis.

Research in 1998 showed that the reduction of some 30 mph United Kingdom speed limits to 20 mph had achieved only a 1 mph drop in speeds and no discernible reduction in accidents; '20 mph speed limit zones' which use self enforcing traffic calming achieved average speed reductions of 10 mph, child pedestrian accidents were reduced by 70% and child cyclist accidents by 48%.

Zones where speeds are set at 30 km/h (or 20 mph) are gaining popularity as they are found to be effective at reducing crashes and increasing community cohesion.

Studies undertaken in conjunction with Australia's move from 60 km/h speed limits to 50 km/h in built-up areas and found that the measure was effective in reducing speed and also the frequency and severity of crashes.

A study of the impact of the replacement of 60 km/h with 50 km/h speed limits in New South Wales, Australia, showed only a 0.5 km/h drop in urban areas and a 0.7 km/h drop in rural areas. The report noted that widespread community compliance would require a combination of strategies including traffic calming treatments.

A 1999 study found that the U.S. states that increased speed limits in the wake of the repeal of federally mandated speed limits had a 15% increase in fatalities.

Information campaigns are also used by authorities to support speed limits, for example the Speeding. No one thinks big of you. campaign in Australia 2007.

Justification
Speed limits are set primarily to balance road traffic safety concerns with the effect on travel time and mobility. Speed limits are also sometimes used to reduce consumption of fuel or in response to environmental concerns.

Road traffic safety


According to a 2004 report from the World Health Organisation a total of 22% of all 'injury mortality' worldwide were 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 said that speed limits should be set appropriately for the road function and design along with physical measures related to the road and the vehicle and 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). They are also occasionally set to reduce vehicle emissions or fuel use.

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.

Typically motorways have higher speed limits than conventional roads because motorways have features which decrease the likelihood of collisions and 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 1000 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 speeds, 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 rate 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 percent of all speeding-related crashes in fatal crashes had "exceeding posted speed limits" among their crash factors, and 45 percent 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 actually 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 crash analysis as there exist 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 without necessarily compromising safety" since design speed is "based on conservative assumptions about driver, vehicle and roadway characteristics".

Vision Zero, which envision reducing road fatalities and serious injuries to zero by 2020, suggests the following "possible long term maximum travel speeds related to the infrastructure, given best practice in vehicle design and 100% restraint use":

"Roads with no possibility of a side impact or frontal impact" are sometimes designated as Type 1 (motorways/freeways/Autobahns), Type 2 ("2+2 roads") or Type 3 ("2+1 roads"). These roadways have crash barriers separating opposing traffic, limited access, grade separation and prohibitions on slower and more vulnerable road users. Undivided rural roads can be quite dangerous even with speed limits that appear low by comparison. For example, in 2011, Germany's 100 km/h-limited rural roads had a fatality rate of 8.7 deaths per billion travel-km, over four times higher than the autobahn rate of 2 deaths. Autobahns accounted for 31% of German road travel in 2011, but just 11% (453 of 4,009) of traffic deaths.

Fuel efficiency
Fuel efficiency sometimes affects speed limit selection. The United States instituted a National Maximum Speed Law of 55 mph as part of the Emergency Highway Energy Conservation Act in response to the 1973 oil crisis to reduce fuel consumption. According to a report published in 1986 by The Heritage Foundation, a Conservative advocacy group, the law was widely disregarded by motorists and hardly reduced consumption at all. In 2009 The American Trucking Associations called for a 65 mph speed limit and also national fuel economy standards claiming that the lower speed limit was not effective at saving fuel. European studies have claimed that, whereas the effects of specific speed reduction schemes on particulate emissions from trucks are ambiguous, lower maximums speed for trucks consistently result in lower emissions of CO2 and better fuel efficiency.

Environmental considerations
Speed limits can also be used to improve local air quality issues or other factors affecting environmental quality for example the "environmental speed limits" in the United States including one in an area of Texas.

The European Union is also increasingly using speed limits as in response to environmental concerns.

Advocacy
Speed limits, and especially some of the methods used to attempt to enforce them, have always been controversial. There are a variety of notable organisations and individuals who, for a variety of often passionately held views, oppose or support the use of speed limits or the way they are enforced.

Opposition
Speed limits, and their enforcement have been opposed by various groups and for various reasons since their inception. Historically, the AA was formed in 1905, initially to warn members about speed traps.

In more recent times some advocacy groups seek to have certain speed limits as well as other measures removed. For example, automated camera enforcement has been criticised by motoring advocacy groups the Association of British Drivers, the North American National Motorists Association,  and the German Auto Club.

Arguments used by those advocating a relaxation of speed limits or their removal include:
 * A 1994 peer-reviewed paper by Charles A. Lave et al. titled Did the 65 mph Speed Limit Save Lives? stated that evidence that a higher speed limit may be positive on a system wide in the United States by shifting more traffic to these safer roads.
 * A 1998 report in the Wall Street Journal title 'Highways are safe at any speed' stated that when speed limits are set artificially low, tailgating, weaving and speed variance (the problem of some cars traveling significantly faster than others) make roads less safe.
 * In 2010, German Auto Club (a major motoring organisation) argued that an autobahn speed limit was unnecessary because numerous countries with a general highway speed limit had worse safety records than Germany, for example Denmark, Belgium, Austria, and the United States.
 * In 2008, the German Automobile Manufacturer's Association called general limits "patronizing", arguing instead for variable speed limits. The Association also stated that "raising the speed limits in Denmark (in 2004 from 110 km/h to 130 km/h)  and Italy (2003 increase on six-lane highways from 130 km/h to 150 km/h) had no negative impact on traffic safety. The number of accidental deaths even declined".
 * Safe Speed, a UK advocacy organisation campaigns for higher speed limits and to scrap speed cameras on the basis that the benefits were exaggerated and that they may actually increase casualty levels; their ePetition to the UK government in 2007 calling for speed cameras to be scrapped received over 25,000 signatures.

Support
Various other advocacy groups press for stricter limits and better enforcement. The Pedestrians Association was formed in the United Kingdom in 1929 to protect the interests of the pedestrian. Their president published a critique of motoring legislation and the influence of motoring groups in 1947 title 'Murder most foul' which laid out in an emotional but detailed way the situation as they saw it and called for tighter speed limits. Historically, the Pedestrians' Association and the Automobile Association were described as "bitterly opposed" in the early years of United Kingdom motoring legislation. More recently organisations such as RoadPeace, Twenty is Plenty, and Vision Zero have campaigned for lower speed limits in residential areas.

Signage
In most of the world speed limit signs display the limitation within a red circle. This design follows the style set out by the Vienna Convention on Road Signs and Signals, with the exception of United States and Canada. All countries globally worldwide (except the United Kingdom and United States) use metric speed limits in kilometres per hour. Metric signs are optional in the United States though less commonly seen, while Samoa uses both units. In Ontario, Canada, the type, location, and frequency of speed limit signs is covered by regulation 615 of the Ontario Highway Traffic Act.

Minimum speed limit
Minimum speed limit signs are usually a blue circle with a white border and the limit in white. Some South American countries (e.g.: Argentina), use a red border. Japan and South Korea use their normal speed limit sign, with a line below the limit.

Special speed limits
In some countries, speed limits may apply to certain classes of vehicles, or special conditions such as night-time. Usually these speed limits will be reduced from the normal limit for safety reasons.

Speed limit derestriction
In some countries, derestriction signs are used to mark where a speed zone ends. The speed limit beyond the sign is the prevailing limit for the general area, for example the sign might be used to show the end of an urban area. In the United Kingdom, the sign means that the national speed limit applies (60 mph on open roads, 70 mph on dual carriageways and motorways). In New Zealand it means you are on an open road, but the maximum legal speed of 100 km/h still applies On roads without a general speed limits, such as German Autobahn, a portion of Stuart Highway, and rural areas on the Isle of Man, it really means end of all quantitative speed limits.

Advisory speed limit
Advisory speed limits may provide a safe suggested speed in an area, or warn of the safe maximum speed for dangerous curves.