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Effects Of Speed On Public Roads Essay

Road safety issues, including speeding, have been subjected to scientific analysis for nearly one hundred years and a vast body of research has been developed. The consensus in road safety  best practice is:

  • The faster you drive, the worse the crash will be if you get in one because of the greater force involved
  • The faster you drive, the more likely you will be in a crash because you will have less time to react to unexpected hazards.
  • Even small decreases in mean speed travelled equal many lives saved.

In a 2006 study, The Organisation for Economic Co-operation and Development (OECD) and the European Council of Transport Ministers determined that excessive or inappropriate speed was the full or partial cause in approximately one third (33%) of all fatal crashes. Put another way, SPEED KILLS.

Know the Difference

“Speeding” is driving above the speed limit whilst “excessive speed” is driving way above the speed limit, and you can be arrested for it. This applies at 30 km/h or more in an urban area and 40 km/h or more outside an urban area or on a freeway. Under Sections 35 and 36 of the National Road Traffic Act, 93 of 1996 your driver’s licence will be suspended if you are convicted of excessive speed.

“Inappropriate speed” is driving too fast for the conditions; for example, driving at the speed limit in heavy rain.

Why Speed Kills

Speed has two main relationships with road safety

  1. The most direct link is aggravation of severity: greater collision speeds mean more force unleashed on the victims and thus speed directly influences the likelihood of death or serious injury. This is particularly so when vulnerable road users such as pedestrians, cyclists and motorcyclists are involved in a collision with another vehicle.
  2. The second relationship, a bit more complex, is the causal role of speed in a road trauma incident. The higher the speed at the point where a crash becomes likely, the less time there is for the driver to react, and the increased chances of skidding or other events influencing loss of control. So higher speeds play a very significant role in causing crashes.


Q: Is keeping to the speed limit safe?

A: Not always. South Africa has very high speed limits compared to countries with low road crash fatalities. In particular, the standard speed limit in urban areas is 60 km/h, which is very high for some areas, particularly where there are many schoolchildren and other pedestrians. In addition, speed limits are determined for good daytime conditions, and you should lower your speed in heavy traffic, at night or in bad weather, or any circumstance where visibility is reduced.

Q: Is driving slower than the rest of the traffic dangerous?

A: Scientists working in road safety in the 1950’s and 1960’s (e.g. Solomon, 1964) established a causal link between speed variation from the norm for a given road, and likelihood of a crash. It was long-believed this held true for both slower vehicles and faster vehicles, although more modern research (e.g. Kloeden et al, 1997; 2001; 2002) shows that the relationship is only significant for vehicles going faster than the mean.

These early findings, however, appear to have left a considerable impact, in that there are many people today who believe that relative speed is the most important consideration when selecting speed, an approach which is open to be manipulated to justify high travelling speeds.

Q: Are lower speed limits safer?

A: Yes. Lower speed limits have been shown to reduce road deaths and injuries considerably. Developed countries have adopted progressively lower speed limits in recent decades, in response to increased traffic volumes and large numbers of deaths, particularly among pedestrians and other vulnerable road users.

Q: Do lower speed limits make drivers more compliant with the speed limit?

A: No, they don’t necessarily have an effect on overall compliance, which is more responsive to law enforcement and behaviour change through education. But they do have an effect on the mean speed travelled, which is very important for crash rates. Small changes in mean speed travelled translate to significant reductions in fatal and serious crashes.

The Nilsson equation (Nilsson, 1982) is still used today to calculate fatalities reductions through speed limit reductions and has been shown to remain true through studies in a wide variety of jurisdictions and conditions. Nilsson’s model, for example, has been used to show that a 1 km/h reduction in mean speed travelled from 120 km/h to 119 km/h equals a fatalities reduction of 3.8%.

Q: Should South Africa lower its speed limits?

A: Transport specialists in South Africa are very divided on this issue, with a majority being opposed. However, those specialists opposed to speed limit reductions appear to be uninformed regarding international research confirming the success of speed limit reductions in bringing down death rates down around the world (Steunenberg, Sinclair, 2014).

The argument is also made that South Africa lacks the capacity to enforce the existing speed limits and should therefore not tamper with them, although high levels of law enforcement have not been proven to be an absolute prerequisite for success. This opposition to lower speed limits reflects broader society where resistance to lower speed limits is even more pronounced. There is also a high level of suspicion among the public regarding the motives of the authorities whenever speed limit reductions are considered.

This is an international trend and it can take many years of reduced road fatalities to convince the more sceptical of the value of lower speed limits. However, international research also largely suggests that speed limits must be credible to be respected, which is the key to compliance. This implies that a great deal of public education about speed and the role it plays in road safety is still required in South Africa before the public will be accepting of lower speed limits.

This article is about road speed limits. For speed of light, see special relativity. For road speed limits in specific countries, see Speed limits by country. For rail speed limits, see Slow zone.

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 speed is unlimited (e.g. on some Autobahn sections 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 or judicial bodies.

The first maximum speed limit was the 10 mph (16 km/h) limit introduced in the United Kingdom in 1861. The highest posted speed limit in the world is 160 km/h (99 mph), which applies to some motorways in UAE.[1] However, some roads have no speed limit for certain classes of vehicles. Best known are Germany's less congested Autobahns,[2] where automobile drivers have no mandated maximum speed. Measurements from the German state of Brandenburg in 2006 showed average speeds of 142 km/h (88 mph) on a 6-lane section of autobahn in free-flowing conditions.[3]Rural roads on the Isle of Man[4] and the Indian states of Andhra Pradesh,[5]Maharashtra,[6] and Telangana,[7] 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.)[n 1] 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 (19 mph) for both safety and efficiency reasons.[8] However, it has also been shown that in some circumstances changing a speed limit has little effect on the average speed of cars.[9]

In situations where the natural road speed is considered too high by governments, notably in urban areas where speed limits below 50 km/h (31 mph) 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.


The United Kingdom Stage Carriage Act 1832 first introduced the offense of endangering the safety of a passenger or person by 'furious driving'.[10] 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 (16 km/h) on open roads in town, reduced to 2 mph (3 km/h) in towns and 4 mph (6 km/h) in rural areas by the 1865 'red flag act'. The Locomotives on Highways Act 1896, which raised the speed limit to 14 mph (23 km/h) (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 (13 km/h). He was fined 1 shilling plus costs.[11][12][13]

In the UK 20 mph speed was allowed in 1903.[14]

In Australia, during the early 20th century, there were people reported for "furious driving" offences. One conviction in 1905 cited furiously driving 20 mph (32 km/h) when passing a tram traveling at half that speed.[15]

In the 1960s, in continental Europe, some speed limit were established based on the V85 speed, (so that 85% of drivers respect this speed).[14]

Later Sweden defined the Vision Zero program.[14]


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. There is an ongoing discussion as to whether they should follow the lead of other countries and switch to using metric units (see Metrication in the United Kingdom and Metrication in the United States).

Basic rule[edit]

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

Vienna Convention on Road Traffic[edit]

In countries bounded by Vienna Convention on Road Traffic (1968 & 1977), article 13 defines a basic rule for Speed and distance between vehicles:[16] "Every driver of a vehicle shall in all circumstances have his vehicle under control so as to be able to exercise due and proper care and to be at all times in a position to perform all manœuvres required of him. He shall, when adjusting the speed of his vehicle, pay constant regard to the circumstances, in particular the lie of the land, the state of the road, the condition and load of his vehicle, the weather conditions and the density of traffic, so as to be able to stop his vehicle within his range of forward vision and short of any foreseeable obstruction. He shall slow down and if necessary stop whenever circumstances so require, and particularly when visibility is not good. "

Reasonable speed[edit]

Drivers are required to drive at a safe speed for conditions. In the United States, this requirement is referred to as the basic rule,[17] but more generally in Britain and elsewhere in common law as the reasonable man requirement.[18] The German Highway Code (Straßenverkehrs-Ordnung) section on speed begins with a statement[19] 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.

In France the law clarifies that even if speed is limited by law and by local authority, the driver assumes the responsibility to control his vehicle’s speed, and to reduce speed in various circumstances, such as overtaking a pedestrian, or bicycles, individually or in a group, when overtaking a stopped convoy, when passing a transportation vehicle loading or unloading people or children, in any case where road does not appear clear, or risky, when visibility is low (rain, fog, ...), in turns, when the road goes rapidly down, in road sections that are small, busy, or with homes, near the top of the road, near a crossing when visibility is not sure, when specific lights are used, when overtaking animals.[20] According to the same article, the fact for a driver to not keep master of its speed or to not reduce it in such cases is penalized.

The US federal government has a similar law—49 CFR 392.14[21]—which applies in all states as permitted under by the commerce clause and due process clause.;[22][23] 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".[24]

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,[25] sharp corners, blinding glare,[26] darkness, crossing traffic,[27] 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).[28][29] Basic speed laws are statutized reinforcements of the centuries-old common lawnegligence doctrine as specifically applied to vehicular speed.[30]

Excessive speed[edit]

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.[31] 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[32] or weather[33] conditions.[31] Examples of conditions[34] where drivers may find themselves driving too fast include: wet roadways (rain, snow, or ice), reduced visibility (fog[35] or "white out" snow[36]), uneven roads, construction zones,[37] curves,[38] intersections, gravel roads,[39] and heavy traffic. Per distance traveled, consequences of inappropriate speed are more frequent on lower speed, lower quality roads;[40] in the United States, for example, the "speeding fatality rate for local roads is three times that for Interstates"[41]

Citations for violations of the basic speed law without a crash[42] have sometimes been ruled unfairly vague or arbitrary, hence a violation the due process of law, at least in the State of Montana.[43] Even within states, differing jurisdictions (counties and cities) choose to prosecute similar cases with differing approaches.[44]

Maximum speed limits[edit]

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 (660 to 1,640 ft) 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.[45]

The speed limit is commonly set at or below the 85th percentileoperating speed (being the speed which no more than 15% of traffic is exceeding)[46][47][48] and in the US is frequently set 4 to 8 mph (6 to 13 km/h) below that speed.[49] 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").[50] 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.[51][52] 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[citation needed]. 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.[53][54]

The maximum speed permitted by statute, as posted, is normally based on ideal driving conditions, and the basic speed rule always applies.[55] Violation of the statute generally raises a rebuttable presumption of negligence.[56]

In international European roads, speed should be taken into account at design stage[citation needed].

Road classification60 km/h80 km/h (50 mph)100 km/h (60 mph)120 km/h (75 mph)140 km/h (85 mph)
Express road6080100120x

Minimum speed limits[edit]

Some roads also have "minimum speed limits", where slow speeds can impede traffic flow or be dangerous.[57]

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".

This minimum speed is not so common, as the risk usually raise with higher speed and not with lower speed.[14]

Variable speed limits[edit]

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.[58] 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, 1,300 km (810 mi) of German motorways were equipped with such systems.[59]

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.[60] More typically, variable speed limits are used on remote stretches of highway in the United States in areas with extreme changes in driving conditions.[61] For example, variable limits were introduced in October 2010 on a 52-mile (84 km) stretch of Interstate 80 in Wyoming, replacing the winter season speed reduction from 75 to 65 mph (121 to 105 km/h) that had been in place since 2008.[62][63] This Variable Speed Limit system has been proved to be effective in terms of reducing crash frequency and road closures.[64][65] 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.[61][66] As a response to fog-induced chain-reaction collisions involving 99 vehicles in 1990, a variable speed limit system covering 19 miles (31 km) of Interstate 75 in Tennessee was implemented in fog-prone areas around the Hiwassee River.[67]

A variable speed limit was introduced on part of Britain's M25 motorway in 1995 (on the busiest 14-mile (23 km) 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.[68] 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.[69] In January 2010 temporary variable speed cameras on the M1 between J25 and J28 were made permanent.[70]

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 (50 mph).[71]

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 (81 to 99 mph).[72] 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.

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.[73]

In 2016 the Oregon Department of Transportation has installed a variable speed zone on a 30-mile 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 is scheduled to be activated November 2016.[citation needed]

Roads without speed limits[edit]

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.[75]German federal highways and any road outside of towns which is either a dual carriageway or features at least two lanes per direction do have a general speed limit of 100 km/h. Usually it is reduced to 80 km/h at Allée-streets (federal highways bordered by trees or bushes on one or both sites).[76] 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 (81 mph). More than 30% of motorists exceed 150 km/h (93 mph)".[77] 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,[78] despite "interim arrangements [which] involved the continuation of the speed limit of 100 km/h (62 mph) on autobahns and of 80 km/h (50 mph) 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 (81 mph) freeway advisory limit, 100 km/h (62 mph) on other rural roads, and 0.05 milligrams BAC).[79]

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. [4]Measured travel speeds on the island are relatively low.[80]

Roads formerly without speed limits[edit]

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 (62 mph) maximum limit in quick response to higher fuel prices; the limit on motorways was increased to 130 km/h (81 mph) later in 1974.[81][82][83]

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.[84] 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.[85] In response, Montana's legislature imposed a 75 mph (121 km/h) limit on rural freeways in 1999.

In Australia's Northern Territory, after the removal of open limits in 2007, sections of the Stuart Highway had no limits as part of an open speed limit trial from 2014 to 2016.


Several methods exist to set up a speed limit:[86]

  • Engineering
  • Harm minimization,
  • Economic optimization
  • Expert system


Main article: Speed limit 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.

In 2012, in UK, 30% of drivers did not comply with speed limits.[14]

In Europe, between 2009 and 2012, 20% of European drivers have been fined for excessive speed.[14] In 2012 in Europe, 62% of people supported the idea to set up speed-limiting devices.[14]

The tolerance level needs to be adequate to not add confusion to the driver. One efficient scheme consist in penalty points and charges for speeding just a few over the speed limit.[14]

Another possibility is to make the necessary work to change the road so that the driver can consider the speed limit is legitimate in regard to the road. This can be achieved by implementing traffic calming measure, vehicle activated signs, or safety cameras.[87]

The city of Munich has adopted Self-explaining roads: roadway widths, intersection controls and crossing types have been harmonized in regard to speed limit, so that the driver can guess the speed limit even with no sign.[87]


To be effective and applied by drivers, the speed limits need to be perceived as credible.[14] This mean that the speed limit should be adequate in regard to some factors such as, for instance, the view ahead and the view to the right.[14]

To be effective, speed limit needs to be set up with road infrastructure, education or enforcement activity.[14]

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:[n 2]

Country (year of research publication)Speed limit reductionReported change
Australia (1992)110 km/h to 100 km/hInjury crashes declined by 19%
Australia (1996)5–20 km/h decreasesNo significant change (4% increase relative to sites not changed)
Denmark (1990)60 km/h to 50 km/hFatal crashes declined by 24%
Injury crashes declined by 9%
Germany (1994)60 km/h to 50 km/hCrashes declined by 20%
Sweden (1990)110 km/h to 90 km/hSpeeds declined by 14 km/h
Fatal crashes declined by 21%
Switzerland (1994)130 km/h to 120 km/hSpeeds declined by 5 km/h
Fatal crashes declined by 12%
UK (1991)60 mph to 40 mph /100 km/h to 65 km/hSpeeds declined by 6 km/h (4 mph)
Crashes declined by 14%
US (22 states) (1992)5 mph to 15 mph (8 km/h to 24 km/h) decreasesNo significant changes
CountrySpeed limit increaseReported change
Australia (1992)100 km/h to 110 km/hInjury crashes increased by 25%
Australia (Victoria) (1996)5–20 km/h increasesCrashes increased overall by 8%, 35% decline in zones raised from 60 km/h to 80 km/h
Netherlands (2012)120 km/h to 130 km/hEffect as of yet unclear, more research needed
US (1989)55 mph to 65 mph (89 km/h to 105 km/h)Fatal crashes increased by 21%
US (1990)55 mph to 65 mph (89 km/h to 105 km/h)Fatal crashes increased by 22%
Speeding increased by 48%
US (40 states) (1990)55 mph to 65 mph (89 km/h to 105 km/h)Fatalities increased by 15%
Decrease or no effect in 12 States
US (Iowa) (1996)55 mph to 65 mph (89 km/h to 105 km/h)Fatal crashes increased by 36%
US (Michigan) (1991)55 mph to 65 mph (89 km/h to 105 km/h)Fatal and injury crashes increased significantly on rural freeways
US (Michigan) (1992)VariousNo significant changes
US (Ohio) (1992)55 mph to 65 mph (89 km/h to 105 km/h)Injury and property damage increased but not fatal crashes.[88]
US (40 states) (1994)55 mph to 65 mph (89 km/h to 105 km/h)Statewide fatality rates decreased 3-5% (Significant in 14 of 40 states)
US (22 states) (1997)5 mph to 15 mph (8 km/h to 24 km/h) increaseNo significant changes

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.[89]

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.[n 3]

Research in 1998 showed that the reduction of some 30 mph (48 km/h) United Kingdom speed limits to 20 mph (32 km/h) 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%.[90]

Zones where speeds are set at 30 km/h (or 20 mph) are gaining popularity[91] as they are found to be effective at reducing crashes and increasing community cohesion.[92]

Studies undertaken in conjunction with Australia's move from 60 km/h (37 mph) speed limits to 50 km/h (31 mph) in built-up areas and found that the measure was effective in reducing speed and also the frequency and severity of crashes.[93]

A study of the impact of the replacement of 60 km/h (37 mph) with 50 km/h (31 mph) 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.[94]

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.[95]

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.


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.[96]

Some speed limit have also been initiated to avoid import too much gas-oil during 1973 oil crisis.

Road traffic safety[edit]

See also: 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[n 4] and without 'increased efforts and new initiatives' casualty rates would increase by 65% between 2000 and 2020.[n 5] The report identified that the speed of vehicles was 'at the core of the problem[n 6] 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.[n 7] 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).[97] 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.[98] 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;[99] 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".[n 8]

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,[100] 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.[101] 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.[102]

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.[103] Furthermore, separate research finds that only 1.6% of crashes are "caused" by drivers that exceed the posted speed limit.[104] 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.[105]

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"[106] However traffic engineers recognize that "operating speeds and even posted speed limits can be higher than design speeds without necessarily compromising safety"[107] since design speed is "based on conservative assumptions about driver, vehicle and roadway characteristics".[50]

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":[108]

Type of infrastructure and trafficPossible travel speed (km/h)
Locations with possible conflicts between pedestrians and cars30 km/h (19 mph)
Intersections with possible side impacts between cars50 km/h (31 mph)
Roads with possible frontal impacts between cars, including rural roads[109]70 km/h (43 mph)
Roads with no possibility of a side impact or frontal impact (only impact with the infrastructure)100 km/h (62 mph)+

"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").[110] 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 (62 mph)-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.[102] Autobahns accounted for 31% of German road travel in 2011,[102] but just 11% (453 of 4,009) of traffic deaths.

Fuel efficiency[edit]

Fuel efficiency sometimes affects speed limit selection. The United States instituted a National Maximum Speed Law of 55 mph (89 km/h) as part of the Emergency Highway Energy Conservation Act in response to the 1973 oil crisis to reduce fuel consumption.[111] 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.[112] 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.[113] 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.[114]


Speed limits can also be used to improve local air quality issues or other factors affecting environmental quality[114] for example the "environmental speed limits" in the United States including one in an area of Texas.[115]

The European Union is also increasingly using speed limits as in response to environmental concerns.[96]


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.


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.[116]

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,[117][118] the North American National Motorists Association,[119][120] and the German Auto Club.[121]

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.[122]
  • 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.[123]
  • 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.[121]
  • In 2008, the German Automobile Manufacturer's Association called general limits "patronizing",[124] 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)[125][126] 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".[127]
  • Safe Speed, a UK advocacy organisation campaigns for higher speed limits[128] and to scrap speed cameras on the basis that the benefits were exaggerated and that they may actually increase casualty levels;[129] their ePetition to the UK government in 2007 calling for speed cameras to be scrapped received over 25,000 signatures.[130]


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.[131] Historically, the Pedestrians' Association and the Automobile Association were described as "bitterly opposed" in the early years of United Kingdom motoring legislation.[132] More recently organisations such as RoadPeace, Twenty is Plenty, and Vision Zero have campaigned for lower speed limits in residential areas. In the United States, advocacy groups favoring stricter limits and better enforcement include the Advocates for Highway and Auto Safety, Insurance Institute for Highway Safety and the National Safety Council.


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

A typical 60 km/h speed limit sign used in Australia
A typical 50 mph speed limit sign in the United States
A typical 50 mph speed limit sign in the United Kingdom and 50 km/h speed limit sign in Hong Kong
Speed limit sign in Germany, showing a 60 km/h restriction. Signs in other European countries are similar but make use of different fonts and sizes
Map of highest-posted speed limits around the world; click to zoom in. Note: UK and US use mph ("SPEED LIMIT xx"); elsewhere km/h (in circle)
In France city sign has value of speed limit
End of speed limit; previous speed limit applies
Maximum speed limits by country in Europe in km/h; highest in UK posted at 70 mph
UK minimum speed limit sign, in mph
Example variable speed limit sign in the United States, in mph
Digital speed limit sign for variable speed limits
German border crossing sign showing 50 km/h (31 mph) limit in built-up areas, 100 km/h (62 mph) in rural areas, but a legal advisory 130 km/h (81 mph) limit for the Autobahn (motorway), increasing liability in the case of an accident from driving faster [74]
The Pan-American Highway with central median and no freestanding obstructions increasing level of safety at high speed
A stack of speed limit signs