Road speed limits are used in most countries to regulate the speed of road vehicles. Speed limits may define maximum, minimum or variable speed limits or no speed limit and are normally indicated using a Traffic sign. Speed limits are commonly set and enforced by the legislative bodies of nations or provincial governments.
The first maximum speed limit was the 10 mph (16 km/h) limit introduced in the United Kingdom in 1861. Currently, the highest posted speed limit in the world is 130 kilometres per hour (81 mph) on Polish motorways, although a variable speed limit up to 160 kilometres per hour (99 mph) was permitted experimentally on a stretch of Austrian motorway in June 2006. and the Isle of Man and Germany are the only places in the world that do not have a general speed limit.
Speed limits are often set with an intention to reduce the number of road traffic casualties from traffic collisions; the World Health Organization estimated that some 1.2m people were killed and 50m injured on the roads around the world in 2004 and that they are the leading cause of death among children 10 - 19 years of age (260,000 children die a year, 10 million are injured).
The first speed limits legislation in the United Kingdom were set by a series of restrictive Locomotive Acts (in 1861, 1865 and 1878). The 1861 Act introduced a 10 mph (16 km/h) limit (automobiles were in those days termed “light locomotives”). The 1865 (the 'red flag act') reduced the speed limit to 4 mph (6 km/h) in the country and 2 mph (3 km/h) in towns and required a man with a red flag or lantern to walk 60 yards (50 m) ahead of each vehicle, and warn horse riders and horse drawn traffic of the approach of a self-propelled machine. The 1878 Act removed the need for the flag and reduced the distance of the escort to 20 yards (20 m). Under pressure from motoring enthusiasts and the emergent UK motor industry the Locomotives on Highways Act 1896 increased of the speed limit to 14 mph (23 km/h) and removed the need for the escort. A celebratory run from London to Brighton was held soon after the act was passed and has been commemorated each year since 1927 by the London to Brighton Veteran Car Run. The speed limit for motor cars was raised to 20 mph (32 km/h) by the Motor Car Act 1903 which stood until 1930 when all speed limits for cars were removed by the Road Traffic Act 1930 which strangely also introduced a 20 mph (32 km/h) speed limits for UK coach services and buses services. A 30 mph (48 km/h) limit was reintroduced in urban areas by the Road Traffic Act 1934 following alarming increases in UK traffic casualties. An upper speed limit on all roads of 70mph was introduced in 1965 together with 50mph limits on some rural roads and the a limit of 60mph for single carriageway roads in 1978. It was made easier for authorities to introduce a 20mph limit in 1999.
The first person to be convicted of speeding in the UK was Walter Arnold of East Peckham, Kent. On January 28, 1896 he was fined for speeding at 8 mph (13 km/h), thus exceeding the contemporary speed limit of 2 mph (3.2 km/h). He was fined 1 shilling plus costs.
Most public roads in most places are legally assigned a default maximum speed limit. The relevance of default speed limits to road users varies; in some places, authorities always post a sign stating the maximum speed limit(s) of a given road with a numerical value which may or may not be the default speed limit. In other places, default speed limits that are relevant to road users may be indicated by a non-numeric sign, a lack of speed limit signs, the presence of streetlights, or the physical arrangement of the road. If a default limit applies everywhere within one country or state, it is known as a national or state speed limit. Different default speed limits usually apply to urban streets, rural highways, and freeway-like roads and these values may also vary according to the type of vehicle. A posted limit that is lower than a default speed limit is generally given precedence. A posted speed limit differing from the default speed limit is typically a linear speed limit and only applies to that road, and not necessarily any intersecting roads. Zonal speed limits apply on all roads beyond the sign that defines them.
The start of a different speed limit is usually marked numerically by posting a speed limit sign. Some places, go even further with a sign that says "speed zone ahead" while approaching a speed limit change. The signs are on both sides of the road, and 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 will often have the same orange flags as well. Speed changes often appear near borders and road intersections, and in some cases, especially the U.S., speed limit reminder signs appear at regular intervals. 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 1,600 ft) after the first sign. The same practice is followed in many US states.
Design of speed limit signage varies between countries. Many nations, some of which are not contracting parties, but especially those in much of Europe use signage which conforms to the standards set forth in the Vienna Convention on Road Signs and Signals. Thus a value in black text circumscribed in red on a white background is fairly common all around the world. In the U.S., the signs are usually rectangular with the words "SPEED LIMIT" (in Canada, "MAXIMUM") and the values in black on a white background, although the color scheme for speed limit signs can be polarized and with differing text for night and day. The U.S. Federal Highway Administration's Manual on Uniform Traffic Control Devices provides guidelines for the appearance of speed limit signs. Australian signs are rectangular but have a red circle like the conventional signs. Sometimes, speed limits are also painted on the road surface as a reminder.
The design of minimum speed signage also varies between countries. Most countries 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 U.S they are also identical to their respective maximum speed limit signs with the exception of the text "MINIMUM SPEED".
Some roads also have "minimum speed limits", where slow speeds can impede traffic flow or be dangerous. Such roads are typically motorways strictly limited to motorized traffic. In the UK such slowness would be charged as "Failure to make safe, reasonable progress".
Recently some jurisdictions have begun experimenting with variable speed limits which change with road congestion and other factors (this is distinct from France's reduction of limits during adverse weather). Typically such speed limits only change to decline during poor conditions, rather than being improved in good ones. One example is on Britain's M25 motorway, which circumnavigates London. On the most heavily-traveled 14-mile (23 km) section (junction 10 to 16) of the M25 variable speed limits combined with automated enforcement have been in force since 1995. Initial results indicated savings in journey times, smoother-flowing traffic, and a fall in the number of accidents, so the implementation was made permanent in 1997. Further trials on the M25 have been thus far proved inconclusive. From December 2008 the upgraded section of the M1 between the M25 and Luton will have the facility for variable speed limits.
In Germany, the first experiments with variable signs took place in 1965 on A8 Munich-Salzburg with signs that were operated manually. Beginning in the 1970s, more and more advanced Streckenbeeinflussungsanlagen (linear 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. By 2007, 1,200 km (750 mi) (10%) of German motorways will be equipped with such systems.
In 2006, Austria began experimenting with a 160 km/h (100 mph) speed limit on a selected test stretch of Autobahn as part of their program of variable speed limit, using the slogan "flexibility with responsibility."
New Zealand has had variable speed limits since 2001. The first installation was on the Ngauranga Gorge, a steep section of dual carriageway on State Highway 1 north of the capital, Wellington. The speed limit is normally 80 km/h, but with an average grade of 8% becomes increasingly more dangerous in wet conditions in the older front-wheel drive Japanese import cars that are common in the country. The downhill section is monitored by a fixed speed camera, which managed to issue 24,835 tickets in 2002, in a section of road with one fatality in the ten years prior to 2004.
In The Netherlands, much of the dense motorway network is equipped with variable speed regulation systems. The electronic signage is commonly posted every 500 m (1,640 ft). The system keeps track of all traffic movement and lowers the speed limit if it detects the start of traffic congestion. When activated the speed limit can be set at 90 km/h (56 mph), 70 km/h (43 mph), or 50 km/h (31 mph) according to the level of expected traffic congestion.
Variable speed limits are used on some stretches of highway in the United States. This has not, however, been implemented on a national basis. On Interstate 90 at Snoqualmie Pass, Washington, (near Seattle) variable speed limits are used to slow traffic in severe winter weather. This is also done on other mountain passes in Washington. Variable speed limit signs, in combination with variable message signs, have been in use since the 1960s on the New Jersey Turnpike, where officials can adjust the speed limit according to weather, traffic conditions, and construction. Other roadways with variable speed limits include the Pulaski Skyway in New Jersey, I-495 in Delaware, and the Missouri part of the I-270/I-255 loop around St. Louis.
In some jurisdictions, public roads have no speed limits:
Montana, United States has had a speed limit since June 1999 (see Montana speed limit). Australia's Northern Territory had no blanket speed limits outside major towns until January 2007, when rural speed limits were reduced to 110 km/h or 130 km/h.
Before the (now-defunct) 1974 national 55 mph (89 km/h) speed limit in the U.S., German autobahns had a higher fatality rate than U.S. Interstates; however, a few years later, the autobahn rate fell below that of (then) 55 mph limited U.S. Interstates. IRTAD records show the U.S. rate remains higher than that on the largely unrestricted German autobahn network. While the fatality rate on the UK's 70 mph (113 km/h) speed-limited motorways is about half of Germany's, the 100 km/h (62 mph) limit in rule-conscious Japan corresponds to a motorway fatality rate greater than Germany's. However, simple comparisons of fatality rates between countries neglect to account for differences in traffic density, quality of medical care, technical conditions of the vehicles involved, and Smeed's law.
After the opening of the East German borders in November 1989 the nature of car use there changed. Prior to German reunification in 1990, the available cars (such as the Trabant) were technically outdated and had small engines. Accidents were prevented by restrictive traffic regulation. Within two years after the opening, motorized traffic increased by 54% and annual traffic deaths doubled, 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 and a blood alcohol limit of 0.0‰". 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 ten years while traffic regulations were raised to western standards (e.g., 130 km/h (81 mph) freeway advisory limit, 100 km/h (62 mph) on other rural roads, and 0.5 milligrams BAC).
Occasionally, different units of speed measurement are used on each side of a border. For example, Northern Ireland (part of the United Kingdom) uses mph for speed limits and miles for distance, whereas the Republic of Ireland uses km/h for speed limits and km for distance. The United Kingdom and the United States are the only large nations still using miles and miles per hour.
The U.S. has shown no intention to convert to SI units, and reverted to miles in states that had both systems such as California and Arizona. However, Ohio, South Dakota, Maine, and Vermont (especially near the Canadian border) still have some SI distances and speeds on their exit distance and speed limit signs (such as "70 mph / 110 km/h", or "3 miles / 5 km to next exit"). When entering Canada, signs are posted reminding drivers that metric signage is in use. Conversely upon entering the U.S. from Canada, some drivers are shown a metric speed limit sign. Some exit distance signs on Interstates in New Hampshire are marked with the distance in miles followed by the distance in kilometres shown in parentheses. Houston, Texas has some signs in both SI and imperial units near its airports and downtown. Delaware Route 1 and Interstate 19 have exits numbered by kilometer: I-19 also has kilometer posts.
Prior to the invention of radar, speed limits were normally enforced by clocking vehicles traveling through speed traps. This is done by timing how long it takes for the automobile to pass between two fixed landmarks along a roadway, from which the vehicle's average speed can be determined. Setting up a speed trap that could provide legally satisfactory evidence was usually time consuming, however, and early speed traps were often difficult to hide. As a result, organizations such as The Automobile Association could often keep fairly accurate records of speed trap locations.
In the early 21st century, police used radar, laser rangefinders, aircraft, and automated devices. Officers also used a method called pacing: following a car for a certain time to establish speed using the calibrated speedometer of the patrol car.
In several countries, notably the Netherlands and the United Kingdom, an increase in automated speed enforcement has resulted in a significant increase in the number of fake, stolen, tampered with, or incorrectly registered number plates. In France, the use of automated enforcement has been credited with contributing to a substantial reduction in fatalities. Most Western European countries now use automated enforcement on at least some roads.
Speed limit policy can affect enforcement. According to the AASHTO, "experience has ... shown that speed limits set arbitrarily below the reasonable and prudent speed perceived by the public are difficult to enforce, produce noncompliance, encourage disrespect for the law, create unnecessary antagonism toward law enforcement officers, and divert traffic to lesser routes".
Governments also prosecute speeds that are unreasonable for conditions even if speeds are within any posted speed limits; for example, drivers are required to adjust their speed when driving in fog or heavy rain, and to not drive faster than they can see to stop in the line of sight. California Vehicle Code section 22350 is typical; it states that "No person shall drive a vehicle upon a highway at a speed greater than is reasonable... and in no event at a speed which endangers the safety of persons or property." This "basic rule", or similar legal language, applies even where no maximum speed limit is in place (such as formerly in the U.S. state of Montana and the majority of German motorways). Typically the burden of proof is upon the government to prove that a given speed is unreasonable for the then prevailing conditions.
Speed limit enforcement often begins at a small amount above the speed limit. For example, speeding citations for 1 unit (mph or km/h) above the limit are rare. In certain cases, such as Houston, Texas, only 1% of speeding citations are for less than 10 mph (16 km/h) above the speed limit.
In the United States, speeding enforcement tolerance is usually up to the discretion of the arresting officer. A small tolerance is almost always allowed, even where traffic signs advise "NO TOLERANCE". Some states (such as Pennsylvania) have official tolerances. Per state law, one cannot be cited by an officer using a radar/laser gun for traveling less than 10 mph (16 km/h) over a speed limit of less than 55 mph (89 km/h) or for traveling less than 6 mph (10 km/h) over a speed limit of 55 mph (89 km/h) or greater.
In Taiwan, even though the Regulations on Establishing Traffic Signs and Indicating Lines (zh:道路交通標誌標線號誌設置規則) define the speed limit signs to show absolute limits, police agencies have generally agreed a tolerance of up to 10 km/h. A notable exception was the Hsuehshan Tunnel opened on June 16, 2006 with automated speeding cameras. After the "zero tolerance" on speeding created controversy, effective 00:00 (UTC+8) on September 16, 2006, a tolerance of 10 km/h (6 mph) has been allowed as on other Taiwanese roads.
In Hong Kong and Poland, there is a tolerance of 10 km/h over the posted speed limit.
In the United Kingdom ACPO guidelines recommend a tolerance level of the speed limit "+10% +2 mph" (e.g., a maximum tolerance in a 30 mph (50 km/h) zone of 30 + (30 × 10% = 3) + 2 = 35 mph). However, each police force or safety camera partnership has the ability to use its discretion when setting the levels at which drivers will be prosecuted.
In the Netherlands drivers can get a fine for driving 4 km/h over the speed limit, after applying a 3 or 4 km/h correction factor to compensate for measuring errors. Police officers are usually not allowed to use their discretion when setting the speeding threshold during enforcement activities by photo radar.
Road safety improvements in the Australian state of Victoria are largely attributed by the state government to infrastructure improvements and speed management including tougher tolerances and enforcement. Low level speeding is targeted because of the overall population effects. This is best explained by the recent Auditor General's independent review which cites:
The relative risk of casualty crash involvement for vehicles travelling only a few km/h above the speed limit is lower than for those travelling a greater amount above the limit. However the contribution of “low level speeders” to the total number of casualty crashes is high because of the high number of motorists travelling at these speeds. Therefore, “low level speeding” represents a substantial risk across the road network.
Victoria has some of the tightest speeding tolerances in Australia, with 3 km/h if the speed is under 100 km/h, or 3% if over 100 km/h. This is despite the fact that the Australian Design Rules only stipulate that a car's speedometer must be accurate within a 10% tolerance.
In Germany, a 3 km/h tolerance (4 km/h when speeding over 100 km/h) in favor of the offender is always deducted. Fines for speeding depend on how high above the speed limit the measured speed is and where the offense occurred. Speeding in built-up areas invariably carries higher fines than outside city limits. While fines for minor offenses tend to be moderate, speeds in excess of 20 km/h (12 mph) above the limit in built-up areas and 30 km/h (19 mph) on other roads result in distinctly higher fines and points on the driver's license, and, depending on the speed at which the offender was clocked, may lead to a driving ban of at least one month.
In New Zealand, the speed limit is not enforced up to 10% over the posted limit, unless the speed was considered dangerous for the road. The tolerance is reduced to 0% within 250 m (820 ft) of schools.
In Romania, exceeding the speed limit by up to 9 km/h (5.6 mph) is not taken into consideration, however this means that the starting fine (at 10 km/h over the posted limit) is relatively high.
Methods for evading enforcement of speed limits have entered popular culture. Among the most familiar techniques is to purchase a radar detector to seek out police radar signals before one enters an enforcement zone. Observers have pointed out a small-scale arms race ensues, as speeders buy radar detectors of greater technology and police purchase equipment that is harder to detect. Such detectors are illegal in certain jurisdictions. Speeders can also alter their traffic behavior according to known police stakeout positions.
Traffic engineers observe that the majority of drivers drive in a safe and reasonable manner, as demonstrated by consistently favorable driving records. A report from the British Columbia Ministry of Transportation includes in its summary the finding that the incidence of crashes depends more on variations in speed between vehicles than on absolute speed, and that the likelihood of a crash happening is significantly higher if vehicles are traveling at speeds slower or faster than the mean speed of traffic.
Speed limits are most frequently set through statutes. Speed limits can usually be lowered, or sometimes raised, from the legislated speed limit through a process called speed zoning. Common factors influencing speed zoning are:
Fuel efficiency sometimes affects speed limit selection. The United States once had a National Maximum Speed Law of 55 mph (89 km/h) to reduce fuel consumption. The law was widely disregarded by motorists and hardly reduced consumption at all.
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.
In the United States the design speed is officially defined as "a selected speed used to determine the various geometric design features of the roadway", according to the 2001 American Association of State Highway and Transportation Officials highway design manual, commonly referred to as the "Green Book." Previous versions of the Green Book referred to design speed as the "maximum safe speed that can be maintained over a specific section of highway when conditions are so favorable that the design features of the highway govern"; however the 2001 edition removed the term "safe" in order to avoid the implication that speeds greater than the design speed were necessarily "unsafe."
Safe operating speeds can exceed the design speed. Example reasons include:
In commonly accepted engineering practice, design speed is considered a "first guess" at an appropriate speed limit.
Traffic engineers may rely on the 85th percentile rule to establish speed limits. The speed limit should be set to the speed that separates the bottom 85% of vehicle speeds from the top 15%. The 85th percentile is greater than a speed that is one standard deviation (SD) above the mean of a normal distribution.
The theory is that traffic laws that reflect the behavior of the majority of motorists may have better compliance than laws that arbitrarily criminalize the majority of motorists and encourage violations. The latter kinds of laws lack public support and often fail to bring about desirable changes in driving behavior. An example is United States's old 55 mph (88 km/h) speed limit that was removed in part because of notoriously low compliance.
Most U.S. jurisdictions report using the 85th percentile speed as the basis for their speed limits, so the 85th-percentile speed and speed limits should be closely matched. However, a review of available speed studies demonstrates that the posted speed limit is almost always set well below the 85th-percentile speed by as much as 8 to 12 mph (13 to 19 km/h). Some reasons for this include:
The 1998 report Synthesis of Safety Research Related to Speed and Speed Limits sponsored by the Federal Highway Administration 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. The report commented that on high-speed roads traffic speeds would change by about one-quarter of any speed limit change, and that international studies suggested that a 1 mile per hour (1.6 km/h) speed change would result in a 5% change in the number of injury accidents. The report noted that traffic calming significantly reduced speeds and injuries in treated areas but that the decrease may be due to reduced traffic volumes. The report also suggests that "variable speed limits that adjust with traffic and environmental conditions could provide potential benefits" as most of the speed related crashes involve speed too fast for conditions.
The report noted a landmark study that observed a "U-shaped curve" – subsequently referred to as the Solomon curve – of crash probability versus speed, where crash rates were lowest for travel speeds near the mean speed of traffic, and increased with greater deviations above and below the mean. Subsequent research has found that "The occurrence of a large number of crashes involving turning maneuver partly explains the increased risk for motorists traveling slower than average and confirms the importance of safety programs involving turn lanes, access control, grade separation, and other measures to reduce conflicts resulting from large differences in travel speeds."
A 1994 study by Jeremy Jackson and Roger Blackman showed, consistent with the risk homeostasis theory, that although increased speed limits and reduced speeding fines significantly increased driving speed, there was no effect on accident frequency, with the 24 participants maintaining the same level of risk and risky behaviour. It also showed that an increased accident cost caused large and significant reductions in accident frequency but no change in speed choice. The abstract states that the results suggest that regulation of specific risky behaviors such as speed choice may have little influence on accident rates.
Where speed limits are set too low for the road, this can encourage aggressive behavior by people attempting to drive at the speed appropriate for the road, passing those driving at the posted limit.
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 ca 55 percent of all speed-ing-related crashes in fatal crashes were due to “exceeding posted speed limits” and 45 percent were due to “driving too fast for conditions.”
Most "speed-related" crashes involve speed too high for conditions such as limited visibility or reduced road traction, rather than speed in excess of the posted speed limit. Most speed-related crashes occur on local and collector roads with relatively low speed limits. However, most speed-related traffic citations involve speeds in excess of posted maximum speed limits. Variable speed limits offer some potential to reduce speed-related crashes, but because of the high cost of implementation they exist primarily on motorways. Speed-related crashes can occur on high speed limit roads at low speeds, e.g., below 30 mph (50 km/h); for example, on exit ramps.
Currently, the highest posted speed limit in the world is 130 kilometres per hour (81 mph) on Polish motorways, although a variable speed limit up to 160 kilometres per hour (99 mph) was permitted experimentally on a stretch of Austrian motorway in June 2006.
In more recent times, other organizations, such as the Association of British Drivers, Safe Speed, the North American National Motorists Association, and German Auto Club ("ADAC"), seek to ban or discredit certain speed limits as well as other measures, such as automated camera enforcement. The debate over speed limit enforcement has become a large part of the road safety and environmental policy debate in some countries.
Organizations critical of speed limits and strict enforcement point to:
In the United Kingdom, the speed limit in towns is usually 30 miles per hour, and the speed limit on dual carriageways and motorways is usually 70 miles per hour. In France, the speed limit on motorways is usually 130 kilometres per hour, which is approximately 80 miles per hour. Also, in United States, most highways are from 65 to 70 miles per hour, and freeways are from 55 to 80 miles per hour. In Canada, the speed limit is usually about 100 kmh.Australia the speed limit on freeways is between 100 - 120 kilometres per hour and in towns and cities between 40 - 60 (kph)