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Speed limit
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A road speed limit is the maximum speed as allowed by law for road vehicles. Speed limits are commonly set and enforced by the legislative bodies of nations or provincial governments, such as countries within the world.
In addition to setting an explicit maximum speed limit, most governments also enforce speed limits that are related to driving conditions; for example, requiring drivers to adjust their speed when driving in fog or heavy rain. California Civil Code 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.
Discussion
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Timeline
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1865 First speed limit introduced in Britain - 2 mph in town and 4 mph in the country
1896 Walter Arnold, of Kent, England, is fined for speeding in excess of the contemporary speed limit of 2 mph
1995 U.S. President Bill Clinton signs a highway bill that ends the federal 55 mph speed limit.
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Encyclopedia
A road speed limit is the maximum speed as allowed by law for road vehicles. Speed limits are commonly set and enforced by the legislative bodies of nations or provincial governments, such as countries within the world.
In addition to setting an explicit maximum speed limit, most governments also enforce speed limits that are related to driving conditions; for example, requiring drivers to adjust their speed when driving in fog or heavy rain. California Civil Code 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). Some roads also have "minimum speed limits", where slow speeds are considered to impede traffic flow or be dangerous.
The first speed limit was the 10mph (16.1km/h) limit introduced by the Locomotive Act of 1861 (or "Red Flag Act") in the United Kingdom (automobiles were in those days termed “light locomotives”). In 1865, the revised Locomotive Act reduced the speed limit to in the country and in towns. The 1865 Act required a man with a red flag or lantern to walk ahead of each vehicle, enforce a walking pace, and warn horse riders and horse drawn traffic of the approach of a self-propelled machine. The replacement of the "Red Flag Act" by the Locomotive Act of 1896, and the increase of the speed limit to has been commemorated each year since 1927 by the London to Brighton Veteran Car Run.
Nepal, the Isle of Man and the Indian state of Uttar Pradesh are the only places in the world that do not have a general speed limit. In Germany, 57% of the autobahn system remains free from speed limits. The highest posted speed limit in the world is 160 km/h (99 mph), which was experimentally applied during 2007 on selected test stretches in Austria and the United Arab Emirates.
Factors in setting speed limitsTraffic 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 set based on many factors, such as road features, crash records, legal statutes, administrative judgment, engineering judgment, and political dictate. Two common measures for setting speed limits are the design speed of the road and the 85th percentile of travel speeds (See ).
Fuel efficiency also affects the choice of speed limits. The United States at one time had attempted a maximum speed limit of to reduce fuel consumption (See National Maximum Speed Law).
It is also estimated that speed limits can be used to reduce emissions and pollution, and some areas have reduced speed limits for improving the air quality (See Environmental Speed Limits).
Design speedIn 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:
- A design speed is not a representative speed of an entire roadway. Rather, the road's design speed is limited by its most restrictive feature, such as a curve, bottleneck, or hill.
- Actual roadway design may exceed the design specifications.
- Current parameters for determining the design speed assumes the capacity of outdated automotive technology.
- The stated design speed for a given road is usually not changed. Therefore, the design speed on older roads, which were calculated with older methodologies, may not factor in improved automotive technology which can maintain designed safety at higher travel speeds.
In commonly accepted engineering practice, design speed is considered a "first guess" at an appropriate speed limit.
85th percentile ruleTraffic 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 slightly greater than a speed that is one standard deviation 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 the federally-mandated 55 mph (90 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 (13 to 19 km/h). Some reasons for this include:
- Political or bureaucratic resistance to higher limits.
- Statutes that restrict jurisdictions from posting limits higher than an arbitrary number.
Signage
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 many places authorities always sign the maximum speed limits of their roads with a numerical value. Elsewhere, 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 street lights, or the physical arrangement of the road. If a default limit applies everywhere within one country it is known as a national speed limit. Different default speed limits usually apply to urban streets, rural highways and motorways. A signed limit overrides a default limit.
The start of a different speed limit is usually marked numerically with a speed limit sign. Speed limit signs can appear near borders and road intersections, and in some cases 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 metres (about 650 to 1,650 ft) after the first sign. The same practice is followed in several U.S. states.
Occasionally, different units of speed measurement are used on each side of a border. For example, Northern Ireland (part of the UK) uses miles per hour (mph) for speed limits and miles for distance, whereas the Republic of Ireland uses kilometres per hour (km/h) for speed limits and kilometres for distance. The UK and the United States are the only major nations still using the customary (imperial) units system.
The U.S. has shown no intention to convert to SI units, and reverted to imperial units in states that had both imperial and SI 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 / 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 US from Canada, some drivers are shown a metric speed limit sign. All 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.
Design of speed limit signage varies between countries. In much of Europe the red circle is most common, while in North America, and in Australia, signs are usually rectangular. 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 obligatory signs. A Japanese minimum speed sign has the same design as a maximum speed sign but with a horizontal line below the number.
Speed limits in specific countriesEnforcementPrior to the invention of radar, speed limits were normally enforced by clocking vehicles travelling through speed traps. Clocking a vehicle simply means timing how long it takes for the automobile to pass between two fixed landmarks along a roadway, from which the vehicle's average speed could easily 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 may also use 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[.]"
ToleranceSpeed 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 have official tolerances, such as Pennsylvania. As per state law, one cannot be cited by an officer using a radar/laser gun for traveling less than over a speed limit of less than or for traveling less than over a speed limit of or greater.
In Taiwan, even though the Regulations on Establishing Traffic Signs and Indicating Lines define the speed limit signs to show absolute limits, the 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 on September 16, 2006, a tolerance of 10 km/h has been allowed as on other Taiwanese roads.
In Hong Kong, 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 tolerance level in a zone of 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 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, travelling at any speed above the posted speed limit constitutes a speeding offense. A 3 km/h tolerance (4km/h when speeding over 100km/h) in favor of the offender, however, is always deducted. Fines for speeding depend on how high above the speed limit the measured speed is, and on 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 above the limit in built-up areas and 30 km/h on other roads not only result in distinctly higher fines, but also points on the driver's license, and, again depending on the speed at which the offender was clocked, may lead to a driving ban of at least one month.
Evading enforcementMethods 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 stake-out positions.
Database of speed limitsDatabases are available containing the GPS location of speed limit signs. One is the Open-Speedlimit-Database. This information can be used to build advanced cruise controls, eventually leading to the driverless car.
Safety and efficacyEssential physicsThe 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 at impact, rising much faster than kinetic energy.
To illustrate these statistics, suppose two vehicles crash into a massive, fixed object, and one vehicle’s speed is 10% greater than the other vehicle. The faster vehicle will release 21% more energy, and its occupants will experience a 46% higher probability of a fatality.
It should be noted that crashes with dramatic, sudden speed changes that terminate almost all velocity are atypical. These kinds of crashes include head on collisions or collisions with massive, fixed objects like trees or concrete bridge piers.
Although the basic relationship between vehicle speed and crash severity is unequivocal and based on the laws of physics, the probability of a crash as well as crash severity can be mitigated. Safety devices like crash attenuators, barriers, or wide medians are examples. The highest degree of mitigation is found on motorways (which may be called freeways, limited access highways, also Autobahns, Interstates or other national names), which are internationally documented as being the safest roads per mile travelled despite their higher speeds, due to designing out of most conflict opportunities as well as restricted access.
Speed limits, actual speeds, and aggregate safetyThe 1998 Synthesis of Safety Research Related to Speed and Speed Management sponsored by the US Federal Highway administration found, "on freeways and other high-speed roads, speed limit increases generally lead to higher speeds and crashes." Increasing a speed limit by 4 mph (6 km/h) would increase the average speed by 1 mph (1.6 km/h) and increase injury accidents by 5%. The report cautions that "changing speed limits on low and moderate speed roads appears to have little or no effect on speed and thus little or no effect on crashes." 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 the landmark study (D. Solomon, "Accidents on Main Rural Highways Related to Speed, Driver, and Vehicle", Federal Highway Administration, Washington, DC, July 1964) that observed a "U-shaped 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.
Speed and crash factorsSome safety factors are not always under the full control of the driver, such as driver alertness and distractions, road conditions, weather, daylight availability, actions and alertness of other drivers, and wildlife. While these factors are not directly related to vehicle speed, the effects of these factors can be more severe with more speed. For example, a deer running across the road has no consequences to a parked vehicle but could have disastrous consequences for a vehicle traveling at 100 mph (160 km/h). This suggests that lower speeds can reduce the frequency and severity of crashes; lower speeds can give the driver more time to respond appropriately in the face of unexpected dangers, and it can reduce the severity of a crash should one happen. However, since the efficacy of speed limits in restraining driver speed is subject to debate, it is not clear how well speed limits can ameliorate these other factors.
Another view is that, while speed can play a part of the causal chain which leads to crashes, speed's role is mostly to magnify the consequences of other unsafe acts. This viewpoint is reinforced by the fact that speed is rarely the sole crash factor. In many cases, removing the other crash factors, such as a right of way violation, would have absolutely prevented the collision. While reducing the speed could have a beneficial effect on the severity and probability of the crash, it usually cannot guarantee crash prevention.
Most 'speed-related' crashes involve speed such as limited visibility or reduced road traction, rather than in excess of the posted speed limit. Most speed-related crashes occur on with relatively low speed limits. However, most speed-related traffic citations involve speeds in . Variable speed limits (q.v.) offer some potential to reduce speed-related crashes, but due to the high cost of implementation exist primarily on motorways. Speed-related crashes can occur on high speed limit roads at low speeds, e.g. below ; for example, .
Variable speed limitsRecently 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). One example is on Britain's M25 motorway, which circumnavigates London. On the most heavily-traveled 22 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.
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, 1200 km (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 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 SH1 north of the capital, Wellington. The speed limit is normally 80 km/h. The downhill section is monitored by a fixed speed camera.
In The Netherlands, much of the dense motorway network is equipped with variable speed regulation systems. The electronic signage is commonly posted every 500 metres. 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, 70, or 50 km/h according to the level of expected traffic congestion.
Variable speed limits are used on some stretches of highway in the United States. For example on I-270 in St Louis MO. 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 and I-495 in Delaware.
OppositionSpeed limits and their enforcement have been opposed by some motorists since their inception. Britain's first motoring organisation, the AA, was formed to warn members about speed traps. Other organizations, such as the Association of British Drivers, Safe Speed, the North American National Motorists Association, and German Auto Club ("ADAC"), have sought to 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.
Critics of speed limits and strict enforcement outside built-up areas point to:
- Inconclusive results from most speed limit studies. For example, a 1972 OECD Road Research Group report entitled 'Speed Limits Outside Built-Up Areas' reviewed most international studies to that date. They concluded that "because of the weaknesses in the research designs of many investigations, scientifically well-established conclusions cannot be drawn." "Indeed, some of the speed limit changes were more in the nature of administrative exercises than scientifically designed experiments and the methods of analysis in these cases were deficient from the statistical point of view." The Group stated that "speed limit policies should be based on reliable research work and generally accepted scientific evidence." They proposed an international co-operative experiment to overcome weaknesses in prior studies. However, the 1973-1974 oil price crisis intervened, and widespread blanket speed limits became more common without exacting study. More recently, a review of the effect of speed on vehicle crash rates noted that the studies and evidence are
- Misleading definition of to combine the concepts of:
- crashes that occur often at relatively low speeds, but excessive for adverse conditions, such as low visibility
- citations that are issued for travel in excess of the posted speed limit
In Australia for instance, Government & Police attribute speed as the main cause in 30% of crashes, even though speeding is a cause in only 20% of those cases (or 6% of total cases). This is due to an extremely wide-ranging definition of speed, in order to explain fixed speed cameras and zero-discretion traffic policing.
- Surprisingly broad range for 'speed-related' fatalities as a percentage of total traffic fatalties, suggesting that categorizing accidents as 'speed-related' is highly subjective. Among the U.S. States, the range is .
- "Evidence that suggests the net effect of [higher motorway] speed limits may be positive on a system wide basis [by shifting more traffic to these safer roads]." This statement from 1998 U.S. Federal is based on the published, peer-reviewed work of Charles A. Lave et al., e.g.
- Motorists generally pick reasonable speeds for conditions, even on motorways. For example, the 75 mph (120 km/h) speed limit in the U.S. State of South Dakota has good compliance: the average speed is less than or equal to the posted limit almost a decade after it was increased.
- "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"
Prior to the (now defunct) 1974 national 55 mph (88 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 (88 km/h) 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 (112 km/h) speed-limited motorways is about half of Germany's, the 62 mph (100 km/h) 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, and Smeed's law.
Roads without speed limitsIn some jurisdictions some public roads have no speed limits:
- The German intercity Autobahn, two-thirds of which have only advisory limits.
- The Isle of Man has no speed limit on many rural roads. A 2004 proposal for 70 and 60 mph (112 and 96 km/h) speed limits was very unpopular., although measured travel speeds are often low.
- Nepal has average traffic speeds of 40-50 km/h and has no statutory speed limits set on most of its roads.
- The state of Uttar Pradesh in India has no default speed limit for cars. (see India Speed Limit)
Montana 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 .
See also
External links- Governments
- A comprehensive UK report into the effects of speeding.
- A major research report into the psychology of the speeding driver.
- Motorists' and other groups
- A U.S. organization arguing for 85th percentile limits.
- A UK road safety organisation which campaigns for lower speed limits.
- An Austrian project that aim at more flexible speed limits and also higher speed limits for a better traffic control and safer driving.
- Other links
- pigradar.com
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- The United States' Transportation Research Board (TRB) National Cooperative Highway Research Program (NCHRP): 2003.
- C. Lave and P. Elias, Accident Analysis and Prevention, Vol. 26, No. 1, 1994.
- , United States Publication No. FHWA-RD-97-084, January 1997.
- , Final Report 551, Arizona Dept of Transportation, October 2004.
- , Transportation Research Board, 1998.
- Why we can't drive safely at high speed
- Technology is being developed to assist (or in some cases, force) drivers keep to the speed limits.
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