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For a discussion of the maximum speed possible in the universe, see speed of light and special relativity.

A speed limit is the maximum speed allowed by law for road vehicles. The highest speed limit is 160 km/h (100 mph), experimentally posted on selected test stretches in Austria and the United Arab Emirates*. Very few public roads have no speed limit. The first speed limit was introduced on 5 July 1865 in England.

Some roads also have minimum speed limits, where slow speeds are considered to impede traffic or be dangerous.

Factors in Setting Speed Limits

Speed limits are set based on many factors, such as road features, crash records, legal statutes, administrative judgement, and engineering judgement. Two common measures for setting speed limits are the design speed of the road and the 85th percentile of travel speeds (See Design Speed, Operating Speed, and Posted Speed Practices).

Design speed

Definition
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 AASHTO 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 misperception that speeds greater than the design speed were "unsafe".

Limitations
Safe operating speeds can exceed the design speed. Example reasons include:
  1. 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.
  2. Actual roadway design may exceed the design specifications.
  3. Current parameters for determining the design speed assumes the capacity of outdated automotive technology.
  4. 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 rule

In the United States, traffic engineers may rely on the 85th percentile rulehttp://www.ite.org/standards/speed_zoning.pdf 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 somewhat less than two standard deviations 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 scrapped in part because of notoriously low compliance.

Traffic engineers observe that the majority of drivers drive in a safe and reasonable manner, as demonstrated by consistently favorable driving records. Studies have shown crash rates are lowest at around the 85th percentile. Vehicles traveling over the 85th percentile speed (or faster than the flow of traffic) have a significantly higher crash risk than vehicles traveling around or modestly below this speed.

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 (see p.88) (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

For more information about traffic signs in general, see Traffic sign. The start of a speed limit is usually marked with a speed limit traffic 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 speed limits of the respective country are usually erected right after border crossings, with a repeater sign placed some 200 to 500 metres 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 imperial units system. The US has shown no intention to convert to SI units, and, in fact, reverted to its current imperial units in states that had both imperial and SI systems such as California and Arizona. However, Ohio still has some SI distances and speeds on its exit distance and speed limit signs (such as 70 mph / 110 km/h, or 3 miles / 5 km to next exit). Houston, Texas has some signs in both imperial and SI units near its airports and downtown.

Design of speed limit signage varies between countries. In the European Union, the red circle is most common, while in North America, signs are usually rectangular. Australian speed limit signs are a combination, with a red circle inside a rectangular sign. 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 of a maximum speed sign plus a horizontal line below the number.

Image:Zeichen 393.png|Informational sign at German border crossings Image:Zeichen 274.1.png|Zonal restriction, common in residential areas Image:Zeichen 274.2.png|End of restricted zone Image:Speed limit 50 sign.svg|United States speed limit sign in miles per hour

Speed limits in specific countries

Europe

The first British motorways did not have speed limits. However, after a series of multiple crashes on motorways mainly in fog, an experimental speed limit of 70 miles per hour (112 km/h) was introduced in December 1965, and made permanent in 1967. It was reduced to 50 mph (80 km/h) in response to the 1973 oil crisis and restored to 70 mph (112 km/h) in 1974. The Association of British Drivers has called for the limit to be increased. The opposition Conservative Party is proposing to raise the limit to 80 mph (130 km/h) where appropriate.

On French autoroutes there is a variable speed limit: in dry weather, 130 km/h (80 mph); when raining, 110 km/h (70 mph). In 2005, a governmental report advised lowering the higher speed to 115 km/h (70 mph) in order to save fuel and reduce accident risks, but this proposal was badly received. Since 2002, the French government has installed a number of automatic radar guns on autoroutes, routes nationales, and other major thoroughfares. These are in addition to radar manned by the French National Police or Gendarmerie. The French authorities have credited this increase in traffic enforcement with a 21% drop in road fatalities from 2002 to 2003.

The German autobahns are famous for having no speed limits for cars over much of their rural sections, although limits as low as 80 km/h (50 mph) can apply within densely trafficked or built-up areas. Blanket speed limits apply for trucks, buses, and cars pulling trailers. Speeds over 200 km/h (125 mph) are not uncommon, but there is a recommended speed (Richtgeschwindigkeit) of 130 km/h. In case of an accident, the insurance company may refuse to settle part of the bill if the driver is found to have driven at a speed considered unsafe for the road conditions. Many car manufacturers (including Mercedes, BMW, and Audi) electronically limit the speed of their cars to 250 km/h (155 mph) in a Gentlemen's Agreement, although this is not a legal requirement. Porsche vehicles, for instance, are known for not having this restriction, and many aftermarket car tuners offer to remove the limiter. While the idea of not having a speed limit on long-distance roads may seem appealing, in recent years more and more federal states in Germany have begun imposing speed limits. Rhineland-Palatinate has imposed a permanent speed limit of 130 km/h on a dangerous stretch of autobahn, and experimented with a variable speed limit on that stretch. Although the concept of a nationwide law restricting Autobahn speeds to 120 or 130 km/h has been on the agenda of various political and environmentalist groups for decades, there are no definite plans for this at present.

Italian Autostradas have a 130 km/h speed limit (80 mph), with 110 km/h (70 mph) limits on curvy roads and in rainy conditions and 150 km/h (95 mph) limits on newer and straighter roads.

Swiss Autobahnen are limited to 120 km/h (75 mph). Semi-motorways, known as "motor roads" or Autostrassen, generally have a limit of 100 km/h (60 mph).

From about 1990 to 1995, Sweden lowered the limit in the large city provinces from 110 km/h (70 mph) to 90 km/h (55 mph), the lowest in Europe at the time, citing environmental reasons. The term "large city province" was defined as a province including one of the three large cities with suburbs. That meant that the west coast motorway E6 had a 90 km/h (55 mph) limit on its (then) about 250 km of motorway, but some ordinary roads in less densely populated provinces had a 110 km/h (70 mph) limit. This reduced limit was later removed because it was neither popular nor well obeyed.

Since May 2002 the Netherlands has been experimenting with 80 km/h (50 mph) zones on motorways crossing suburban areas. The first zone to be implemented was on the A13, connecting Rotterdam to the Hague, at the Rotterdam suburb of Overschie. This was generally accepted as a success, so in 2005 the experiment was expanded, with four new zones in Rotterdam, the Hague, Utrecht and Amsterdam. DCMR report (Dutch). The new zones have had mixed results, causing great controversy and calls for the removal of them.

In Europe, "traffic calming" is gradually becoming a regular part of urban traffic management, after a long evolution of opinions and attitudes towards car use and vulnerable road users. Regulations for "30 km/h zones" have been enacted from about 1980 and have been widely applied. New urban policies have been defined with a view to encouraging a switch from car use to public transport and non-motorised modes (cycling, walking), with the additional condition of lower speeds to improve safety of vulnerable road users, for example national policies such as "sustainable safety" in the Netherlands or "vision zero" in Sweden.

The following table shows the respective speed limits (excepting the local 30 km/h or lower limits in many countries) in km/h (mph for the United Kingdom and Northern Ireland in brackets):


Zeichen_310.png  Sinnbild_Kraftrad.pngSinnbild LKW.svg/Zeichen_331.png/Zeichen_331.png || 50 || 90 || 130 || 90 || 130 || 50 || 80 || 100 || ||
Country
-
- 		Austria 50 100 130 /160 14 100 4 100 5
- 		Belgium 50 90 120 90 120
-
-
- 		Croatia 50 80 /100 130 80 80
- 		Cyprus 50 80 100 80 100
- 		Czech Republic 50 90 /130 130 80 80
- 		Denmark 50 80 130 80 80
- 		Estonia 50 90 110 90 90
- 		Finland 50 80 /100 120 6 80 /100 120
- 		France 50 90 /110 130 90 /110 130
- 		Germany 50 100 /∞1 ∞1 80 80 /100 7
- 		50 90 120 80 80
- 		50 70 90
- 		Hungary 50 90 /110 130 70 80
- 		Iceland12 50 90 80 80 80
- 		Isle of Man ∞ ∞
- 		Ireland8 50 80 /10011 120 80 /100 80
- 		Italy 50 90 /130 ² 130 /150 ³ 70 80
- 		Liechtenstein 50 80 80
- 		Malta 50 80 60
- 		Netherlands 50 80 /100 120 80 80
- 		Norway 50 80 90 /100 9 80 80
- 		Poland 50 90 13 130 70 80
- 		Portugal 50 90 /100 120 70 /80 100
- 		Romania 50 90 / 100 130 80 100
- 		Russia 60 90 / 110 110 70 / 90 90
- 		Serbia 60 80 / 100 120
- 		Slovakia 60 90 130 80 80
- 		Slovenia 50 90 /100 130 80 80
- 		South Africa 60 60 120 100 100
- 		Spain 50 90 /100 120 70 /80 80
- 		Sweden 50 70 /90 110 80 80
- 		Switzerland 50 80 /100 120 80 80
- 		Turkey 50 90/130 130 70 70
- 		United Kingdom10 48 (30) 96 (60) /112 (70) 112 (70) 80 (50)/96 (60) 96 (60)
*Motor routes: roads with two or more lanes (dual carriageway), a median, and a minimum speed of 60 km/h.

Remarks:
1 130 km/h is the recommended maximum speed on motorways, as indicated by a blue sign. Many sections of the German motorway network are now covered by speed limits, usually ranging from 80 to 130 km/h depending on local conditions (i.e., frequent traffic, terrain, etc.). It is usual for drivers involved in crashes who were exceeding the 'recommended' speed limit to be held to be at least partly at fault, regardless of the circumstances of the crash and insurance companies have the right to withhold payment.
² 110 km/h for motorcycles.
³ Two-lane expressways: 130 km/h; three-lane expressway: 150 km/h (since 2003, the speed limit of 150 km/h is only valid when signed).
4 Cars with heavy trailer: 80 km/h; lorries with heavy trailer: 70 km/h.
5 Cars with heavy trailer: 100 km/h; lorries with heavy trailer: 80 km/h.
6 During winter, when conditions are often bad, all Finnish motorways have a speed limit of 100 km/h or less.
7 Additional trailer checkup (TÜV) and special speed plaque required on vehicle.
8 Effective January 20, 2005
9 A provisional increase of the speed limit on motorways from 90 to 100 km/h was made permanent when the number of accidents decreased.
10 Signs are posted in miles per hour, a situation unlikely to change in the near future.
11 100 km/h is default limit on all National Routes regardless of design standard when local limits do not apply.
12 Iceland does not have expressways/motorways in the traditional sense. There is really only one such road, with three and four lanes and no traffic lights. It is within city limits, and the maximum speed is 80 km/h.
13 Between 90 and 110 km/h depending on how many lanes the road has.
14A provisional increase on a 12 km stretch.

North America

United States

An American sign reads "SPEED LIMIT XX", such as "SPEED LIMIT 50" for 50 mph (80 km/h). A minimum speed sign reads "MINIMUM SPEED XX", such as "MINIMUM SPEED 45" for 45 mph (70 km/h). Speed limits on United States roads are usually:

  • 25–30 mph (40–50 km/h) on residential streets
  • 35–45 mph (55–70 km/h) on urban arterial roads
  • 50–65 mph (80–105 km/h) on major highways inside cities
  • 45–65 mph (70–105 km/h) on rural two-lane roads
  • 55–70 mph (90–110 km/h) on rural expressways
  • 65–80 mph (105–130 km/h) on rural Interstate highways

The more urbanized east generally has lower speed limits than the more spread-out west.

Canada
Since 1977, Canadian speed limits have been in kilometres per hour - they were in miles per hour before. A Canadian sign reads "MAXIMUM XX", such as "MAXIMUM 80" for 80 km/h. A minimum speed sign reads "XX MINIMUM", such as "60 MINIMUM" for 60 km/h. Typical speed limits are:
  • 30–50 km/h (20–30 mph) within school and playground zones
  • 40–50 km/h (25–30 mph) on residential streets within cities and towns
  • 60–70 km/h (35–45 mph) on major arterial roads in urban and suburban areas
  • 80–90 km/h (50–55 mph) on highways outside of cities and towns and urban expressways
  • 90–120 km/h (55–75 mph) on freeways and rural expressways

Note that where more than one limit is given per road, it usually indicates a difference between provinces; however, within provinces, different roads of the same classification have different speed limits. For example, in Alberta and Nova Scotia some freeways have a limit of 100 km/h, while others have a speed limit of 120 km/h (70 mph). In Ontario, all freeways have a maximum speed limit of 100 km/h unless there is a lower posted limit. Speed limits are generally lower in Ontario and Quebec on comparable roads than in other Canadian provinces, except perhaps British Columbia. Examples of this disparity include rural two-lane highways in Ontario which have a standard speed limit of 80 km/h, while comparable roads in other provinces have standard speed limits of 90–100 km/h.

In British Columbia, a review of speed limits conducted in 2002 and 2003 for the Ministry of Transportation found that posted limits on investigated roads were unrealistically low for 1309 km and unrealistically high for 208 km. The reports recommended to increase speed limits for multi-lane limited-access highways constructed to high design standards from 110 km/h to 120 km/h. MoT Speed Review Report As described in that report, the Ministry is currently using "...Technical Circular T-10/00 * to assess speed limits. The practice considers the 85th percentile speed (the speed at or below which 85% of the motorists are traveling), road geometry, roadside development, and crash history."

In Canada, as in most other locales, speed violation fines are double (or more) in construction zones.

Asia

China
Previously, all expressways in the People's Republic of China were limited to 110 km/h. With the passage of the PRC's first road-related law, the Road Traffic Safety Law of the People's Republic of China, the speed limit was raised to 120 km/h from May 1, 2004; however, the updating of signs will still take some time.

Semi-expressways and city express routes (called kuàisù gōnglù () in Chinese, meaning "high speed public road") generally have lower speed limits of 100 km/h: in some cases the speed limit may be lower.

On China National Highways (which are not expressways), a common speed limit is 80 km/h. In localities, speed limits may drop to 40 km/h.

In reality, few people drive according to the speed limits, and on most roads, enforcement cameras are non-existent. Where an enforcement camera does exist, it is marked "speeding detection camera" ().

On some designated "fast through routes" in cities, speed limits are up to 80 km/h. Otherwise, speed limits are 70 km/h on roads with two uninterrupted yellow lines and 60 km/h or 50 km/h otherwise. Signage in towns and on expressways is often present.

Minimum speed limits on expressways vary. A general minimum speed limit of 60 km/h is in force at all times (although traffic jams more than thwart it). The maximum speed limit, as posted on Chinese motorways is 120 km/h. This is a recent change.

India
India has a speed limit in towns and this is usually signed. It is a contracting party to the "United Nations Conventions on Road Traffic, Road Signs and Signals". Road condition is historically poor, discouraging high speeds, but in Andhra Pradesh and Tamil Nadu in particular, the roads have been substantially improved in recent years due a buoyant economy.

), known by catalogue number in the UN Convention as a C,17a 'End of all local prohibitions imposed on moving vehicles'.

Japan
The general limit is 60 km/h except for divided national highways where the limit is 100 km/h. Urban areas are usually zoned at 40 km/h. Limits in Japan are different from most countries by:
  • having no separate urban limit, with urban limits being set by zoning rather than statute.
  • emergency vehicles are not exempt but have a higher speed limit
  • there are many lower limits set for vehicle classes other than ordinary cars and motorcycles.

Malaysia
The speed limit in Malaysia is 110 km/h on closed toll expressways. Speed limit on federal, state and municipal roads is between 50 km/h and 90km/h depending on geographical factors along the road. The default speed limit is 90 km/h and it is reduced to 60 km/h in urban areas. The highway police monitor the speed of passing cars from beneath a bridge or signboard.

Taiwan

The Act Governing the Punishment of Violation of Road traffic Regulations (zh:道路交通管理處罰條例) is the basic law. The Road Traffic Security Rules (zh:道路交通安全規則) are the basic administrative regulations. When no other limits are posted, the default speed limits are:

  • 15 km/h when approaching a railroad crossing
  • 40 km/h on roads without lane markings or on slow lanes () separated by single solid white lines from nearby fast lanes ()
  • 50 km/h on other roads and lanes

Speed limits on freeways are posted by signs, generally 100 km/h. Limited segments are posted at 90, 80, or 70 km/h. Most segments of the National Highway No. 3 are now posted at 110 km/h, the highest speed limit in Taiwan. A truck with a gross weight of 20 tonnes or more is limited to 90 km/h. Except on approaches to toll stations and work areas, minimum speeds are usually posted at 60 km/h.

Oceania

Australia
Speed limits in Australia range from 10 km/h Shared Zones to 110 km/h. Speed limit signage is in km/h since metrication on 1 July 1974. All speed limits (with the sole exception of the South Australian school zone) are multiples of 10 km/h - the last digit in all speed signs is zero. Advisory speed signs for curves or other road obstacles all end in the digit five.

Australian states and territories use two "default" speed limits. These apply automatically in the absence of 'posted' speed restriction signage. The two default speed limits are:

  • within built-up areas, 50 km/h.
  • outside built-up areas, 100 km/h. Two exceptions are Western Australia at 110 km/h, and the Northern Territory (NT), which does not have a rural default speed limit, here 'speed derestriction' applies. (See below).

Common speed zones are:

  • Shared zones (signposted areas where pedestrians and motorised traffic share the same space) are 10 km/h
  • School zones are 40 km/h during posted school times, except in South Australia, where they are 25 km/h when children are present.
  • 40 km/h zones. A number of councils have implemented lower speed limits, typically 40 km/h, in certain areas, such as shopping precincts, or even in whole suburbs such as Balmain and Rozelle in Sydney.
  • Many sub-arterial roads have a 60 km/h limit
  • Major connector roads and smaller highways are zoned 70 km/h, 80 km/h or 90 km/h.
  • Highways and freeways are 100 or 110 km/h unless otherwise signposted.

). This is an international road traffic sign held in all "United Nations Conventions on Road Traffic, Road Signs and Signals", where it is catalogued as a C,17a - meaning "End of all local prohibitions imposed on moving vehicles". That said, certain license holders, such as learner drivers are restricted in speed by 'license conditions'. Heavy vehicles are also speed restricted by way of separate vehicle construction (speed limiters) and other legal regulations. NT police do NOT tolerate dangerous speeds or behaviour when in Zeichen_282.png areas. The speed derestriction sign has the same UN meaning in Australian Standard 1742.4 of 1999. This is still current.

Also used in growing numbers to signal the end of a posted speed restriction - leading to a rural area default speed limit - is the ‘END’ speed-limit sign. This is a unique Australian-designed sign which contains the word "END" and a number in a circle beneath this, which represents the ceasing speed limit. It is typically used where the road beyond has certain hazards such as hidden driveways, poor camber, soft edges and other hazards where the road authority feels a posted speed limit sign might be too dangerous or otherwise unwarranted. It is intended to invoke particular caution. Again, the rural default applies as a maximum.

Speed traps are used in almost all areas of the country including NT above. Tolerance is from 8% to 10% in most states but only 3 km/h in Victoria, an issue which has caused a lot of controversy in that state. Measures used are police radar, fixed speed cameras, unmarked stationary cameras, fixed 'point to point' cameras and laser.

=Historical limits
= Prior to the 1930s the urban default speed limit was 30 mph (48 km/h). This was increased to 35 mph (56 km/h) over the next 30 years by each of the states and territories, with NSW the last to change, in May 1964. The rural default speed limit speed limit (where it existed) was 65 mph (105 km/h). With metrication on 1 July 1974 all limits and advisory signs were converted to the nearest 10 km/h, so the urban limit became 60 km/h and the rural limit 110 km/h, a slight increase for both. These limits were altered to their current values after the introduction of uniform national road rules across the states in 1999.

In most states the introduction of an urban default limit of 50 km/h was watered down by the 60 km/h urban speed limit being retained on most roads already signed as 60 km/h. There is also a 70 km/h limit on some better arterial roads. Some non-signed roads were made 60 km/h. A few 60 km/h urban arterials or sub-arterials have since been limited to 50 km/h, after representations by local councils. Some councils have also sought and established lower speed limits, typically 40 km/h, to apply throughout certain areas. An example is the Balmain/Rozelle peninsula in Sydney and parts of the Sydney CBD.

New Zealand
Speed limits in New Zealand range from 20 km/h to 100 km/h. Specifically:

  • 100 km/h, sometimes called the "open road limit". Used in rural areas, motorways, expressways and highways and places where there is little development on the roadside
  • 80 km/h on urban arterial routes passing through rural areas if there is cause for it to not be faster
  • 70 km/h in small country towns, urban fringes (often preceding a change from the open limit to urban limit), or where development is only on one side of the road
  • 60 km/h for many urban arterial routes that meet specific design requirements
  • 50 km/h in most urban or built-up areas; LSZ (see next paragraph) under adverse conditions
  • 40 km/h variable speed zone past a school
  • 30 km/h past some roadworks
  • 20 km/h traffic travelling both ways past school buses that have stopped to unload or pick up passengers; accident sites

Signage tends to follow the European model of a number inside a red circle. Sometimes "open road limit" occur as a black bend sinister inside a thin black ring. The letters LSZ (Limited Speed Zone) indicate that the limit is 100 km/h unless conditions (visibility, road condition, rain, many other road users) would make this unwise, in which case it is 50 km/h.

A local bylaw in Auckland produced the curious speed limit of 16 km/h in Waikumete Cemetery. When New Zealand converted from Imperial to metric measure the statues concerning speed in this location were not updated even though the signage legally had to be altered from Imperial units. This limit existed into the early 21st century. Typically speeds in such areas were 10, 15, or 20 km/h after adoption of the metric system.

There is no minimum speed limit but vehicles travelling less than the maximum and which are followed by other vehicles must keep to the side of the road and pull over to allow others to pass as soon as is safe.

Some vehicles are restricted to lower speeds:

  • 90 km/h for trucks and vehicles with trailers
  • 80 km/h for school buses
  • 70 km/h for motorcyclists with learner licences
  • heavy vehicles
    • with suspension
      • 20 km/h if it has solid rubber tyres
      • 15 km/h if it has metal tyres
    • with no suspension
      • 45 km/h if it has pneumatic tyres
      • 10 km/h if it has solid rubber tyres
      • 7.5 km/h if it has metal tyres

There are also has variable speed limits in some areas, such as school zones.

Africa

South Africa
The general speed limits in terms of the South African National Road Traffic Act, 1989 and its Regulations are:

  • 60 km/h on a public road within an urban area
  • 100 km/h on public road outside an urban area which is not a freeway; and
  • 120 km/h on every freeway.

Enforcement

Prior 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, LIDAR, planes, 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. Recently, Automatic Number Plate Recognition (ANPR) camera systems have been used which time a vehicle between long sections of road (approximately one mile), calculating the average speed between two points. This method eliminates the risk of heavy braking at the locations of conventional speed cameras, but may raise privacy issues.

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 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*"* Arbitrarily low limits can turn otherwise reasonable drivers into habitual speed limit violators.

Enforcement tolerance

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 exceedingly 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 (Houston Chronicle, "It's really true: Drivers going less than 10 mph over limit rarely ticketed", Nov. 24, 2002).

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."

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, the police agencies have generally agreed a tolerance of up to 10 km/h. A notable exception is the newly opened Hsuehshan Tunnel with automated speeding camera and zero tolerance that has created controversiesPChome Online: Speeding for 1 km/h fined 3000 TWD, the people heavily scold the bandit government (in Chinese).

Safety and efficacy

Essential physics

The kinetic energy involved in a motor vehicle collision is proportional to the square of the speed change (sometimes referred to as "delta-V", symbolized as δv) at impact. The probability of a fatality is empirically correlated to the fourth power of the speed change 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.

When interpreting this, 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 safety

The 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."Synthesis of Safety Research Related to Speed and Speed Management 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."

Speed and crash factors

Some 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. However, 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 too fast for conditions such as limited visibility or reduced road traction, rather than 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 (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 30 mph; for example, truck rollovers on exit ramps.

Variable speed limits

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). One example is on Britain's M25 motorway, which circumnavigates London. On the most heavily-traveled 22 km section of the M25 variable speed limits combined with automated enforcement have been in force since 1995. Initial results of the 1995 trial indicated savings in journey times, smoother flowing traffic and a fall in the number of accidents, so the trial implementation was made permanent in 1997. Further trials on M25 have been thus far inconclusive.*.

One dangerous stretch of German autobahn in the state of Rhineland-Palatinate has had a variable speed limit since 1994.

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 SH1 north of the capital, Wellington. The speed limit is normally 80 km/h. The downhill section is monitored by a fixed speed camera.

Opposition

Speed limits and their enforcement have been opposed by some motorists since their inception. Britain's first motoring organization, 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 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.

Critics of speed limits and strict enforcement outside built-up areas point to:

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 60 mph (96 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 limits

A few public roads still have no speed limit:
  • The German intercity Autobahn, two-thirds of which have only advisory limits (Richtgeschwindigkeit).
  • Australia's Northern Territory has no blanket speed limits outside major towns.
  • 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 *.
  • Some roads in India have no speed limits.

Montana has had a speed limit since June 1999. Please see the Montana section of the Speed limits in United States page for more information.

See also

References

External links

Traffic law | Transportation | Controversial road traffic legislation

Hastighedsbegrænsning | Tempolimit | Vitesse maximale autorisée | 最高速度 | Lijst van maximumsnelheden in Europa | Hastighetsbegränsning

 

This article is licensed under the GNU Free Documentation License. It uses material from the "Speed limit".

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