Speed limit

A speed limit is the maximum speed allowed by law for vehicles on a road.

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

Signage

Speed limits are usually marked with a traffic sign. Speed limit signs often appear near political borders and road intersections, and in some cases speed limit signs appear at regular intervals. Political borders can range from country borders to city limits.

In some cases, mainly borders surrounding the United Kingdom and the United States, different forms of speed measurement are used on each side of the border. For example, Northern Ireland (currently part of the UK) uses miles-per-hour (MPH) for speed limits and miles for distance, while the Republic of Ireland uses the standard international system of kilometres per hour (km/h) for speed limits and kilometres for distance. The Irish changeover from miles to kilometres on roads, finally completed in early 2005, was described by the Irish Minister for Transport as a "mini-euro" and a huge logistical operation. Britain, too, will likely switch to kilometres per hour in the foreseeable future (as they are required to under an EU directive), though this is unlikely before 2008. The United States has no intentions to convert to international measurements in the forseeable future, and in fact, reverted to its current English measurements in states that had both English and International measurements such as California and Arizona. However, Ohio still has some signs listed with metric distances and speeds on its exit distance and speed limit signs (such as 70 MPH / 120 KM/H, or 3 miles / 5 km to next exit).

Design speed

Speed limits are only peripherally related to the design speed of the road.

In the United States, the design speed is "a selected speed used to determine the various geometric design features of the roadway" according to the 2001 AASHTO Green Book, the highway design manual. It has been changed from previous versions which considered it 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."

Current U.S. standards for design speed assume the characteristics of outdated, decades-old automotive technology. Also, the design speed of a given roadway is the theoretical maximum safe speed of the roadway's worst feature (e.g., a curve, bottleneck, hill, etc.). Finally, on modern road networks, capacity and congestion are a primary limiting factor on speeds. The design speed is therefore considered only a "first guess" at a limit.

85th percentile rule

Since the 1950s, traffic engineers have relied on the 85th percentile rule. The idea is that the speed limit should be set to the speed that separates the bottom 85% of vehicle speeds from the top 15%. The 85th percentile closely corresponds to one standard deviation above the mean of a normal distribution.

In practical terms, the 85th percentile speed is slightly higher than the flow of traffic. It has been observed that vehicles traveling over the 85th percentile speed (or faster than the flow of traffic) have a higher crash risk than vehciles traveling around or modestly below the 85th percentile speed.

In theory, the speed limit should be set below the 85th percentile speed only if crash records suggest an unusual hazard that cannot be mitigated by other measures such as restriping or engineering improvements. However, in practice, speed limits in the United States are generally 5 to 20 MPH below the 85th percentile speed. This is partly due to political pressure, arbitrary laws or regulations^[1], and the fact that many jurisdictions use the 85th percentile speed only as "guidance" or a "starting point."

One theory in support of 85th percentile speed limits, advanced by traffic engineers, is that drivers are, in whole, reasonable and prudent; drivers are able to discern road and traffic conditions, and thus able to select appropriate travel speeds in most cases. Therefore, even if the 85th percentile speed is above the posted limit, it can be prudent to increase the limit to the 85th percentile speed. (Note that traffic engineers do not argue that all drivers make perfect choices in all situations. Rather, they argue that the aggregate speed choices are usually reasonably safe.)

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). A well-studied example is Britain's M25 motorway, where variable speed limits combined with automated enforcement have now been in force since 1995. Indications are that the reduced limits are widely obeyed down to around 50mph. This improves traffic flow, reducing the pressure wave effect of congested highways, resulting in both higher average speeds and increased safety, with collisions down by 20% and injuries down 10%.

Opposition

Speed limits have been opposed by 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 and SafeSpeed, have sought to discredit speed enforcement. At the same time, other organizations such as the Safer Streets Coalition and RoadPeace have supported reduced speeds, especially in residential areas and around schools. The debate over speed enforcement has become a large part of the road safety policy debate in some countries.

Activists in the USA are keen to point out that the Interstate freeway network may have a worse safety record than Germany's mostly "speed limit-less" Autobahns. However, IRTAD (see below) shows that Britain's speed limited motorways have only half the fatality rate of the Autobahn network.

Opposition to speed limits in the United States is made obvious by the frequently low compliance with speed limits and the open use of radar detectors, which are legal in all jurisdictions except Virginia and Washington, DC.

Speed limits in specific countries

Australia

File:60kmh Sign.JPG

Australian speed zone sign

Speed limits in Australia range from 10 km/h (6.25 mph) Shared Zones to 110 km/h (68.75 mph). Speed limit signage is always displayed in km/h, and speeds increase or decrease by a minimum interval of 10 km/h. That is, the last digit in all speed signs will be a zero, excepting advisory speed signs for curves or other road obstacles, which end in the digit five.

The general "rules of thumb" are:

• Shared zones (areas where pedestrians and motorised traffic share the same space) are 10 km/h (6.25 mph)
• School zones are 40 km/h (24.85 mph) when children are present, except in South Australia, where they are 25 km/h (15.625 mph). This speed limit is also enforced in some shopping precincts.
• The default urban speed limit is 50 km/h (31.25 mph) except in the Northern Territory where it is 60 km/h (37.5 mph).
• Major suburban routes are zoned 60 km/h (37.5 mph) or 70km/h (43.75 mph).
• Major connector roads and smaller highways are zoned 80 km/h (50 mph) or 90km/h (56.25 mph).
• Highways and freeways are 100 or 110km/h (62.5 or 68.75 mph).

The default open road speed limit is 100 km/h for all states except for Western Australia where it is 110 km/h and the Northern Territory, which has no set open road speed limit. In the Northern Territory, many of the roads are only suitable for speeds up to 120km/h.

Speed traps are used in almost all areas of the country. 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.

Canada

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
• 90–100 km/h (55–62 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 may 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 110 km/h, but in Ontario, all freeways have a speed limit of 100 km/h unless posted as lower. Speed limits are generally lower in Ontario and Quebec on comparable roads than in other Canadian provinces. An example being rural, two-lane highways in Ontario having a standard speed limit of 80km/h, while comparable roads in other provinces having standard speed limits of 90–100km/h. According to the Government of Ontario, "Ontario has the safest roads in North America," (reference.) In general Canadian speed limits are much lower than on comparable roads in Europe or the U.S. Also, note that these speed limits are reduced by varying amounts within construction zones.

China

Previously, all expressways in the People's Republic of China were limited to a maximum speed limit of 110 km/h (70 mph). With the passage of the PRC's first road-related law, the Road Traffic Safety Law of the People's Republic of China, speed limits were raised nationwide to 120 km/h (75 mph) as of May 1, 2004; however, the updating of signs (if and when it becomes complete) will still take some time.

Semi-expressways and city express routes (uniquely called kuaisu gonglu 快速公路 in Chinese, meaning "fast public road") generally have lower speed limits topping out at around 100 km/h (65 mph), and 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 (50 mph). In localities, speed limits may drop to 40 km/h (25 mph). In reality, few people drive according to the speed limits, and on some roads, enforcement cameras are nearly non-existent.

On some designated "fast through routes" in cities, speed limits can go all the way up to 80 km/h (50 mph). Otherwise, speed limits remain 70 km/h (45 mph) on roads with two uninterrupted yellow lines and 60 km/h (40 mph) or even 50 km/h (30 mph) 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 (40 mph) is in force at all times (although traffic jams more than thwart it). According to law, the overtaking lane has a minimum speed limit of 110 km/h (70 mph), and a second lane often has a minimum speed limit of 90–100 km/h (55–65 mph). (The second lane, though, should only be set a minimum speed limit if third, fourth, and subsequent lanes exist.)

Europe

Table

See following table for the speed limits in European states:

Units are km/h (mph in parenthesis).

State	Automobile and Motorcycle		Automobile with Trailer
	outside towns/motor routes*	Expressway/Motorway	outside towns/motor routes*	Expressway/Motorway
Austria	100 (65)	130 (80)	100 (65)4	100 (65)5
Belgium	90 (55)	120 (75)	90 (55)	120 (75)
Croatia	80 (50)/100 (65)	130 (80)	80 (50)	80 (50)
Cyprus	80 (50)	100 (65)	80 (50)	100 (65)
Czech Republic	90 (55)/130 (80)	130 (80)	80 (50)	80 (50)
Denmark	80 (50)	130 (80)	80 (50)	80 (50)
Finland	80 (50)/100 (65)	120 (75)6	60 (40)/80 (50)	80 (50)
France	90 (55)/110 (70)	130 (80)	90 (55)/110 (70)	130 (80)
Germany	100 (65)/none1	none1	80 (50)	80 (50)/100 (65)7
Greece (Cars)	90 (55)	120 (75)	80 (50)	80 (50)
Greece (Motorcycles)	70 (45)	90 (55)
Hungary	90 (55)/110 (70)	130 (80)	70 (45)	80 (50)
Iceland12	90 (55)	80 (50)	80 (50)	80 (50)
Ireland8	80 (50)/10011 (65)	120 (75)	80 (50)/100 (65)	80 (50)
Italy	90 (55)/130 (80)2	130 (80)/150 (95)3	70 (45)	80 (50)
Liechtenstein	80 (50)		80 (50)
Malta	64 (40)	80 (50)	64 (40)	80 (50)
Netherlands	80 (50)/100 (65)	120 (75)	80 (50)	80 (50)
Norway	80 (50)	90 (55)/100 (65)9	80 (50)	80 (50)
Poland	90 (55)	130 (80)	70 (45)	80 (50)
Portugal	90 (55) /100 (65)	120 (75)	70 (45)/80 (50)	100 (65)
Romania	90 (55) / 100 (65)	130 (80)	80 (50)	100 (65)
Russia	90 (55)	120 (75)
Slovakia	90 (55)	130 (80)	80 (50)	80 (50)
Slovenia	90 (55)/100 (65)	130 (80)	80 (50)	80 (50)
Spain	90 (55)/100 (65)	120 (75)	70 (45)/80 (50)	80 (50)
Sweden	70 (45)/90 (55)	110 (70)	80 (50)	80 (50)
Switzerland	80 (50)/100 (65)	120 (75)	80 (50)	80 (50)
Turkey	90 (55)/130 (80)	130 (80)	70 (45)	70 (45)
United Kingdom10	95 (60) /110 (70)	110 (70)	80 (50)/95 (60)	95 (60)

^*Motor routes: Roads with two or more lanes (dual carriageway), a median, and a minimum speed of 60 km/h (40 mph).

Remarks:
¹ 130 (80) is the recommended maximum speed on motorways as indicated by a blue sign. Many sections of the German motorway network are now also covered by enforcable speed limits, usually ranging from 80 to 120 km/h depending on local conditions. 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.
² For motorcycles 110 (70).
³ Two-lane expressways: 130 (80); three-lane expressway: 150 (95) (since 2003, the speed limit of 150 km/h (95 mph) is only valid when signed).
⁴ Automobile with weighty trailer: 80 (50); Truck with weighty trailer: 70 (45).
⁵ Automobile with weighty trailer: 100 (65); Truck with weighty trailer: 80 (50).
⁶ During the winter months, when conditions are often bad, all Finnish motorways have a speed limit of 100 km/h (65 mph) or less.
⁷ Need to be licensed from the German Technical Inspection Authority (TÜV).
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 it turned out the number of accidents decreased.
¹⁰ Signs are posted in miles per hour, a situation unlikely to change in the near future.
¹¹ 100km/h is default limit on all National Routes regardless of design standard.
¹² Iceland doesn't 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 (50). You are not pulled over if you keep below 100 (60). A general rule of thumb in Iceland is if you exceed the speed limit by no more than 20 (10) you are not likely to be pulled over and fined. In some parts of the country this is closer to 10 (5).

In most European states there is a general speed limit of 50 km/h (30 mph) inside towns.

Comments

The first British motorways did not have imposed speed limits. However, after a series of severe crashes a temporary speed limit of 70 miles per hour (110 km/h) was enforced in 1965, which was made permanent in 1967. It was reduced to 50 mph in response to the 1973 oil crisis and restored to 70 mph in 1974. The Association of British Drivers have called for the limit to be increased. The opposition Conservative Party are now proposing to raise the limit to 80 miles per hour (130 km/h) where appropriate.

On French autoroutes, there is a de facto variable speed limit. In dry weather an autoroute has a speed limit of 130 km/h (80 mph), when raining the speed limit is reduced to 110 km/h (70 mph). In 2005, a governmental report advised lowering this speed to 115 km/h 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 thoroughfare, in addition to radar manned by the Police or Gendarmerie. The French authorities have credited this increase in traffic enforcement for a 21% drop in road fatalities from 2002 to 2003.

The German Autobahnen are famous for having no speed limits for cars over much of their length. Blanket speed limits do 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 (80 mph). In case of a crash, insurance payments can be dropped where the recommended speed is exceeded. Some areas have compulsory speed limits to reduce the noise or for safety reasons. Many car manufacturers (including Mercedes, BMW and Audi) limit the speed of their cars electronically to 250 km/h (155 mph)although this is not a legal requirement.

The Italian Autostradas have a 130 km/h (80 mph) speed limit, 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) as a maximum speed limit. Semi-motorways, known as "motor roads" or Autostrassen, have a generally lower speed limit of 100 km/h (65 mph).

For a period about 1990 to 1995, Sweden banned the highest limit 110 km/h (68 mph) in the large-city provinces, citing environmental reasons. 90 km/h (56 mph) limits were introduced on most motorways, the lowest in Europe. The term "large-city province" was defined as any province having one of the three large cities with suburbs. That meant that the west coast motorway E6 had 90 km/h all along its (then) about 250 km of motorway, but some ordinary roads in less densely populated provinces had 110 km/h. This ban was later removed because the limit was neither popular nor much obeyed.

India

India has a speed limit, though is very loosely enforced to a level where the speed you travel at is left to your discretion. In northern states where road conditions are better, one need not worry of ruining their vehicles' suspension much like in the southern states of India.

New Zealand

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

• 20 km/h (10 mph) past school buses and accident sites
• 30 km/h (20 mph) past roadworks
• 50 km/h (30 mph) in most urban areas
• 60 km/h (40 mph) for many city arterial routes
• 70 km/h or 80 km/h (45–50 mph) on highways through built-up areas, or on dangerous or older roads.
• 100 km/h (65 mph) on expressways and highways

Some vehicles are restricted to lower speeds:

• 90 km/h (55 mph) for trucks and vehicles with trailers
• 80 km/h (50 mph) for school buses
• 70 km/h (45 mph) for motorcyclists with learner licences

United States

Further information: Speed limits in United States

Speed limits on United States roads are usually as follows:

• 25–30 mph (40–50 km/h) on residential streets in cities and towns
• 35–45 mph (55–70 km/h) on major arterial roads in urban and suburban areas
• 50–65 mph (80–105 km/h) on major divided highways inside cities
• 45–65 mph (70–105 km/h) on rural two-lane roads
• 55–70 mph (90–110 km/h) on non-Interstate highways and rural expressways.
• 65–75 mph (105–120 km/h) on rural Interstate highways

Generally, western states have higher limits than eastern states.

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 use 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.

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 licence 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[.]"[2] Arbitrarily low limits can turn otherwise reasonable drivers into habitual speed limit violators.

Safety and efficacy

File:Polizei geschwindigkeitsmessung.jpg

German police officer enforcing speed limit

The research record is mixed on the efficacy of speed limits in creating safe roadways.

Essential physics

Forces in a motor vehicle collision are proportional to the square of the speed change at impact. This means that crash forces rise much faster than speed. The probability of a fatality is proportional the fourth power of the speed change at impact [3], rising much faster than crash forces.

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 experience 21% higher in crash forces, 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 atypical crashes can include head on collisions or collisions with massive, fixed objects like trees or concrete bridge piers. Furthermore, the severity and probability of these atypical crashes can be reduced with safety devices like crash attenuators, barriers, or wide medians.

Speed limits, actual speeds, and aggregate safety

It is usually assumed that lowering speed limits decreases speeds and increases safety. Several United States studies and most international studies find straightforward, positive correlations between speed limits, speed, and safety.[4]

However, several United States speed studies find a weak association between speed limits and actual speeds and aggregate safety.[5][6][7] In fact, the most comprehensive study, one of 98 sites in 22 states (Parker 1997), found on average a subtly inverted relationship between crashes and speed limit changes. One reason for explaining this finding is that the study validated the weak relationship between speeds and speed limits; the speed limit changes produced little actual change in speeds.[8]

A theory explaining system wide crash reductions in the face of higher speed limits is that higher limits can reduce average speed differentials, meaning the speed difference between any two cars is reduced. This meshes with long-term observations of engineers that altering the speed limit in accordance with the 85th percentile rule, which often requires a speed limit increase, may reduce speed differentials by encouraging slower drivers to catch up to the flow of traffic.

This viewpoint is reinforced by an observed "U-shaped curve" of crash probability versus speed: the probability of crashing increases dramatically for drivers traveling much slower or faster than the mean traffic speed.[9]

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 vehicle parked at 0 mph but could have disastrous consequences for a vehicle traveling at 100 mph. 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 an beneficial effect on the severity and probability of the crash, it usually cannot guarantee crash prevention.

Roads without speed limits

A speeding car at 140 mph (225 km/h)

A few public roads still have no speed limit.

The most famous are the German intercity Autobahn, much of which has no speed limit or only advisory limits.

Australia's Northern Territory has no blanket speed limits outside major towns.

The Isle of Man has no speed limit on most rural roads. A 2004 proposal for 70 and 60 mph (112 and 96 km/h) speed limits was very unpopular[10].

India still does not enforce a speed limit on is highway system. Speeds above 55 mph (88 km/h) are generally not possible because of the quality of the road system, although speeds of up to 125 mph (200 km/h) are still achievable in the right places, such as the Mumbai-Pune Expressway.

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

Footnotes

1. ^ The usage of arbitrary here is not meant to convey a point of view. Rather, it is meant to convey that the selection of speed limits in the United States is often constricted or absolutely specified by broadly applicable legislative fiat. For example, it is no coincidence that virtually all rural speed limits match a speed that is prescribed in statutes. Commonly accepted speed zoning procedures would usually result in higher limits.

See also

• Speed limits in United States
• Hierarchy of roads
• Interstate highway
• Speed trap
• Road-rule enforcement camera
• Traffic Violations Reciprocity

External links

• Governments
  • Speed Cameras:10 Criticisms and Why They Are Flawed From The UK Parliamentary Advisory Council for Transport Safety
  • Transport, Local Government and the Regions - Ninth Report A comprehensive UK report into the effects of speeding.
  • The Speeding Driver: Who, How and Why? A major research report into the pyschology of the speeding driver.
• Motorists' and other groups
  • Association of British Drivers' views on speed limit setting an enforcement
    • They call themselves the voice of the driver. But who do they really represent?, A Guardian expose of 'The Association of British Drivers'.
    • Speed and Accidents - Lets Put the Record Straight The UK Transport Research Laboratories response to misrepresentation of their work by the ABD and others.
  • Aren Cambre's Speed Limit Facts Database (a list of references for a research project)
  • National Motorists Association A U.S. organization arguing for 85th percentile limits.
  • Slower Speeds Initiative A UK road safety organisation which campaigns for lower speed limits.
• Other links
  • Controlreader.org: what speeding in France will cost you
  • John F. Carr's State Traffic and Speed Laws
  • Speed Limits by State, Insurance Institute for Highway Safety