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Revision as of 10:46, 6 February 2006 by Alex Sims (talk | contribs) (→Speed limits, actual speeds, and aggregate safety)(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff)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 foreseeable 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 vehicles 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, 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 Safe Speed, 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.
Simple comparisons of fatality rates between countries neglect to account for differences in traffic density, quality of medical care, and Smeed's law. Nevertheless U.S. activists are keen to point out that the fatality rate on Germany's mostly "speed limit-less" Autobahns is now better than on U.S. Interstates -- a reversal of the relationship two decades prior. However, IRTAD 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
Speed limits in Australia range from 10 km/h (5 mph) Shared Zones to 110 km/h (70 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 (5 mph)
- School zones are 40 km/h (25 mph) when children are present, except in South Australia, where they are 25 km/h (15 mph). This speed limit is also enforced in some shopping precincts.
- The default urban speed limit is 50 km/h (30 mph) except in the Northern Territory where it is 60 km/h (40 mph).
- Major suburban routes are zoned 60 km/h (40 mph) or 70 km/h (45 mph).
- Major connector roads and smaller highways are zoned 80 km/h (50 mph) or 90km/h (55 mph).
- Highways and freeways are 100 or 110 km/h (65 or 70 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 (75 mph).
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
- 90–110 km/h (55–70 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 110 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. The Ontario Ministry of Transportation claimed that "Ontario has the safest roads in North America," in a media relase promoting a bill to increase safety in school zones through increased fines.
In British Columbia, a review of speed limits was conducted in 2002 and 2003 for the Ministry of Transportation found that posted limits on investigated roads were unrealistically low for 1309 km and unrealisticly high for 208 km. The reports recommended to increase speed limits for multilane 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 percent 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.
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) | 100 (65) |
| 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) | 60 (40)/80 (50) | 80 (50) |
| France | 90 (55)/110 (70) | 130 (80) | 90 (55)/110 (70) | 130 (80) |
| Germany | 100 (65)/none | none | 80 (50) | 80 (50)/100 (65) |
| 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) |
| Iceland | 90 (55) | 80 (50) | 80 (50) | 80 (50) |
| Ireland | 80 (50)/100 (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 | 80 (50) | 80 (50) | 60 (37) | 60 (37) |
| Netherlands | 80 (50)/100 (65) | 120 (75) | 80 (50) | 80 (50) |
| Norway | 80 (50) | 90 (55)/100 (65) | 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 Kingdom | 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:
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).
Automobile with weighty trailer: 80 (50); Truck with weighty trailer: 70 (45).
Automobile with weighty trailer: 100 (65); Truck with weighty trailer: 80 (50).
During the winter months, when conditions are often bad, all Finnish motorways have a speed limit of 100 km/h (65 mph) or less.
Need to be licensed from the German Technical Inspection Authority (TÜV).
Effective January 20, 2005
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.
Signs are posted in miles per hour, a situation unlikely to change in the near future.
100km/h is default limit on all National Routes regardless of design standard.
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 (70 mph) in the large-city provinces, citing environmental reasons. 90 km/h (55 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 StatesSpeed 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 (60–70 km/h) on major arterial roads in urban and suburban areas
- 50–65 mph (80–100 km/h) on major divided highways inside cities
- 45–65 mph (70–100 km/h) on rural two-lane roads
- 55–70 mph (90–110 km/h) on non-Interstate highways and rural expressways.
- 65–75 mph (100–120 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" Arbitrarily low limits can turn otherwise reasonable drivers into habitual speed limit violators.
Safety and efficacy
Essential physics
Forces in a motor vehicle collision are proportional to the square of the speed change (sometimes referred to as "delta-V", symbolized as δv) 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 , 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.
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 limited access highways, Autobahns, Interstates or other national names), which are typically safest roads per mile travelled while at the same time delivering higher speeds. Motorways improve safety by:
- prohibiting vulnerable road users
- prohibiting slow-moving vehicles, thus reducing speed variation and potential δv for same-direction travel
- segregating opposing traffic flows with median dividers or crash barriers, thus reducing potential δv for opposite-direction collisions
- separating crossing traffic by replacing intersections with interchanges, thus reducing potential δv into the side, most vulnerable vehicle section (side impacts are also responsible for some of the most serious traumatic brain injuries)
- removing roadside obstacles.
Although these roads may experience greater severity than most roads to due higher speeds in the event of a crash, the probably of a crash is reduced by removing interactions (crossing, passing, slower and opposing traffic), and crash severity is reduced by removing massive, fixed objects or surrounding them with energy attenuation devices (e.g. guardrails, wide grassy areas, sand barrels). These mechanisms deliver lower fatalities per vehicle-kilometer of travel than other roadways, as documented in the following table.
In general, motorway fatality rates are inversely related with AADT. It is unclear if higher AADT are generally correlated with lower fatality due to better access to medical care, lower speed variances, lower speeds, or other mechanisms such as Smeed's Law.
| KILLED per 1 BILLION Veh·Km | ||||||
| year 2003 | Motorway | Motorway Usage | Maximum Motorway | |||
| Country | Motorways | Non-Motorways | AADT | (% of Road Travel) | Speed Limit in 2003 | |
| Austria | 5.9 | 13.4 | 30,077 | 23% | 130 (80) | |
| Czech Republic | 9.9 | 34.3 | 25,714 | 11% | 130 (80) | |
| Denmark | 3.0 | 11.9 | 29,454 | 25% | 130 (80) | |
| Finland | 1.4 | 8.3 | 22,780 | 10% | 120 (75) | |
| France | 4.0 | 12.8 | 31,979 | 21% | 130 (80) | |
| Germany | 3.8 | 12.4 | 48,710 | 31% | 130 (80) (advisory) | |
| Ireland | 7.4 | 11.0 | 26,730 | 4% | 120 (75) | |
| Japan | 4.0 | 11.9 | 26,152 | 9% | ? | |
| The Netherlands | 2.1 | 11.7 | 66,734 | 41% | 120 (75) | |
| Slovenia | 8.1 | 18.7 | 15,643 | 19% | 130 (80) | |
| Sweden | 2.5 | 9.9 | 24,183 | 21% | 110 (70) | |
| Switzerland | 2.8 | 11.8 | 43,641 | 33% | 120 (75) | |
| United Kingdom | 2.0 | 9.3 | 85,536 | 23% | 110 (70) | |
| United States | 5.2 | 10.7 | 39,634 | 24% | 120 (75) |
definitions: AADT - average annual daily traffic. The bi-direction traffic count representing an average 24-hour day in a year. Sometimes called "traffic density" although it ignores or assumes a constant number of travel lanes.
source: English version of the International Road Traffic and Accident Database (IRTAD), 2003 Risk Values and Selected References Values for 2003 -- courtesy of the Bundesanstalt für Straßenwesen, that is, the (German) Federal Highway Research Institute. Travel was computed by dividing the fatality rate by the number of fatalities; AADT by dividing travel by the length of the network. 2003 speed limits were obtained from this page and verified with other sources.
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. Increasing a speed limit by 4 mi/h would increase the average speed by 1 mi/h and increase injury accidents by 5 percent. 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 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 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 a beneficial effect on the severity and probability of the crash, it usually cannot guarantee crash prevention.
Roads without speed limits
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 (110 and 95 km/h) speed limits was very unpopular.
India still does not enforce a speed limit on is highway system. Speeds above 55 mph (90 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
- 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.
- The Malta Highway Code: Rules and Regulations This government website informs very well about the rules and regulations that apply on Maltese roads. Explanations of traffic signs are given as well as speed limits.
- 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.
- Association of British Drivers' views on speed limit setting an enforcement
- Other links