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A speed limit is the maximum speed allowed by law by vehicles on a road. (Also, an axiom of Einstein's relativity theories states that the speed limit of the Universe is the speed of light in a vacuum, i.e. 299,792,458 metres per second.)
Signage Speed limits are usually, but not always, marked with a traffic sign. Near border crossings between countries using different units (for example, between Canada and the United States), they may be marked on the first few signs that are encountered after crossing.  New style Irish speed limit sign in km/h A similar situation now exists at the border between Northern Ireland and the Republic of Ireland. Northern Ireland use miles per hour for speed limits, and since 20 January 2005 the Republic has used kilometres per hour. The Republic uses kilometres for distance (although some old signs with miles have not been removed they are scheduled to be replaced before the end of 2005), and the United Kingdom uses miles. This changeover from miles to kilometres on roads was described by the then Irish Minister for Transport, Seamus Brennan, on October 6, 2003 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. More information on the Irish metrication of road and speed limit signs can be found at the official website: http://www.gometric.ie/  Standard Binnacle for Automobile speedometers used in most countries
Design speed Speed limits are generally peripherally related to the design speed of the road. In the United States this 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." This distinction reflects the fact that the design speed has been discredited as a basis for establishing a speed limit; the design speed is largely based on characteristics of outdated automotive technology and is only a very conservative "first guess" at a limit.
85th percentile rule Since the 1950s, United States traffic engineers have been taught the 85th Percentile Rule, which claims that the maximum speed limit on a previously unrestricted road should be set to the speed below which 85% of vehicles are traveling. The 85th percentile closely corresponds to one standard deviation above the mean of a normal distribution. This rule has been used for many years, yet no scientific evidence has been produced that this particular rule is safer than any other.
Speed limits on United States roads and highways On interstate highways in the United States speed limits range from urban limits as low as 40 mph (60 km/h) to rural limits as high as 75 mph (120 km/h). Before the 1973 energy crisis, some states posted no speed limit on the interstate highways. At one time Kansas had an 80 MPH (130 km/h) speed limit on its turnpike system. In 1974, Congress imposed a nationwide 55 mph (90 km/h) speed limit by threatening to withhold highway funds from states that did not adopt this limit. It was estimated that a speed of 55 mph (90 km/h) used 17% less fuel per mile than a speed of 75 mph (120 km/h). This limit was unpopular, especially in Western states. In 1987 states were permitted to raise speed limits to 65 mph (100 km/h) on rural interstate highways.
All federal speed limit controls were lifted on November 28, 1995, leaving the task of setting maximum speeds to the states. Immediately, all states except Montana imposed numerical speed limits on their interstate highway segments, many higher than 65 mph (100 km/h). However, no Interstate highway, freeway, or expressway is currently signed for over 75 mph (120 km/h), and within major city limits, few freeways have speed limits over 65 mph (100 km/h).
For four years, Montana had a "reasonable and prudent" speed limit during the daytime. As a result, drivers of high-performance automobiles began to regularly visit Montana for the specific purpose of driving at high speeds on its freeways. In June 1999, Montana joined the rest of the nation and imposed a maximum speed limit of 75 mph (120 km/h) on its Interstate highways.
In addition to the legally defined maximum speed, minimum speeds are posted occasionally. However, minimum speed laws are rarely enforced.
On most other roads and highways, the general speed limits are as follows:
- 15-25 mph (20-40 km/h) in school zones
- 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
- 45-65 mph (70-100 km/h) on highways outside cities and towns and urban expressways
- 55-70 mph (90-110 km/h) on non-Interstate freeways and rural expressways.
Speed limits on European motorways, autoroutes and equivalent
Table See following table for the speed limits in European states:
Unit: km/h (in parenthesis mph)
| State | Automobile and Motorcycle | Automobile with Trailer | | outside towns/motorroutes* | Expressway/Motorway | outside towns/motorroutes* | 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) | 60 (40)/80 (50) | 80 (50) | | France | 90 (55)/110 (70) | 130 (80) | 90 (55)/110 (70) | 130 (80) | | Germany | 100 (65) | 130 (80)1 | 80 (50) | 80 (50)/100 (65)6 | | Greece (PKW) | 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) | | Ireland7 | 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 | 64 (40) | | 64 (40) | | | Netherlands | 80 (50)/100 (65) | 120 (75) | 80 (50) | 80 (50) | | Norway | 80 (50) | 90 (55)/100 (65)8 | 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) | | 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 Kingdom9 | 95 (60) /110 (70) | 110 (70) | 80 (50)/95 (60) | 95 (60) | *Motorroutes: Two or more line roads with median (dual carriageway) with a minimum speed of 60 km/h (40 mph).  Speed Equivalent chart issued to Irish motorists Remarks:
1 130 (80) is the recommended speed (though more and more motorways in Germany are signposted for 120 (75) as the maximum).
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 Need to be licensed from the German Technical Inspection Authority (TÜV).
7 Effective January 20, 2005
8 A provisional 100 km/h limit on some motorways was made permanent when it turned out the number of accidents decreased.
9 Signs are currently posted in miles per hour, although that could change in the near future in the United Kingdom.
In most European states there is a generally speed limit of 50 km/h (30 mph) inside towns.
Comments When British motorways were first built, there was no speed limit imposed on them. However, after a series of horrendous crashes, a speed limit of 70 miles per hour (110 km/h) was enforced. The Association of Chief Police Officers in Scotland and the Association of British Drivers have called for the limit to be increased. The Conservative Party are now proposing to raise the limit to 80 miles per hour (130 km/h), but it remains unclear whether this proposal will eventually became law.
On French autoroutes, there is a variable speed limit. In dry weather an autoroute has a speed limit of 130 km/h (80 mph), where when raining the speed limit is reduced to 110 km/h (70 mph).
The German Autobahns are famous for not having speed limits for cars except where indicated by traffic signs. Blanket speed limits do apply for trucks, buses and cars pulling trailers. Speeds over 300 km/h (185 mph) are not uncommon, but there is a recommended speed (in German: "Richtgeschwindigkeit) of 130 km/h (80 mph). In case of an accident insurance payments can be dropped by exceeding the recommended speed. Some areas have compulsory speed limits to reduce the noise produced by cars when driving through residential areas.
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 Autobahns are limited to 120 km/h (75 mph) as a maximum speed limit. Semi-motorways, known as motorroads or Autostrasse, have a generally lower speed limit of 100 km/h (65 mph).
Speed limits on Canadian roads and highways  Canadian speed limit posting Typical speed limits are: - 30 km/h (20 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
- 100-110 km/h (65-70 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 may have different speed limits. For example, in Alberta, 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. Also, note that these speed limits are reduced by varying amounts within construction zones.
Speed limits on Australian roads and highways Speed limits in Australia range from 40 km/h to 110 km/h at 10 km/h intervals. Generally: - School zones are 40 km/h during school hours (25 km/h in South Australia)
- Suburban roads are 50 km/h (in most states)
- Major suburban routes are 60 km/h
- Major connector roads and smaller highways are 80 km/h
- Highways and freeways are 100 km/h
- National highway routes are 100 or 110 km/h
Some states do not have 50 km/h zones, and those that do have only introduced them in recent years.
The Northern Territory is an exception, as there is no speed limit on the highways.
Speed cameras are used in almost all areas of the country. Tolerance is from 8%-10% in most states but only 3 km/h in Victoria, an issue which has caused a lot of controversy.
Speed limits on Chinese roads and expressways Previously, all expressways in the People's Republic of China were limited to a maximum speed limit of 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, speed limits were raised nationwide to 120 km/h 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, 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. In localities, speed limits may drop to 40 km/h. 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. Otherwise, speed limit remains 70 km/h on roads with two uninterrupted yellow lines and 60 km/h or even 50 km/h otherwise. Signage in towns and expressways are often present.
Minimum speed limits on expressways are varied. A general minimum speed limit of 60 km/h 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, and a second lane often has a minimum speed limit of 90 - 100 km/h. (The second lane, though, should only be set a minimum speed limit if third, fourth, and subsequent lanes exist.)
Speed limits in New Zealand Speed limits in New Zealand range from 20 km/h to 100 km/h. Specifically: - 20 km/h past school buses and accident sites
- 30 km/h past roadworks
- 50 km/h in most urban areas
- 70 km/h or 80km/h on highways through built-up areas, or on dangerous or older roads.
- 100 km/h on expressways and highways
Some vehicles are restricted to lower speeds: - 90 km/h for trucks
- 80 km/h for school buses and vehicles with trailers
- 70 km/h for motorcyclists with learner licences
Safety It is argued that lower speeds save lives since crash forces rise exponentially with speed. However, the raising of U.S. speed limits in the 1980s and 1990s produced mixed empirical evidence. Sometimes there were more fatalities on the roads immediately affected, but other times no effect was noticed. Overall systemwide roadway death rates went down. It is theorized that higher speed limits encourage more travel on safer, Interstate-class roads instead of more dangerous, non-limited access highways. Thus fatalities on non-Interstates were reduced. Others argue that it is speed variance that kills; higher speed limits encourages slow drivers who strictly obey speed limits to catch up to the flow of traffic. (Vehicles traveling the same speed in the same lane will not hit each other.)
However, it is commonly believed that unrealistic or unreasonably low speed limits lead to disrespect for the law, contempt of law enforcement, and larger speed differences between faster and slower drivers. They will make violators out of citizens who would be otherwise law-abiding.
It has also been argued that the human instinct for survival means that allowing drivers to drive at fast speeds encourages better concentration, whereas in cars with modern suspension 55 mph (90 km/h) feels slow, has the effect of letting the mind wander, and yet hitting an immovable object at 55 mph will almost certainly be fatal.
Roads without speed limits There remain a few public roads where blanket speed limits do not apply, often due to low traffic levels. The most famous of these are German intercity Autobahnen. Australia's Northern Territory also has no blanket speed limits outside major towns.
See also
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