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A speed limit is the maximum speed of travel permitted by a vehicle on a road by law. Speed limits are applied elsewhere on different modes of transport e.g. on stretches of railroad/railway, on boats in harbours, some bicycle routes, etc.
Speed limits vary by type of road, including the number of lanes. Residential streets, with primarily an access function, typically have much lower maximum speeds than intercity roads, with primarily a movement function.
Most commonly, speed limits refer to maximum speed limits, i.e., the maximum speed a vehicle is legally allowed to travel at. However, minimum speed limits also exist, most noticeably on motorways, freeways, and expressways.
Speed limits are usually marked with a speed limit sign.
| International speed limit sign |
| United States speed limit sign |
In most of the world, speed limits are given in kilometres per hour, while in the Ireland, the United Kingdom, and the United States they are in miles per hour. These units are usually omitted on signs (they previously were on signs in Germany, but now have been omitted too). 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.
The situation is, if anything, even more confusing at the border between the United Kingdom and the Republic of Ireland. Both countries currently use miles per hour for speed limits. However, the Republic uses kilometres for distance (although old signs with miles have not been removed), and the United Kingdom uses miles. The situation will become similar to that of the U.S./Canada border in the future. The Irish government has announced that all speed limits will be changed to km/h on 20 January 2005. Remaining non-metric signs with distances will also be replaced This changeover from miles to kilometres on roads, was described by the Irish Minister for Transport, Seamus Brennan, on October 6, 2003 as a "mini-euro" and a huge logistical operation.
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."
Since the 1950s, American 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.
On interstate highways in the United States speed limits range from 55 mph to 75 mph (about 89 km/h to 121 km/h). Before the 1973 energy crisis, some states posted no speed limit on the interstate highways. In 1974, Congress imposed a nationwide 55 mph (89 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 used 17% less fuel per mile than a speed of 75 mph. This limit was unpopular, especially in Western states. In 1987 states were permitted to raise speed limits to 65 mph (104 km/h) on rural interstate highways. The federal restriction on speed limit was lifted on November 28, 1995, leaving speed setting to the states. All states except Montana imposed numerical speed limits (Montana had a "reasonable and prudent" speed limit during the daytime, until June of 1999 when Interstates were generally posted at 75 mph), many higher than 65 mph. However, no Interstate Highway or express way is signed for over 75 mph, and within Major City Limits, few are over 65 mph. South Carolina raised the speed limit to 70 mph in 1999, but the 65 mph speed limit is used on certain Interstate highways, and the speed limit has been reduced to 60 mph (96 km/h) in some sections.
In addition to the legally defined maximum speed, there is often also a minimum speed on certain roads. Vehicles are expected to travel above 40 mph (about 64 km/h) under normal conditions. However, most states do not state this nor enforce it.
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 (62) | 130 (81) | 100 (62)4 | 100 (62)5 |
| Belgium | 90 (56) | 120 (75) | 90 (56) | 120 (75) |
| Croatia | 80 (50)/100 (62) | 130 (81) | 80 (50) | 80 (50) |
| Cyprus | 80 (50) | 110 (68) | 80 (50) | 100 (62) |
| Czech Republic | 90 (56)/130 (81) | 130 (81) | 80 (50) | 80 (50) |
| Denmark | 80 (50) | 130 (81) | 80 (50) | 80 (50) |
| Finland | 80 (50)/100 (62) | 120 (75) | 60 (37)/80 (50) | 80 (50) |
| France | 90 (56)/110 (68) | 130 (81) | 90 (56)/110 (68) | 130 (81) |
| Germany | 100 (62) | 130 (81)1 | 80 (50) | 80 (50)/100 (62)6 |
| Greece (PKW) | 90 | 110 (68) | 80 (50) | 80 (50) |
| Greece (Bikes) | 70 (43) | 90 (56) | ||
| Hungary | 90 (56)/110 (68) | 130 (81) | 70 (43) | 80 (50) |
| Ireland7 | 96 (60) | 112 (70) | 80 (50) | 80 (50) |
| Italy | 90 (56)/130 (81)2 | 130 (81)/150 (93)3 | 70 (43) | 80 (50) |
| Liechtenstein | 80 (50) | 80 (50) | ||
| Malta | 64 (40) | 64 (40) | ||
| Netherlands | 80 (50)/100 (62) | 120 (75) | 80 (50) | 80 (50) |
| Norway | 80 (50) | 90 (56) | 80 (50) | 80 (50) |
| Poland | 90 (56) | 130 (81) | 70 (43) | 80 (50) |
| Portugal | 90 (56) /100 (62) | 120 (75) | 70 (43)/80 (50) | 100 (62) |
| Slovakia | 90 (56) | 130 (81) | 80 (50) | 80 (50) |
| Slovenia | 90 (56)/100 (62) | 130 (81) | 80 (50) | 80 (50) |
| Spain | 90 (56)/100 (62) | 120 (75) | 70 (43)/80 (50) | 80 (50) |
| Sweden | 70 (43)/90 (56) | 110 (68) | 80 (50) | 80 (50) |
| Switzerland | 80 (50)/100 (62) | 120 (75) | 80 (50) | 80 (50) |
| Turkey | 90 (56)/130 (81) | 130 (81) | 70 (43) | 70 (43) |
| United Kingdom | 96 (60) /112 (70) | 112 (70) | 80 (50)/96 (60) | 96 (60) |
*Motorroutes: Two or more line roads with median (dual carriageway) with a minimum speed of 60 km/h (37 mph).
Remarks: 1 130 (81) is the recommended speed (though more and more motorways in Germany are signposted for 120 (75) as the maximum) 2 for motorcycles 110 (68), 3 two lane-expressways: 130 (81); three lane-expressway: 150 (93) (since 2003, the speed limit of 150 km/h (93 mph) is only valid when signed) 4 Automobile with weighty trailer: 80 (50), Truck with weighty trailer: 70 (43) 5 Automobile with weighty trailer: 100 (62), Truck with weighty trailer: 80 (50), 6 need to be licensed from the German Technical Inspection Authority (TÜV). 7 Irish speed limits will be changed to metric from 20 January 2005, which together with a review of current limits, will result in amended speed limits.
At the most European states there is a generally speed limit of 50 km/h (31 mph) inside towns.
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 (112 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 (129 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 (81 mph), where when raining the speed limit is reduced to 110 km/h (69 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 (186 mph) are not uncommon, but there is a recommended speed (in German: "Richtgeschwindigkeit) of 130 km/h (81 mph). In case of an accident insurance payments can be dropped by exceeding the recommended speed.
The Italian Autostradas have a 130 km/h (81 mph) speed limit, with 110 km/h (68 mph) limits on curvy roads and in rainy conditions and 150 km/h (94 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 (62 mph).
Speed limits in Australia range from 40 km/h to 110 km/h at 10 km/h intervals. Generally:
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.
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.)
The question of speed limits and safety is also an important one. It is argued that lower speeds save lives. Vehicles crashing at slow speeds rarely cause deaths. However, the evidence from raising speed limits in the 1980s and 1990s found mixed empirical evidence. While there were more fatalities on the interstate roads immediately affected, overall roadway death rates went down. This is because high speed drivers switched from even more dangerous non-interstate facilities to interstates, now that the risk of being caught for speeding was diminished. Thus fatal accidents on non-interstates were reduced. Others argue that it is speed variance that kills, and accidents are caused by vehicles traveling at very different speeds. (Vehicles traveling the same speed in the same lane will not hit each other).
However, unrealistically (or unreasonably) low speed limits, especially interstate, lead to disrespect for the law, contempt of law enforcement, and high difference between faster and slower drivers. They will make violators out of citizens who would be otherwise law-abiding. If the posted speed limit is unrealistically low, violating the 85th percentile rule, then more than 15% of vehicles will be likely to ignore the limit while fewer than 85% will abide by it.
It has also been argued that the human instinct for survival means that allowing drivers to drive at a speed that feels fast enough to them and also feels more dangerous encourages them to concentrate intensely on the job, whereas in cars with modern suspension 55 mph (89 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.
There still remain a few public roads where blanket speed limits do not apply. The most famous of these are German intercity Autobahns. The Northern Territory, Australia also has no blanket speed limits outside major towns. Traffic levels on the Territory's roads are extremely light.
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