Speeding

A speed limit is the maximum speed allowed by law for 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 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 (part of the UK) uses miles per hour for speed limits, and since 20 January 2005, the Republic of Ireland 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/

Design speed

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."

The design speed has largely been discredited as a sole basis for establishing a speed limit. Current U.S. standards for design speed derive from outdated, less-capable automotive technology.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.). The design speed usually underestimates the maximum safe speed for a roadway and is therefore considered 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. The idea is that the speed limit 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.

Every state in the United States statutorily or administratively picks a particular speed for a speed limit cap, meaning that no speed limit in that state may be set higher than the cap. A practical effect of this cap is that nearly every rural roadway in the U.S. has a speed limit that is well below the 85th percentile speed.

Speed limits in specific countries

Australia

Speed limits in Australia range from 40 km/h (25 mph) to 110 km/h (70 mph) 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.

It is not uncommon to see 70 km/h and 90 km/h limits on some roads, where a higher limit is deemed too dangerous, yet a lower limit is unreasonable for the traffic.

The Northern Territory is an exception, as there is no speed limit on the highways.

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.

Canada

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. 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. 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

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

^*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 (though many motorways in Germany are signposted for 100, or 120 km/h and virtually all motor routes are signposted for 60 to 120 km/h 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 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.
¹⁰Signs are posted in miles per hour, a situation unlikely to change in the near future.

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 horrendous crashes, a supposedly temporary 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 become 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). 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 freeways, highways and other major thoroughfare, in addition to radars manned by the Police or Gendarmerie.

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 200 km/h (125 mph) are not uncommon, but there is a recommended speed (in German: "Richtgeschwindigkeit") of 130 km/h (80 mph). In case of a crash, insurance payments can be dropped by exceeding the recommended speed. Some areas have compulsory speed limits to reduce the noise cars produce when driving through residential areas. Many car manufacturers (including Mercedes, BMW and Audi) limit the speed of their cars electronically to 250 km/h (155 mph); this is only a gentlemen's agreement, 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 Autobahns are limited to 120 km/h (75 mph) as a maximum speed limit. Semi-motorways, known as "motor roads" or Autostrasse, have a generally lower speed limit of 100 km/h (65 mph).

New Zealand

United States

On Interstate highways in the United States, speed limits range from urban limits as low as 40 mph (65 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 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 a speed of 55 mph used 17% less fuel per mile than a speed of 75 mph (120 km/h). It was also believed, based on a noticeable drop the first year the limit was imposed, that it cut down on highway deaths, but later studies were more mixed on this point. This limit was unpopular, especially in Western states. In 1987 states were permitted to raise speed limits to 65 mph (105 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, Montana reverted to non-numerical speed limits on most rural highways. Many other states reverted to older policies allowing speed limits higher than 65 mph (105 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 (105 km/h).

Montana

For four years, Montana had a "reasonable and prudent" speed limit during the daytime on most rural roads. However, the police enforced an approximately 90 mph (140 km/h) speed limit as "reasonable and prudent" at this time. This action was considered to be unconstitutional due to the vague nature of the "reasonable and prudent" laws. Thus, for the last half-year of non-numerical speed limits there was no enforced speed limit. In June 1999, a Montana law went into effect whose practical effect was to require posted limits on all roads and disallow any limit to be posted higher than 75 mph (120 km/h).

California

In California many speed limit signs are identified as "Maximum Speed", usually when the limit is 55 mph (90 km/h) or more.

Texas

Texas is the only state whose speed limit laws or rules do not prescribe a specific limit for each type of roadway. Any rural road—two lane, four lane, Interstate, or otherwise—that is numbered by the state or federal government has a 70 mph (110 km/h) statutory limit[1]. The law generally allows changing the 70 mph limit only if a study recommends a different limit[2].

75 mph and 80 mph limits

In 2001, the Texas Legislature allowed the Texas Department of Transportation to post 75 mph (120 km/h) speed limits in counties with fewer than 10 people per square mile[3]. This has the practical effect of only allowing 75 mph speed limits in the most sparsely populated counties, all of which are generally well west of a line stretching from San Antonio to Odessa. In 2005, the Texas Legislature revised this law, allowing 80 mph (130 kmh) limits on I-10 and I-20 in certain rural counties in west Texas[4]. This law also revised the eligibility for 75 mph speed limits: now eligible counties can have up to 15 persons per square mile.

Texas law does not disallow 75 mph speed limits on two-lane roads. Several west Texas 2 lane roads carry 75 mph limits, including portions of US 90[5]. No other state has a limit higher than 70 mph on any 2 lane road.

Environmental speed limits

55 mph speed limit signs photographed on August 20, 2002. These signs were on southbound I-45 at the north border of Montgomery County. Immediately before this point, the speed limit was 70 mph. These were part of a grossly unpopular 55 mph speed limit cap in the Houston-Galveston area. The speed limit at the Montgomery County border now only drops to 65 mph.

Same location as prior picture with 65 MPH speed limit, photographed on August 4, 2005.

Texas is the first state to lower speed limits for purportedly environmental reasons since the 1995 repeal of federal speed limit controls. In roughly a 50-mile (80 km) radius of the Houston-Galveston and Dallas-Ft. Worth regions, the Texas Commission on Environmental Quality co-opted the Texas Department of Transportation to reduce the speed limit on all roads with 70 mph (110 km/h) or 65 mph (100 km/h) speed limits by 5 mph[6]. This was instituted as part of a plan to reduce smog-forming emissions in areas out of compliance with the federal Clean Air Act.

Initial studies suggested the lower speed limits could bring the areas roughly 1.5% closer to compliance[7]. However, follow-up studies suggest that the actual benefit is only a fraction of this original estimate. First, the emissions modeling software used for intitial estimations, MOBILE 5a, overestimated the emissions contribution of speed limit reductions. Rerunning the models with the next generation software, MOBILE 6, produced dramatically lower emissions reductions. Second, speed checks in the Dallas area performed 1 year after implementation of speed limit reductions show that actual speed reductions are only about 1.6 MPH, a fraction of the anticipated 5.5 MPH speed reduction. With both of these facts combined, it is possible that the speed limit reductions only provide a thousandth of the total emissions reductions necessary for Clean Air Act compliance.[8]

The Houston-Galveston area briefly had all roadways capped at 55 mph (90 km/h) in mid-2002[9]. Facing immense opposition and the realization that lowered speed limits produced only a fraction of the originally estimated emissions reductions, the TCEQ relented and reverted to the 5 mph reduction scheme[10].

In 2003, the Texas Legislature prospectively banned environmental speed limits effective September 1, 2003. The wording of the bill allows environmental speed limits already in place to remain indefinitely; no new miles of roadway may be subjected to environmental speed limits, however[11].

This law has allowed interesting inconsistencies. Generally, all primary arterial roadways within the inner loops of Texas cities have speed limits of 60 mph (95 km/h) or lower, so they were not subjected to environmental speed limits. Arterial roads between the inner loop and the outer loop generally have 65 mph (100 km/h) limits, and arterial roads outside the outer loop generally have 70 mph (110 km/h) limits. In at least one case—TX 121 between I-35W and I-820 in Ft. Worth—the speed limit rises from 60 mph to 65 mph as you cross IH-820 approaching downtown[12], contravening the standard. (Note that this "standard" is not the law, and in a strict sense is likely illegal; it is only an observed pattern.)

Night speed limits

Texas is one of very few states that widely use separate night speed limits. Texas statutorily prescribes a blanket 65 mph (100 km/h) night speed limit on roads with a speed limit of 70 mph (110 km/h) or higher[13]. While the Texas Department of Transportation has the power to raise or lower this night speed limit, it in fact rarely does, so nearly every 70 mph or higher speed limit sign has an accompanying 65 mph night speed limit sign.

Minimum speeds

In addition to the legally defined maximum speed, minimum speeds may be posted. In practice they are rarely used and seldom enforced. In California the minimum speed by regulation on freeways (in free flowing traffic) is 45 mph, although generally not posted.

In general

Enforcement

There are various means of enforcing set speed limits. Usually police and state highway patrol officers use radar, laser detection equipment, planes, and automated devices to enforce speed limits.

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.

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[.]"[15] Arbitrarily low speed limits also make violators out of otherwise reasonable drivers.

Safety

Crash forces are proportional to the square of the speed. While it is true that higher speed usually increases the severity of crashes, this scientific fact has an unclear correlation to aggregate safety.

There are many factors that go into discerning the safety value of speed limits, many of which have nothing to do with vehicle speed. For instance, driver alertness, various distractions, road conditions, weather, and wildlife often play vital roles in the safe operation of motor vehicles.

Second, several studies performed in the U.S., including studies performed after speed limit increases in the late 1980s and mid- to late 1990s, produced mixed empirical evidence on the relationship between speed and safety[14]. Sometimes no overall effects was noticed, and sometimes overall fatalities and crashes declined. Even the studies that found more fatalities often fail to explain why adjacent roads often had fewer crashes or why systemwide safety effects were minimal or sometimes positive.

Some have suggested that higher speed limits on Interstate roads would encourage travel on roads that are generally safer than non-limited-access highways. Others suggest that higher speed limits would encourage slower drivers to match the faster flow of traffic, thus removing possible obstacles and frustrations from interfering with faster drivers.

Roads without speed limits

India still does not enforce a speed limit. Speeds above 90 mph are generally not possible because of the quality of the road system, although such speeds are still achievable in the right places. India's poor road standards means the Isle of Man roads are still the best place for high-speed driving.

It is a myth that Montana had no speed limit from 1995 to 1999. In fact, Montana just had no numeric daytime speed limit. It still had a "basic rule" in effect requiring that all speeds be "reasonable and prudent", and the night speed limit was 65 MPH or 55 MPH, depending on road type. Speeding tickets were still written both day and night.

Contents