Speed limit

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

A speed limit is the maximum speed allowed by law for road vehicles. The highest speed limit is 160 km/h (100 mph), experimentally posted on selected test stretches in Austria and the United Arab Emirates[1]. Very few public roads have no speed limit. The first speed limit was the 10 mph limit introduced by the Locomotive Act of 1861 in the United Kingdom.

Some roads also have minimum speed limits, where slow speeds are considered to impede traffic or be dangerous.

Factors in Setting Speed Limits

Speed limits are set based on many factors, such as road features, crash records, legal statutes, administrative judgement, and engineering judgement. Two common measures for setting speed limits are the design speed of the road and the 85th percentile of travel speeds (See Design Speed, Operating Speed, and Posted Speed Practices).

Design speed

Definition

In the United States, the design speed is officially defined as "a selected speed used to determine the various geometric design features of the roadway", according to the 2001 AASHTO highway design manual, commonly referred to as the "Green Book". Previous versions of the Green Book referred to design speed as 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"; however the 2001 edition removed the term "safe" in order to avoid the misperception that speeds greater than the design speed were "unsafe".

Limitations

Safe operating speeds can exceed the design speed. Example reasons include:#A design speed is not a representative speed of an entire roadway. Rather, the road's design speed is limited by its most restrictive feature, such as a curve, bottleneck, or hill.#Actual roadway design may exceed the design specifications.#Current parameters for determining the design speed assumes the capacity of outdated automotive technology.#The stated design speed for a given road is usually not changed. Therefore, the design speed on older roads, which were calculated with older methodologies, may not factor in improved automotive technology which can maintain designed safety at higher travel speeds.

In commonly accepted engineering practice, design speed is considered a "first guess" at an appropriate speed limit.

85th percentile rule

In the United States, traffic engineers may rely on the 85th percentile rulehttp://www.ite.org/standards/speed_zoning.pdf to establish speed limits. The speed limit should be set to the speed that separates the bottom 85% of vehicle speeds from the top 15%. The 85th percentile is somewhat less than two standard deviations above the mean of a normal distribution.

The theory is that traffic laws that reflect the behavior of the majority of motorists may have better compliance than laws that arbitrarily criminalize the majority of motorists and encourage violations. The latter kinds of laws lack public support and often fail to bring about desirable changes in driving behavior. An example is the federally mandated 55 mph (90 km/h) speed limit that was scrapped in part because of notoriously low compliance.

Traffic engineers observe that the majority of drivers drive in a safe and reasonable manner, as demonstrated by consistently favorable driving records. Studies have shown crash rates are lowest at around the 85th percentile. Vehicles traveling over the 85th percentile speed (or faster than the flow of traffic) have a significantly higher crash risk than vehicles traveling around or modestly below this speed.

Most U.S. jurisdictions report using the 85th percentile speed as the basis for their speed limits, so the 85th-percentile speed and speed limits should be closely matched. However, a review of available speed studies demonstrates that the posted speed limit is almost always set well below the 85th-percentile speed by as much as 8 to 12 mph (see p.88) (13 to 19 km/h). Some reasons for this include:
*Political or bureaucratic resistance to higher limits.
*Statutes that restrict jurisdictions from posting limits higher than an arbitrary number.

Signage

For more information about traffic signs in general, see Traffic sign.

Speed limit sign common to much of Europe, showing a 60 km/h restriction

The start of a speed limit is usually marked with a speed limit traffic sign. Speed limit signs can appear near borders and road intersections, and in some cases speed limit reminder signs appear at regular intervals. In the European Union, large signposts showing the national speed limits of the respective country are usually erected right after border crossings, with a repeater sign placed some 200 to 500 metres after the first sign. The same practice is followed in several U.S. states.

Occasionally, different units of speed measurement are used on each side of a border. For example, Northern Ireland (part of the UK) uses miles per hour (mph) for speed limits and miles for distance, whereas the Republic of Ireland uses kilometres per hour (km/h) for speed limits and kilometres for distance. The UK and the United States are the only major nations still using the imperial units system. The US has shown no intention to convert to SI units, and, in fact, reverted to its current imperial units in states that had both imperial and SI systems such as California and Arizona. However, Ohio still has some SI distances and speeds on its exit distance and speed limit signs (such as 70 mph / 110 km/h, or 3 miles / 5 km to next exit). Houston, Texas has some signs in both imperial and SI units near its airports and downtown.

Design of speed limit signage varies between countries. In the European Union, the red circle is most common, while in North America, signs are usually rectangular. Australian speed limit signs are a combination, with a red circle inside a rectangular sign. Sometimes, speed limits are also painted on the road surface as a reminder.

The design of minimum speed signage also varies between countries. Most countries use blue circles based on obligatory signs. A Japanese minimum speed sign has the same design of a maximum speed sign plus a horizontal line below the number.

Image:Zeichen 393.png|Informational sign at German border crossingsImage:Zeichen 274.1.svg|Zonal restriction, common in residential areasImage:Zeichen 274.2.svg|End of restricted zoneImage:Speed limit 50 sign.svg|United States speed limit sign in miles per hour

Speed limits in specific countries

Europe

UK speed limit upon entry to village, Fivemiletown in Northern Ireland (30 mph)

The first British motorways did not have speed limits. However, after a series of multiple crashes on motorways mainly in fog, an experimental speed limit of 70 miles per hour (112 km/h) was introduced in December 1965 Austria ¦¦ 50