Speeding, traffic signs and metric In 1865, the revised Locomotive Act reduced the speed limit to 4 mph (6 km/h) in the country and 2 mph (3 km/h) in towns. The 1865 Act required a man with a red flag or lantern to walk 60 yards (50 m) ahead of each vehicle, enforce a walking pace, and warn horse riders and horse drawn traffic of the approach of a self-propelled machine. The replacement of the “Red Flag Act” by the Locomotive Act of 1896, and the increase of the speed limit to 14 mph (23 km/h) has been commemorated each year since 1927 by the London to Brighton Veteran Car Run.Nepal, the Isle of Man and the Indian states of Uttar Pradesh and Kerala are the only places in the world that do not have a general speed limit.

In Germany, 57% of the autobahn motorway system remain free from speed limits. The highest posted speed limit in the world is 160 km/h (99 mph), which has been experimentally applied on selected test stretches in Austria and the United Arab Emirates.

A road speed limit is the maximum speed allowed by law for road vehicles. Speed limits are commonly set and enforced by the legislative bodies of nations or provincial governments, such as countries within the world.In addition to setting a maximum speed limit, most governments also enforce speed limits that are relative to the driving conditions experienced; that is, requiring drivers to adjust their speed when driving in fog or heavy rain.

For example, the California Civil Code 22350 states that “No person shall drive a vehicle upon a highway at a speed greater than is reasonable… and in no event at a speed which endangers the safety of persons or property.” This “basic rule”, or similar legal language, applies even where no maximum speed limit is in place (such as formerly in the U.S. state of Montana). Some roads also have “minimum speed limits”, where slow speeds are considered to impede traffic flow or be dangerous.The first speed limit was the 10 mph (20 km/h) limit introduced by the Locomotive Act of 1861 (or “Red Flag Act”) in the United Kingdom (automobiles were in those days termed “light locomotives”).

Factors in setting speed limitsTraffic engineers observe that the majority of drivers drive in a safe and reasonable manner, as demonstrated by consistently favorable driving records. A report from the British Columbia Ministry of Transportation includes in its summary the finding that the incidence of crashes depends more on variations in speed between vehicles than on absolute speed, and that the likelihood of a crash happening is significantly higher if vehicles are traveling at speeds slower or faster than the mean speed of traffic.Speed limits are set based on many factors, such as road features, crash records, legal statutes, administrative judgment, engineering judgment and political dictate. 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).

Fuel efficiency also affects the choice of speed limits. United States at one time had attempted a maximum speed limit of 55 mph to reduce fuel consumption (See National Maximum Speed Law).It is also estimated that speed limits can be used to reduce emissions and pollution, and some areas have reduced speed limits for improving the air quality (See Environmental Speed Limits).

Design speed DefinitionIn 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 American Association of State Highway and Transportation Officials 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 implication that speeds greater than the design speed were necessarily “unsafe.” LimitationsSafe 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 ruleIn the United States, traffic engineers may rely on the 85th percentile rule 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 slightly greater than a speed that is one standard deviation 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 removed in part because of notoriously low compliance.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. SignageThe 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 immediately after border crossings, with a repeater sign some 200 to 500 metres (about 650 to 1,650 ft) 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 customary (imperial) units system.The U.S. has shown no intention to convert to SI units, and reverted to imperial units in states that had both imperial and SI systems such as California and Arizona. However, Ohio, South Dakota, and Vermont (especially near the Canadian border) still have some SI distances and speeds on their exit distance and speed limit signs (such as 70 mph (110 km/h) / 110 km/h, or 3 miles / 5 km to next exit). When entering Canada, signs are posted reminding drivers to use metric signage. Conversely upon entering the US from Canada (at least in Vermont), drivers are shown a 100 km/h speed limit sign. All exit distance signs on Interstates in New Hampshire are marked with the distance in miles followed by the distance in kilometres shown in parentheses. Houston, Texas has some signs in both imperial and SI units near its airports and downtown. Delaware Route 1 and Interstate 19 have exits numbered by kilometer - I-19 also has kilometer posts.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 as a maximum speed sign but with a horizontal line below the number. Speed limits in specific countriesThe following table shows the respective speed limits (excepting the local 30 km/h.Remarks:1 130 km/h is the recommended maximum speed on motorways, as indicated by a blue sign.

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