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About Road Speeds


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

Speed limits are only peripherally related to the design speed of the road.

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 Current U.S. standards for design speed derive from outdated, less-capable automotive technology.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
An automobile dashboard showing the speedometer with primary markings in miles per hour.

An automobile dashboard showing the speedometer with primary markings in miles per hour.

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.



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