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Through urban areas, at least one routing is to have 16-foot (4.9 m) clearances, but others may have a lesser clearance of 14 feet (4.3 m). Sign supports and pedestrian overpasses must be at least 17 feet (5.2 m) above the road, except on urban routes with lesser clearance, where they should be at least 1 foot (30 cm) higher than other objects.
In civil engineering, clearance refers to the difference between the loading gauge and the structure gauge in the case of railroad cars or trams, or the difference between the size of any vehicle and the width/height of doors, the width/height of an overpass or the diameter of a tunnel as well as the air draft under a bridge, the width of a lock or diameter of a tunnel in the case of watercraft.
The MUTCD's W12 series of signs is for warning signs relating to low clearance. Metric low bridge signs in meters used to be authorized but extremely rare, usually seen near the borders with Canada and Mexico, both of which use the metric system. [5] They have been discontinued as of 2012.
The code also defines the clearance that is shorter than the physical clearance to account for sag curves, bridge deflection and expected settlements with a recommendation of minimum clearance of 5 metres (16 ft 5 in). [2] In UK, the "standard minimum clearance" for structures over public highways is 16 feet 6 inches (5.03 m). [3]
The clearance below required under bridges for the largest ships—container ships, ocean liners and cruise ships—is around 220 feet (67 m) so there are often bridges with approximately that height located in coastal cities with bays or inlets, such as New York City's Verrazzano-Narrows Bridge and San Francisco's Golden Gate Bridge.
For example, the United States requires a height of 14 feet (4.27 m) for highway bridges. [1] Some vehicle standards are made to conform to these expectations. In much of the United States, the maximum height of a semi truck, in the absence of an approved overheight permit, is 13 feet, 6 inches (4.12 m). [2]
US standards specify the height of the driver's eye is defined as 1080 mm (3.5 ft) above the pavement, and the height of the object the driver needs to see as 600 mm (2.0 ft), which is equivalent to the taillight height of most passenger cars. [6]
The first edition of the Highway Capacity Manual was released in 1950 and contained 147 pages broken apart into eight parts. It was the result of a collaborative effort between the Transportation Research Board (TRB) and the Bureau of Public Roads, the predecessor to the Federal Highway Administration. [1]