A controlled-access highway provides an unhindered flow of traffic, with no traffic signals, intersections or property access. They are free of any at-grade crossings with other roads, railways, or pedestrian paths, which are instead carried by overpasses and underpasses across the highway. Entrance and exit to the highway are provided at interchanges by slip roads, which allow for speed changes between the highway and arterial roads and collector roads. On the controlled-access highway, opposing directions of travel are generally separated by a median strip or central reservation containing a traffic barrier or grass.
Controlled-access highways evolved during the first half of the 20th century. The Long Island Motor Parkway, opened in 1908 as a private venture, was the world's first limited-access roadway. Italy opened its first autostrada in 1925. Germany began to build its first 30-kilometre autobahn controlled-access highway without speed limits in 1932 between Cologne and Bonn. It then rapidly assembled a nationwide system of such roads in anticipation of their use in World War II. The first North American freeways opened in the New York City area in the 1920s. Britain, heavily influenced by the railways, did not build its first motorway, the Preston By-pass, until 1958.
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