Hairpin turn

A nearly 180-degree turn in a road, trail, or ramp is called a hairpin turn (also hairpin bend or, if in a trail, a switchback). It is named for its geometric resemblance to a hairpin. These turns are often built when a route climbs up or down a steep slope, so that it can travel mostly across the slope with only moderate steepness, and are often arrayed in a zigzag pattern. An alternative in trail-building is the stairway.

Highways with hairpin turns allow easier, safer ascents and descents of mountainous terrain than a direct, steep climb and descent, at the price of greater distances of travel. Highways of this style are also generally less costly to build and maintain than highways with tunnels.

Some highways with switchback (hairpin) turns include:

• California State Highway 92 east of Half Moon Bay, California
• U.S. Highway 93 on the Arizona side of Hoover Dam
• California State Highway 1 south of Bodega Bay; it is shown in Alfred Hitchcock's film The Birds and is still in use and looks much as it did during the filming.
• The World War II-era Burma Road, constructed over the rugged terrain between the (then) British colony of Burma and China has many hairpin curves to accommodate traffic to supply China, then otherwise isolated by sea and land.
• The roads above Monaco, on the foothils of the Alps; also seen in Hitchcock's To Catch a Thief.