Hairpin turn


A hairpin turn is a bend in a road with a very acute inner angle, making it necessary for an oncoming vehicle to turn about 180° to continue on the road. It is named for its resemblance to a bent metal hairpin. Such turns in ramps and trails may be called switchbacks in American English by analogy with switchback railways.

Description

Hairpin 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. Highways with repeating hairpin turns allow easier and safer ascents and descents of mountainous terrain than does a direct and steep climb and descent at the price of greater distances of travel and usually lower speed limits because of the sharpness of the turn. Highways of this style are also generally less costly to build and maintain than highways with tunnels.
On occasion, the road may loop completely, using a tunnel or bridge to cross itself at a different elevation. When this routing geometry is used for a rail line, it is called a spiral or a spiral loop.
In building trails, an alternative to switchbacks is the stairway.

Notable hairpin turns

If a railway curves back on itself like a hairpin turn, it is called a horseshoe curve. The Pennsylvania Railroad built one in Blair County, Pennsylvania, which ascends the Eastern Continental Divide from the east. However, the radius of curvature is much larger than that of a typical road hairpin. See this example at Zlatoust or Hillclimbing for other railway ascent methods.

Skiing

Sections known as hairpins are also found in the slalom discipline of alpine skiing. A hairpin consists of two consecutive vertical or "closed gates", which must be negotiated very quickly. Three or more consecutive closed gates are known as a flush.