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Ahairpin turn (alsohairpin bend orhairpin corner) 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 metalhairpin. Such turns in ramps andtrails may be calledswitchbacks inAmerican English, by analogy withswitchback railways.
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, safer ascents and descents of mountainous terrain than a direct, steep climb and descent, at the price of greater distances of travel and usually lowerspeed limits, due to the sharpness of the turn. Highways of this style are also generally less costly to build and maintain than highways withtunnels.
On occasion, the road may loop completely, using a tunnel or bridge to cross itself at a different elevation (example onReunion Island:21°10′52″S55°27′17″E / 21.18111°S 55.45472°E /-21.18111; 55.45472; example nearAshland, Oregon42°05′31″N122°35′21″W / 42.09194°N 122.58917°W /42.09194; -122.58917). When this routing geometry is used for a rail line, it is called aspiral, or spiral loop.
Intrail building, an alternative to switchbacks is thestairway.
If a railway curves back on itself like a hairpin turn, it is called ahorseshoe curve. ThePennsylvania Railroadbuilt one inBlair County, Pennsylvania, which ascends theEastern Continental Divide from the east. However, theradius of curvature is much larger than that of a typical road hairpin. See this example at Zlatoust[1] orHillclimbing for other railway ascent methods.
Sections known as hairpins are also found in theslalom discipline ofalpine 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 aflush.[2]
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