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A hairpin turn (also hairpin bend or hairpin 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 metal hairpin. Such turns in ramps and trails may be called switchbacks in American English, by analogy with switchback railways.
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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 lower speed limits, due to 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 (example on Reunion Island: 21°10′52″S 55°27′17″E; example near Ashland, Oregon 42°05′31″N 122°35′21″W). When this routing geometry is used for a rail line, it is called a spiral, or spiral loop.
In trail building, an alternative to switchbacks is the stairway.
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- Devil's Elbow, Isle of Man – hairpin bend in A4 Peel to Kirk Michael coast road, Isle of Man
- Governor's Bridge (Isle of Man) – hairpin bend adjacent to a road junction on the primary A18 Mountain Road in the parish of Onchan in the Isle of Man
- Ramsey Hairpin – hairpin bend on the course of the Isle of Man TT Races
- Fairmont Hairpin – hairpin bend at the Fairmont Monte Carlo on the Circuit de Monaco
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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[1] or Hillclimbing for other railway ascent methods.
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.[2]
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