Hairpin curve
A hairpin turn (also hairpin bend, hairpin corner, etc.), named for its resemblance to a hairpin/bobby pin, is a bend in a road with a very acute inner angle, making it necessary for an oncoming vehicle to turn almost 180° to continue on the road. The hairpin turns are commonly seen in mountaineous area, and often appear continuously. Such turns in ramps and trails may be called switchbacks in American English, by analogy with switchback railways. In British English "switchback" is more likely to refer to a heavily undulating road—a use extended from the rollercoaster and the other type of switchback railway.
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 / 21.18111°S 55.45472°E). 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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