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. 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 (example on Reunion Island: 21°10′52″S 55°27′17″E / 21.18111°S 55.45472°E / -21.18111; 55.45472).
In trail building, an alternative to switchbacks is the stairway.
... A hairpin turn then occurs and takes riders through the first Barrel Roll inversion ... The train then hits another hairpin turn into another short drop ... Another hairpin turn then leads the train into the second Barrel Roll and turns again ...
... The forestry continues until approaching a hairpin turn along the lake ... with Outlet Road, Route 415 makes the hairpin turn, crossing over an outlet from Harveys Lake, and continuing to the northeast along the shoreline of Harveys Lake once again ... After crossing on another gradual hairpin turn, the route passes a large beach and park, entering the community of Laketon ...
... 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 ...
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