Hairpin turn


A hairpin turn, 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 about 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.

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, 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. 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.

Roads with hairpin turns

Some roads with switchbacks include:

Europe

United Kingdom:
Mexico:
Canada:
The eastern ramp of the Liniebrug, a bike and footbridge built over the Amsterdam-Rhine Canal near Nigtevecht in the Netherlands in 2018, consists of a pair of hairpin bends.

Railways

If a railway curves back on itself like a hairpin turn, it is called a horseshoe curve. 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.