Hairpin turn

A hairpin turn is a road bend with such a sharp inner angle that a vehicle must make nearly a full 180-degree rotation to stay on course. The name comes from its similarity to a bent metal hairpin. In American English, similar turns on ramps and trails are often called switchbacks, a term borrowed from switchback railways.

These turns are commonly used to reduce the steepness of a road climbing a slope, arranged in a zigzag pattern. This design makes ascents and descents in mountainous areas easier and safer than a direct, steep route, though it increases travel distance and usually requires lower speed limits due to the curve's sharpness. Roads built this way are also generally cheaper to construct and maintain than those using tunnels.

Occasionally, a road may loop completely over itself, using a tunnel or bridge to cross at a different elevation—examples exist on Réunion Island and near Ashland, Oregon. When this geometry is applied to a railway, it is called a spiral or spiral loop. On trails and footpaths, stairways may replace switchbacks.

Notable hairpin turns include Clarks Summit, Pennsylvania, at the north end of Interstate 476, which eases the grade for trucks; Devil's Elbow and Governor's Bridge on the Isle of Man; the Ramsey Hairpin on the Snaefell Mountain Course; the Fairmont Hairpin at the Circuit de Monaco; Sitinjau Lauik on the Padang City and Solok road in Sumatra; and Trollstigen in Norway.

In railways, a curve that bends back on itself like a hairpin is called a horseshoe curve. The Pennsylvania Railroad built one in Blair County, Pennsylvania, to ascend the Eastern Continental Divide from the east, though its radius is much larger than a typical road hairpin. Another example is at Zlatoust.

In alpine skiing, the slalom discipline includes sections called hairpins, which consist of two consecutive vertical or closed gates that must be navigated quickly. Three or more such gates in a row are known as a flush.