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The term Hairpin, when used to describe a bend, has perhaps become more generic that originally intended. The term originates as a descriptor for the sharpest switchbacks found on roads, where the plan view resembled a hairpin, with two neighbouring parallel roads joined by a wider sweeping curve, doing a full 180 degree turn. They are also known as Switchbacks. Of course, as they are only found on steep hills, the elevational change differs substantially from a hairpin! However, these days the term has come to be used for roads that do a 180 degree turn even when there is a wide space between the two roads, such as the A82 climbing onto Rannoch Moor above Loch Tulla.
The sharpest hairpin bends have the two parts of the road separated by a vertical retaining wall, which tapers to nothing at the apex of the joining bend. However, a very steep bank or rock face is also commonly found, particularly when the two parts of the road diverge in both plan and elevation.
The joining curve of a hairpin is typically much wider than the two pieces of road that it is joining. This is to allow for larger vehicles to make the turn with the minimum of reversing. The bend, whilst generally extremely steep at the apex, can often be virtually level at the outside of the roadway, a point which can lead to inappropriate parking from people wishing to enjoy the views. Whilst there may not be such an example in the UK, on the island of St Helena in the South Atlantic, there is a hairpin which is so steep and sharp that it is impossible to turn at the apex. Instead, a roadway continues around the hill a short distance to a turning space, allowing traffic to return to the hairpin and continue their journey!
The outside edge of the bend at a hairpin is occasionally laid with gravel, or soft grass. This seems to be a basic form of Escape lane, to minimise the danger to vehicles suffering from brake failure. In the case of Porlock Hill on the A39 in Somerset, there is an Escape lane which leads off into the trees from the curve on the hairpin.
Due to the need to climb lengthy steep hills, a true hairpin bend is rarely found in full isolation. There will often be a series of sharp bends around the hairpin, perhaps a series of hairpins, to carry the road up the hill. Also, as they require traffic to slow to a crawl for safe negotiation, very few remain on the main road network. Those that do, such as on the A39 at Porlock Hill and Barton Steep being unusual survivors, even if the former are not true hairpins. Most, such as the Devils Elbow on the A93 or the A83 Rest and be Thankful have long since been straightened by new road lines. Whilst many do survive on B roads, most true hairpins still in use are on unclassified minor roads in the more mountainous regions of the country.