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Left-hand traffic (LHT) and right-hand traffic (RHT) are the practice, in bidirectional traffic, of keeping to the left side or to the right side of the road, respectively. A fundamental element to traffic flow, it is sometimes referred to as the rule of the road.RHT is used in 163 countries and territories, with the remaining 78 countries and territories using LHT. Countries that use LHT account for about a sixth of the world's area with about 35% of humanity and a quarter of its roads. In 1919, 104 of the world's territories were LHT and an equal number were RHT. From 1919 to 1986, 34 of the LHT territories switched to RHT.Many of the countries with LHT were formerly part of the British Empire. In addition, Japan, Indonesia, Thailand, Suriname and other countries have retained the LHT tradition. Conversely, many of the countries with RHT were formerly part of the French colonial empire or, in Europe, were subject to French rule during the Napoleonic conquests.
For rail traffic, LHT predominates in Western Europe (except Germany, Austria, Spain and the Netherlands), Latin America, and in countries formerly in the British and French Empires, whereas North American and central and eastern European train services operate RHT.
According to the International Regulations for Preventing Collisions at Sea, water traffic is effectively RHT: a vessel proceeding along a narrow channel must keep to starboard (the right-hand side), and when two power-driven vessels are meeting head-on both must alter course to starboard also. For aircraft the US Federal Aviation Regulations suggest RHT principles, both in the air and on water.In healthy populations, traffic safety is thought to be the same regardless of handedness, although some researchers have speculated that LHT may be safer for ageing populations since humans are more commonly right-eye dominant than left-eye dominant.

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