<P> 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 . </P> <P> 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 . </P> <P> 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 . </P> <P> Ancient Greek, Egyptian, and Roman troops kept to the left when marching . In 1998, archaeologists found a well - preserved double track leading to a Roman quarry near Swindon, in southern England . The grooves in the road on the left side (viewed facing down the track away from the quarry) were much deeper than those on the right side, suggesting LHT, at least at this location, since carts would exit the quarry heavily loaded, and enter it empty . </P>

Who decided what side of the road to drive on