The origins of road etiquette, who has right of way and on which side of the road should transport pass, are lost, although traditionally vehicles tended to stick to the left-hand side in order to avoid collisions. It has been suggested that this may have been to do with the need to keep your sword arm facing a potential opponent travelling in the opposite direction, although this suggests that all cart-drivers were armed and homicidal.
In Western Europe this all changed following the French Revolution, when those keen to overthrow the old order abolished the habit of the aristocracy to travel on the left, which had forced the peasantry over to the right. After the storming of the Bastille and the subsequent events, aristocrats preferred to keep a low profile and joined the peasants on the right. By 1794, an official ‘keep right’ order had been imposed, something which extended to other countries as both revolutionary fervour (and the armies of Emperor Napoleon) spread across Europe.
Needless to say, the British stubbornly stayed on the left-hand side of the road, extending this practice around their Empire. Following independence, a number of territories, the USA included, switched to the right as a way of escaping their colonial past.