About a quarter of the world drives on the left, and the countries that do are mostly old British colonies. This strange quirk perplexes the rest of the world; however, there is a perfectly good reason.

Up to the late 1700s, everybody traveled on the left side of the road because it's the sensible option for feudal, violent societies of mostly right handed people. Jousting knights with their lances under their right arm naturally passed on each other's right, and if you passed a stranger on the road you walked on the left to ensure that your protective sword arm was between yourself and him. Revolutionary France, however, overturned this practice as part of its sweeping social rethink. A change was carried out all over continental Europe by Napoleon. The reason it changed under Napoleon was because he was left handed; his armies had to march on the right so he could keep his sword arm between him and any opponent. From then on, any part colonized by the French was right hand.

In America, the French colonized the southern states (Louisiana for instance) and the Canadian east coast (Quebec, also the Maritimes -formerly known as Acadia). The Dutch colonized New York (or New Amsterdam). The Spanish and Portuguese colonies the southern Americas. So the British were a minority in shaping the "traffic". USA adopted the drive-on-the-right policy, which was anxious to cast off all remaining links with its British colonial past. Once America drove on the right, left-side driving was ultimately doomed. If you wanted a good reliable vehicle, you bought American; for a period they only manufactured right-hand-drive cars. From then on many countries changed out of necessity.

Today, EC would like Britain to fall into line with the rest of Europe, but this is no longer possible. It would cost billions of pounds to change everything round. In 1967 Sweden changed from driving on the left to driving on the right. Everybody expected the number of deaths on Swedish roads to go up in the first months after the change, but the opposite happened: Fewer people died! Why? Because everybody was driving more carefully than they used to. Thus, changing side actually saved lives, rather than killing people.

snaund pointed out to me how similar the above info is to what was posted on this website:
I would like it known that I gleaned the above contents for this w/u from the Nethercutt Collection Museum in San Sylmar, CA. I used key phrases from one of their brochures, and I suppose they copied their info from the above website.