Traffic has almost always been keep-to-the-left since at least the time of Rome until relatively recently.
Some historians, such as C. Northcote Parkinson, believed that ancient travellers on horseback generally rode on the left side of the road. As more people are right-handed, a horseman would thus be able to hold the reins with his left hand and keep his right hand free—to offer in friendship to passing riders or to defend himself with a sword, if necessary.
According to Wikipedia:
There is a popular story that Napoleon changed the rule of the road in the European countries he conquered from keep-left to keep-right. Some justifications are symbolic, such as that Napoleon himself was left- (or right-) handed, or that Britain, Napoleon's enemy, kept left.
According to an article on eHow:
This practice was common throughout medieval Europe, until it was changed by the French. During the French Revolution in a bid to turn absolutely everything on its antithesis, the driving laws were changed to "keep right." Napoleon quickly adopted the right-hand rule as law for two reasons--a display of absolute power in even the minutest of details, and the singular fact that the emperor was left-handed-- according to Ed Wright's "A Left Handed History of the World."
Is there any basis for this claim? Did Napoleon forcibly introduce driving on the right side simply because he was left-handed (or even because the olde enemy drove on the left)?