I found this blog post and thought it was interesting...
Before we go into why Americans drive on the right side of the road, let's understand the reasoning behind driving on the left side.
Since most people are right-handed, swordsmen preferred to keep to the left in order to have their right arm nearer to an opponent and their scabbard [the sheath for the sword] further from him [or her]. Moreover, it reduced the chance of the scabbard (worn on the left) hitting other people. So it is natural that once cars were introduce, you would drive on the left side of the road.
Americans began driving on the right side of the road following the revolution [I'm assuming they were "driving" their horse and buggy]. In the early years of English [colonization] of North America, English driving customs were followed and the colonies drove on the left. After gaining independence from England, however, they were anxious to cast off all remaining links with their British colonial past and gradually changed to right-hand driving.
The first law requiring drivers to keep right was passed in Pennsylvania in 1792, and similar laws were passed in New York in 1804 and New Jersey in 1813.
There you have it.
