While on holiday with some americans recently I got into a discussion about driving on the left or right of the road. Wikipedia has a good entry on the subject.

• 66% of world population drives on the right
• 72% of the world’s roads are designated for driving on the right
• There is good archaeological evidence that the Romans drove on the left of the road
• It is believed that driving on the left was more common so that a horseman could hold the reins in his left hand leaving his right free for a sword
• The switch to the right in the US happened when wagons pulled by several teams of horses came into use. The drivers then switched to sitting on the left (and driving on the right) so that their right hand could be used for a whip
• In England where smaller wagons were used no such change occurred