Ever wonder why we drive on the right, and Brits drive on the left?

This site has some answers.

The papal system prevailed until the late 1700s, when teamsters in the United States and France began hauling farm products in big wagons pulled by several pairs of horses. These wagons had no driver's seat; instead the driver sat on the left rear horse, so he could keep his right arm free to lash the team. Since you were sitting on the left, naturally you wanted everybody to pass on the left so you could look down and make sure you kept clear of the other guy's wheels. Ergo, you kept to the right side of the road.