Although I'm not sure where it fits into cami's timeline, part of the shift to driving on the right was also motivated by the majority of the population being right-handed. A person driving a horse-drawn cart or carriage would typically sit toward the left side of the cart, so that they could easily reach each of the horses with the whip held in their right hand. When it came time to pass carriages coming in the other direction, the drivers would keep to the right, providing themselves a better vantage point (as they were seated on the left) to insure they didn't brush against the other carriage while passing.