Simple physics. The earth rotates in a clock wise motion of course. Thus, the drivers must drive to the left in an attempt to offset the latent centrifugal forces at work. The quantum aspects of this are thus far unknown, however and many speculate that this could---in theory, mind you----cause a potential increase in the amount of ozone in the atmosphere.This in turn could eventually lead to the oft-feared 'pole tippage' refered to in the works of Greenbergar of Sweden. God...let's hope that doesnt happen.
Good question, btw.