In Britain we drive on the Correct side of the road. As do the Japanese and as did the Austrians (who used to until the Nazis took over) the Swedes until the 1950's and of course lots of places like Australia, New Zealand, South Africa, India, etc, etc still do.
How the drive on the right came about is not too clear, it seems to have something to do with the French revolution, Napoleon and so on.
Some say that in the days of the French monarchy the aristocrats kept to the left and everyone else had to use the right. Come the revolution it was no longer healthy to be thought an "Aristo", might have to "kiss" Madame Guillotine.
Keep left made sense if you were on horseback and wore your sword on your left hip, easier to mount, easier to engage any opponent hand-to-hand. See a film showing jousting, they keep to the left of the "fence" thing, couch the lance under right arm, shield in the left hand (or so I think).
(I am sure Eulalia can correct me if wrong.)
Incidently I understand that the earliest reference in England to this matter was a ruling about old London Bridge. Traffic from Southwark bound northwards into the City had to keep to the West side of the bridge and South bound traffic to the East. Thus keep left. My guess is maybe no general rule, just custom applied elsewhere.