In Britain, we had stricter rules on the use of the roads in horse-drawn times than most other countries did. For the first few decades of motor vehicle use, there was a mix of horse-drawn and motorised vehicles on the roads so it made even more sense to keep these rules going. In early cars the brake levers were outside of the cab and it made sense to be able to use your right hand to operate them, as most people's right is the stronger., and also used for hand signals. Many other countries started out driving on the left of the road , both for the same reasons and because the vehicles they were driving were made in Britain. Quite why some of continental Europe began driving on the left I'm not too sure, but obviously the ease with which you could drive from one country to another meant that they all soon ended up on the same side ! You can find early cars from many European countries which are right-hand drive, so most of them changed at some time. Quite why America started out on the right of the road with left-hand drive cars I'm not too sure - I'll try to find out though. Good question.