Keep left

“Keep Left” first started in Rome, but the first law on it was made in 1756 in England. In 75 countries, you have to drive on the left, in 167 countries, on the right. The countries in which you 'keep right' follow the convention first started in France by Napoleon. Supposedly, he wanted people to cross to the right of others, to minimise injuries due to agitated people whipping out swords and attacking those on their right while passing. It is not so easy for most people to slash those on their left, especially while on horseback.

Rule: You must drive on the left of the road.

Traffic Logic: To avoid collisions, people driving in opposite directions should be on opposite sides of the road. The rule of "keep left" is mandatory in countries where the driver's seat is on the right of the vehicle (RHD vehicles). See more here.

Do note that the rule for pedestrians is exactly the opposite, that is, "keep right".