Assume you are a Roman soldier. If you pick up a sword and a shield, your sword goes in your right hand and your shield in the left. In order that every Legionaire move together in a Phalanx, it meant that their shields had to be very tightly packed together; the best method of doing this was to standardise battle dress.
The Roman empire was exclusively right-handed by decree and travelled down the left hand side of the road for the above reason. Because the loose hand is the attacking position (in this case the right hand) then the defensive seat must be on the left; this applied to all horses, chariots, wagons and pushcarts. Thus the rules of driving down the left hand side remainded uniform across the Roman Empire and remained as the de facto rule of the road for well over 1800 years; in fact all of Europe up to and including the time of Napoleon obeyed a rule of the road on the left.
Britain was the first country in the world to formally enact this standing rule in 1835 with the Highways Act.
Section 78 of the act is of some import.
Drivers causing hurt or damage to others, or quitting the road, or driving carriage without owner’s name, or not keeping the left or near side, or interrupting free passage, if not the owner to forefeit 20s; if he be the owner, 40s.Woah, those are some steep charges! £2 in 1835 was a lot of dosh. Bearing in mind that Karl Benz' motor car was 50 years into the future at least, it would appear that Britain merely formally adopted what already existed.
Although I've heard the story that Napoleon changed the rule of the road in the countries he conquered from keep-left to keep-right. The justifications mentioned are usually symbolic, such as that Napoleon himself was left- (or right-) handed, or that Britain, Napoleon's enemy, kept left. This story has never been shown to have a factual basis and it appears to be a legend from what I can gather.
Keep-right road rules appear to be a political thing in Philadelphia in 1792, New York in 1804 and New Jersey in 1813. Certainly by the time roads moved westwards in America, traffic was on the right. Most of Europe however switched over between 1882 and 1939 either as a result the Triple Alliance, the Weimar or later Nazi Occupation. Sweden switched over to right hand drive in 1967 to bring it in line with Europe and their experience shows that in reality no "side" of the road is safer.
One Word Weather With Nelson Mandela
... yesterday my life was filled with pa-ain!