Originally Posted by Aardwolf

Explanation I've heard is:

• Most people are right handed
• Castles have spiral staircases because that's the only way to build for defense.
• Defending a castle you want to descend the spiral staicase (to fight invaders) standing near the middle with sword in the right hand (near the outside) so you can stab them first.
• So 'drive on the left' on spiral staircases.
• The rest is history.

Speaking of history, me on a Triumph 650cc T120 Bonneville some years back (when photos were square)
(foot brake on left, right is 1 down + 3 up)