[Fat Charlie]

I'd guess a horseman would have his spear in the right hand and shield in the left because that's how a righty would carry his stuff.

As to angles and force, I'm probably giving cavalrymen way too much credit (easy to do) but if I were on a horse in that situation I'd make sure my shield was facing the other guy no matter what. You can always make another run or get another stick but you probably can't take being gored more than once, so oncoming horses would be on my left every time. That might possibly explain why most places drive on the right, but I don't think it would explain driving on the left.

The one time I took a Land Rover for a spin I was so relieved to find that the pedals were in the same places that "close to me or away from me" wasn't a problem. I'd feel safer with "close to me or away from me" than "up to the left no matter where you're sitting," though.