system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did nations pick which side of the road to drive on? Why don't we all use the same side?
gpt
I remember reading somewhere that people originally starting riding horses/carriages on the left side of the road was to make it easier for them to use their swords against others on the road. by being on the left side of the road they could hold the reins in their left hand and a sword in their right.

This switched in America because more wagons with multiple horses were used. To control all the horses they'd use a whip, which would be held in the right hand. If you're holding a whip in your right hand you want to be sat on the left side of the wagon so you can best reach all the horses with it, and if you're sat on the left side of the wagon you want to be driving on the right side of the road.