I used to love listening to the radio show Car Talk. So I went into their archives and sifted though their puzzlers. Most of them I think too hard or obscure, but this one should be ok.
Some time ago, a team of British archaeologists came upon some ancient Roman ruins. Among the ruins was a stone quarry, a nearby fortress of some kind, and what appeared to be a small town, with some houses, some livestock pens, and so on.
All of these things were connected by roads -- those famous stone roads built by the Romans. In fact, they probably obtained the stones for the roads from the very quarry they discovered. After some careful observation, these archaeologists determined that the Romans drove their carts and wagons, and probably chariots too, on the left side of the road, just like the Brits do.
How did they know this?