[slivym]

I think there's a serious point to what you're saying. I was bought up in the UK so I have metres for short distances and miles for long distances. If you say to me 5 miles I instantly know what you mean. If you say 8km, I have an academic understanding but not really an intuitive feel for what you mean (other than 8km=5mi, feel 5mi).

[nicoburns]

That's just familiarity though. People in countries that primarily use kilometers have the same intuitions about them.

[gspetr]

I doubt that in practice an average person would be able to (reasonably) accurately differentiate between 4 miles and 5 miles or 6km and 8km without external measurement tools in an unfamiliar environment.

The heuristics at play that the human brain would use is likely: "5 miles is way more than I'd normally want to walk on my feet, since the trip would take me about 2 hours."