>I've heard that in the USA / Canada, the average "personal space bubble" that people find comfortable is around 1.5 feet

"comfortable" is a stretch. But yes, I won't think much of an unassuming person 2 feet away from me. School desks are about that much spaced out when reaching behind or in front of you (close enough to tap the back of the person in front of you), so I suppose that's how we form our bubbles.