Informal exceptions would have to be made for the International Peace Garden, and for the houses and commercial buildings bisected by the border. Or else you'd have to report to Customs before going from one room to the other. (Or before shooting pool in that NY-PQ bar, where the table is in the US, hi.)
It's baffling, IMHO, that US Customs can't come up with a simple, inexpensive, border control scheme in places where the crossing isn't staffed 24/7. Something similar to the passcard and gate arrangement used by gated apartments. Even the public library in Flint, which shares part of its parking lot with a large high school, that's always crying poverty, manages to control access to its parking in the early mornings when the kids arrive before it opens.
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