I saw in the demand modeling that borders are (generally) considered to be "hard" (at least, in the model example given, which used Helsinki and the Finnish/Estonian sea border). Given the presence of a body of water there, I was wondering if the same thing applied on all-land borders. In particular, I was wondering if the same thing applied with (for example) the US/Canadian border (or the US/Mexican border) since IIRC Bellingham (Vancouver) and Detroit (Windsor) generate a modest stream of traffic from large metros across the border (partly due to nosebleed prices in Canada). I think Burlington does the same (Montreal), and I can imagine that San Diego or El Paso also generate some cross-border traffic.
I'm trying to think of obvious (non-EU/EEC/EFTA-related) cases elsewhere in the world...potentially setting aside Singapore and Hong Kong as special cases (is Hong Kong a "country", for example) and I'm coming up a little short.