> Montreal has a similar system, while Tokyo, Osaka, Seoul, Hong Kong, Singapore and Houston have systems that resemble the Path in some respects. A few European cities also make considerable use of pedestrian tunnels, including Helsinki, Stockholm and Munich.

Japan's northernmost major city, Sapporo, has a very extensive one -- of those I've seen, it's the one that's most comparable to Toronto's.

The other Japanese tunnel/undercity complexes are mostly subterranean malls around subway stations. (This also applies to all of the ones in Hong Kong.) But Sapporo's is seriously huge.

I think the common denominator is that people would rather walk in a heated underground space when it gets cold.

We were running late for our train in winter - the Sapporo underground system let us walk to the station so much faster than trying to navigate ice, snow and road crossings

Helsinki's underground spaces exist for one reason: Russian bombs.

Sapporo's one stretches for a good mile in a straight line. Quite convenient when going from the entertainment district to the train station with a suitcase in heavy snow.