A surface road being cheaper than a tunnel shouldn’t surprise anybody, but a single highway lane can move about 2,000 cars an hour (much less for city streets).

A surface road being cheaper than a tunnel shouldn’t surprise anybody, but a single highway lane can move about 2,000 cars an hour (much less for city streets). Assuming all the cars have two people (they don’t) that’s 4,000 people an hour. A modern metro line can do 40,000 an hour (in Tokyo they’ll do 60K/hour packing people in like sardines). So you need a 10 lane road in one direction just to match the single line’s people movement. Of course, the whole concept is moot considering the built form around said subway line needs to be able to support having ridership like that.