Boston's "Big Dig" (Central Artery) Interstate highway tunnel project cost a whopping $14.6 billion (that's billion with a B) for about 8 miles. It carries approximately 200,000 vehicle-trips a day.
Assuming all these vehicles travel the full 8 miles, with an average occupancy of 1.6 persons, that's about 2.6 million passenger-miles a day – and includes both local, commute-type trips as well as lots of through, intercity trips (so they're not readily comparable to Boston's local mass transit). According to the latest study (2005) from the Texas Transportation Institute, the Boston urban area experiences about 90 million vehicle-miles/day, or roughly 144 million person-miles (using the average occupancy shown above).
Thus the "Big Dig" project carries only 2.6/144 = a "puny" 1.8% of total urban area road traffic! And for a nearly $15 billion investment! Yet this project – far from being denounced for this ostensibly minuscule travel impact (and in stark contrast to the incessant denunciation of rail and mass transit) – has been widely hailed and favorably cited by the Road Warrior community ... including Wendell Cox (a major advocate of urban roadway tunnels as a "solution" for congestion and alternative to public transport investment).
While not all transit projects pencil out, there are a lot of them that are held to a higher standard than road projects. For the Big Dig project, that $14.6 billion dollars would have bought 486 miles of Light Rail at 30 million per mile. Of course that's just a simple estimation, but imagine what that could have done for travel in Boston Proper. How many trips over 1.8% could have been made by transit. The larger question is how much VMT could have been reduced by this investment. This should be brought out and hung before the urban road warriors. More roads for more sprawl doesn't cut it anymore.