In a recent post, I wrote that we would expect transit patronage to expand if:
(a) transit is more efficient at providing access for subsidies than roads are
(b) people patronize systems depending on the access that they provide
(c) subsidies are distributed based on usage
Here's an example from another domain. The Wii, with its rotation-sensing technology, is more efficient at providing entertainment than the XBox 360 (and the Sony Playstations 2 and 3). People buy game consoles based on the entertainment they provide. (c) is not necessary because we're talking about the private sector. The result is an increase in the market share of the Wii at the expense of the other consoles (NPD data adapted from the Video Game Sales Wiki):
In transportation, with regard to (a), there are a number of factors. In a spread-out rural area with 90% car ownership, a million dollars spent on a new road might well get more people to work/shopping/soccer practice/school than spending that same million on buses and people to run them. But I think in New York City, a dense city where drivers are in the minority, a million dollars spent on transit will go a lot further than a million spent on roads.
So I think assumption (a) is sound. Do people patronize transportation systems based solely on access? Are subsidies based solely on patronage ... of transit systems?