This is the wrong way to go. I would guess that high-speed trains will only reduce people's reliance on airplanes, which are themselves a form of mass-transit.

This is the wrong way to go. I would guess that high-speed trains will only reduce people's reliance on airplanes, which are themselves a form of mass-transit. This particular change should be easy to effect, as many casual travelers will be swayed by the (hopefully) greater convenience of train travel versus the airport hassle. However, many who are over-reliant on their cars will continue to be so, as they are less likely to be the people traveling frequently by plane. Also, people will have no incentive to reduce their car dependence so long as they still have to drive everyday for work/groceries/practically everything, unless they are lucky enough to already live in one of the rare (at least in the USA) cities with public transportation that could be rationally argued as superior to individual cars. So, I would argue that massive investment in intra-city public transport infrastructure would have a more significantly positive long-term effect, as its availability would be relevant to more people, and has a the potential to cause a significantly larger reduction in the number of miles driven per person in the USA. Unfortunately, the negative stigma that seems to linger over public transport in many parts of this country would likely retard these benefits. The real utility of the simple bus systems that would be a reasonable first step, as opposed to metros/subways, would not become apparent until the use of mass-transit reached a critical mass, at which the density of cars on the road would be reduced to a point allowing buses to travel quickly. Though, the importance of reaching this critical mass, which creates a sort of chicken<->egg problem, could be reduced by dedicated bus lanes, as some others have already mentioned.