No particularly good statistic about long distance car travel leaps immediately to mind, but the seven lane Tappan Zee bridge in New York, on a route perhaps comparable to the sort that a high speed interstate railroad might take, carries 135,000 cars per day. If there are perhaps 1.2 people per car average, that's about 60 million passengers per year--a favorable comparison to a train running in Japan's heaviest traffic corridor.

The Shinkansen Tokyo-Osaka line has carried an average of 104 million people per year, is much safer, is more environmentally friendly, and is more sustainable. That's over the Tokyo-Osaka line's entire lifespan - when one considers that passengers/year will only have increased since the line opened, the figure becomes even more favourible. I'm willing to bet that your 1.2 people per car average is a little on the high side - that means one in five cars has two passengers, on average.