Also, think about how many people (and businesses) rely upon a superhighway.
Interstate 405 (San Diego - L.A.) carries some 374,000 vehicles (not people - VEHICLES) per day. That's 136,510,000 per year. In other words, it's fairly safe to assume that an amount of people equal to 1/2 the population of the entire United States travels that stretch of interstate every year.
Compare that to the estimates for the high-speed rail which are 40,500/day, or 14,600,000. Basically, 11%.
In other words, an interstate is 10x more important than a rail.