1. High speed rail is an order of magnitude more expensive than interstate highways. The cost effectiveness of the mass moving people via train vs. via highways (buses, carpool lanes included) is still an active debate.
2. The author cites that many folks use interstate highways daily. That may be true but that vast majority of those commutes are 50 miles or less. High speed rail only becomes cost and time effective over much longer distances. Which fraction of the population need to frequently travel to cities greater than 50 miles away? It's a small subset of the population (and skewed towards wealthier citizens). Roads are much more democratic -- they are available to anyone with a car.
3. While his argument about traffic is correct... where is most of the traffic on interstate highways? Answer: around major cities. The traffic is mostly local commuter traffic that high speed rail would not solve.