Cal-HSRA predicts that by 2040 the train will reduce highway miles traveled by 10 million miles a day[1]. For comparison, in May 2013, there were 15.3 billion miles of travel just on California highways[2]. A reduction of 10 million miles a day would be about 2%, and assuming people continue to move into the state, the number of highway mile traveled per day will likely be quite a bit higher in 2040 than it is today.

I don't know, it just doesn't seem like a very compelling argument to me. The state doesn't have unlimited resources. Spend the money where it will do the most good.

[1] http://www.hsr.ca.gov/docs/newsroom/fact%20sheets/Good%20for...

[2] most recent month with available statistics, lots more data available here http://traffic-counts.dot.ca.gov/