Without actually checking a map, I think that Santa Cruz is a lot farther south of San Francisco than 30 miles -- more like 130 miles! And that's from the airport which is south of the city.
My husband has been long haul trucking for over 30 years and I constantly have to correct his sense of distance, lol.
When he tells me the rest area is only 15 miles, rest assured it will usually always be atleast a minimum of an hour before the potty break! :)