I'm in San Diego too, and I know I've ridden at least 5,000 miles since adopting vehicular cycling, all without a flat (I did get 3 flats in a row in one week on my front wheel of a new bike, but I'm not counting this because they were all due to a faulty rim tape issue).

I don't think it has anything to do with San Diego being clean. We have plenty of thorns, metal clippings, broken glass, etc, but, just like everywhere else, they're all swept by moving traffic off of the traveled way into the curbs, shoulders, islands, "missed triangles in intersections" and bike lanes. In other words, all the puncture causing debris is where vehicular cyclists do not ride, and most cyclists do ride.

Reports of thousands of miles between flats are the norm on vehicular cycling forums, regardless of location. And, like I said, I used to get flats much more often. It's all about where you ride - just a few inches can make a huge difference because of how debris is scattered and how it tends to concentrate in the untraveled sections of the roadway. Avoid those, and you'll avoid 99% of the areas where you're likely to pick up puncture-causing debris.

Giant reductions in puncture frequency is one of the great secret side benefits of vehicular cycling (I have not seen it addressed in any of the books).

Serge