Where did the film "American Graffiti" take place?

"American Graffiti" cruises through Modesto, CA, where Lucas spent $777,000 and 28 days to show off his own teenage years burning rubber and living for those sweet Mel's Drive-In milkshakes. And while he might've nailed the spirit of his hometown perfectly, Lucas had to film most scenes in Petaluma because 1970s Modesto had traded its hot rods and drive-ins for bell bottoms and disco fever. Even the Mel's we see in the film was in San Francisco.