I commute by bike in San Diego and I use to take public transportation.
UCSD has a good bus system that links the campus to a main shopping plaza called UTC (University Town Center) Public transportation runs right through the main street of campus. The buses can get crowded during rush hour, but that's to be expected. I always saw a handful of bike commuters whenever I would pass through campus. There's bike lanes throughout La Jolla. San Diego Bike Club has group rides that starts out at a nearby shopping center near campus. Beautiful Highway 101 with its wide bike lanes is right near campus.
The biggest plus for UCSD is that you can bike year round in mild 50-70degrees weather.