The list of cities I can get to from Oakland on BART in under 30 minutes is pretty long.
Suppose you live in the premium housing really close to the BART, so your walk is only five minutes to the station. Then another five getting through the fare gate and waiting for your train. Then your destination is an easy ten-minute walk from the BART. Great, but that's not a 30-minute trip, it's a 50-minute trip.