Building an entirely new bridge isn't going to be cheaper than adding more cars to BART. Even assuming it was (which it wasn't), it's not going to help as much as you think it is. Not sure if you've ever commuted on the San Mateo bridge, but it's basically fully backed up from the exits onto 101, because 101 is also grid-locked. So in addition to building an entirely new bridge (which, again, more expensive than adding more cars to BART), you also need to add more lanes to 101.