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Bay Bridge 260K ppl/day + San Mateo Bridge 93K ppl/day is within spitting distance of BART (411K ppl). If there was another whole bridge across the bay (well, maybe two or three) it would alleviate the Bay Bridge and the traffic around it.

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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.

I live right by the 92/101 interchange. Adding a Southern Crossing would change the dynamics in ways that you probably wouldn't have to add anything to 101, it would just distribute the traffic that's on 101 into better locations. https://en.wikipedia.org/wiki/Southern_Crossing_(California). For example, for the set of people who are using 92W to get to SF, the southern crossing would move them north and alleviate traffic around san Mateo.

Note that I didn't make my point to say that we should build another bridge, just the pedantic detail that BART traffic isn't that much larger than Bay Bridge + San Mateo Bridge, which is a more apt comparison. I think actually you'd want to add in the Golden Gate Bridge, another 110K. So basically, there are more people commuting into SF driving over bridges than taking BART in.

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