[dekhn]

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.