well CF3, they are building a giant dallas sized Hyatt on the south side of 670 to access the kauffman center on the south side of 670 and the convention center which straddles 670 so i don't think it's a problem bud
also (and this is more important) you can't have a downtown without highway access, in any city, ever
670 is god awful and completely unnecessary. you could also do without the 71 to 35 connection on the east side of downtown. There is no need to route the freeways like they did.
SF only has one downtown freeway and it's like a mile from the financial district, and it's impossible to access via freeway from the north (Marin) where tons of commuters live. (transit, yes, but crap loads of those bankers still drive).