Now that one bridge is built, it is time to add a second...LOL! Seriously though, Seattle traffic has got to rank up there with the rest of congested metro areas in this country. How much capacity can be added in such a narrow isthmus? It seems like WSDOT has done about all they can with I-5 short of paving the entire city into a 100 lane freeway. When I look at Seattle and SoCal, I see designs emerging for metroplexes that I never would have imagined in the 20th century. It will be interesting to see which ones really work and how they get integrated with public transit additions so people can keep on the move.
WSDOT seems to be a bit bi-polar when approaching ways to solve congestion in Seattle. On one hand, they're working tirelessly to add HOV/bus/toll facilities to major roadways that don't currently have them, so as to ensure faster travel for those who are choosing a more economical transport choice (bus, carpool, motorbike, etc). But, on the other hand, they are very willing to add general purpose capacity (a decidedly less economical mode of transport). Two new freeways in the Seattle area will begin construction in a couple years, and neither, at this stage, will include any sort of transit/HOV accommodation, which is definitely unfortunate (perhaps the legislature will assign more money to the projects to get them built out completely from the get-go).
As for the 520's capacity, even with the lane drop approaching the Montlake area, traffic seems to be flowing pretty well on the new bridge (likely due to the addition of shoulders and HOV lanes). There were several studies before building the new bridge that suggested 8-lane alternatives (3+HOV on either side), but it was determined that it would encourage too many additional vehicles to use the bridge, would create massive tail-backs approaching the 5 (where the interchange is less than appropriate even for the 6-lane bridge), and was prohibitively expensive anyways.