The Puget Sound region has the same problem as San Diego. You have water on one side and mountains on the other, which means that you have to build upward, not outward, as the area grows. This creates a situation where the highway system can't keep up, and the cost of widening in an urban area makes a grand stroke of expansion financially impossible.
This is why the region is investing in rail-based transportation. When you have a high enough concentration of population, you need an alternative to the highways. You need transit.
Adding another “Toll” road on the 405 section.... WTH... These do nothing to solve traffic density or congestion but it does stuff those WSDOT coffers... what a mess.
Actually, the key problem is their assumptions. Their assumptions about Bellevue are idiotic. Oh, parking is expensive so people will get out of their cars. They admit that the choke points are 520 and I-90, and those are commuters that are getting on 405, and the bulk majority are going from somewhere on one side of Bellevue to somewhere on the other side of Bellevue. I live S of Bellevue, and when I go into the office, it is in Redmond. 15 miles from the office, and my commute is regularly more than 1 hour, even with back roads. If you are not on 405, your choices to get between 520 and I-90 are Bellevue Way, 108th (which merges w Bellevue Way), 148th, 156th, and W Lake Samm. Only 2 of those are 2 lanes each way the full length (Bell Way and 148th).