I really don't think a tunnel will pass any kind of cost-benefit analysis. It would directly serve Marquam Hill, sure, but beyond that most of the corridor is too low-density for such an expensive project. There are a few areas like Tigard that are starting to plan for more density, but it is not very ambitious. Light rail on the Barbur viaducts will also be very difficult to pull off. My preference would be for a really good BRT system that has dedicated lanes on Barbur. Since it runs parallel to I-5, there is really no need to have it be so car-heavy.

I am in the Netherlands currently studying transportation, and one thing they do here is whenever they build a freeway, they basically close the older local parallel road to through traffic or make it undesirable for through traffic. For example, there is a road that has an electronic bollard that only allows one car through every 30 seconds! Because of that, only local access traffic uses that road, and everyone else uses the freeway. Oh yeah, and buses can make the bollard retract automatically.