My real problem with the tunnel is the outcome if everything goes exactly to plan. The huge risk (financial, civic and environmental) being forced onto the city—and the nasty, triangulating political calculation behind it—are assuredly there, as crisply laid out by Dom. The tunnel's risks are likely to be huge; most pertinently, we're expected to accept these risk before they are even defined.
What if it succeeds? The tunnel is a fifty year (at least) investment. The sheer magnitude of the project (as planned, before any potential cost overruns) establishes it as the centerpiece of our transportation vision—a doubling down on single-occupancy car commuters. This is a transportation system focused on petroleum, cars, trucks, suburban commuters—epitomized by a tunnel that bypasses the urban core of our region, whisking its users right through the city like it isn't even there.
As we're learning, maintaining the petroleum economy isn't so easy. WIth maximum efforts, maximum opening of potential deposits to exploitation, acceptance of maximal environmental risk (both potential and actual), we can expect global oil reserves to last approximately sixty more years. The fighting over the last few drops is likely to be vicious. What will follow—what could take the place the energy-dense hydrocarbon fuels we enjoy now—isn't clear. In the fifty year lifespan of this tunnel, we can expect the cars it's being built for to run out of fuel. It's a dead end.
The problem is moving people and freight—not creating capacity for cars. From that perspective, our investment into the nascent light rail network (another fifty year investment) is prescient. The light rail system does not depend upon petroleum to move people; nor does it depend upon some miraculous, dubious from the limits of physics, leap forward in battery technology. The tunnel would make more sense if its goal was—instead of moving cars from Kent to Shoreline—to move freight from the port via heavy rail out from the city core unmolested. (The tunnel-is-pro-freight argument for the tunnel has always seemed particularly obtuse to me. Rail is vastly more efficient that diesel-fueled trucks. Rail will exist in 100 years. Diesel trucks, we should hope not.)
Tearing down the Viaduct through downtown—replacing it with a surface street and no tunnel—would help the entire region make a gentler transition to the post-petroleum world we're heading towards. Patterns of land development will adjust slowly over time. We're sealing our fate (one way, or the other) by this choice.