There's a great post at the Daily Sightline by Roger Valdez on the detrimental cycle of highway building that leads to more gas tax revenue that leads to more money dedicated exclusively to more highways that leads to more...

And here is where we are today. Highway advocates—road builders, the automotive industry—pushed hard to get gas taxes dedicated to highway construction. In Washington, for example, the state constitution actually enshrines the position that gasoline tax revenue’s can only be used on highways, a provision that now forms the basis of efforts to stop light rail from running on highway lanes. ...

Leaders in the region have their work cut out for them. First, they face the inevitable economic development worries often cited by highway advocates—“if we don’t build it, we will be committing municipal suicide.” And then there is almost 100 years of history, of “that’s the way we do things around here.” Closing the Sustainability Gap is going to require some serious changes in the thinking of a lot of people who grew up in an economy driven—literally—by accommodating growing numbers of cars. Politicians and highway advocates need to read the memo—reducing driving means putting fewer cars on the road, which means we shouldn’t be investing scarce resources in more highways.

But, of course, if we don't build the tunnel, a wider 520 bridge, a second Montlake bridge, Seattle will simply perish.