A common argument made in support of a deep-bore tunnel to replace Seattle's Alaskan Way viaduct is that by putting all those cars underground, we'll end up with a better pedestrian and cycling environment on the city's downtown streets, the waterfront street in particular. That position may sound logical, but not unless you disregard several key realities of cars and cities.
First of all, focusing on how the tunnel would impact downtown streets ignores the impact it will have elsewhere. As I discussed in a previous post, car infrastructure inherently sabotages travel by walking, biking, and transit. The reinforcement of car dependence caused by the tunnel will dwarf any progress on alternative modes that might be made in isolated pockets of downtown Seattle.
Furthermore, there is a major flaw in the underlying premise that with a surface-only viaduct replacement scheme, utilizing the downtown street grid to make up for lost car capacity along the waterfront would force us to take space away from bikers and pedestrians. Because that premise only holds if you accept that car capacity is sacred.
New York City's removal of car travel lanes along Broadway is an unqualified success story. They didn't have anywhere else to put all those displaced cars, but that didn't stop them from doing it anyway. And this rejection of the "car capacity is sacred" mindset is the path that Seattle policy makers will also have to get on if we ever hope to make a meaningful transition from our current state of unsustainable car-dependence.
The second big hole in the pro-tunnel position is that any perceived gains achieved by a reduction of car volumes on downtown streets will only be temporary. Population growth and induced demand guarantee it.
The central Puget Sound region is projected to grow by about 40 percent by 2040. By that time, every important arterial in the downtown Seattle area will be jammed with cars. We're already seeing this start to happen on arterials like Denny Way.
And this is the inevitable endpoint whether or not we have an underground bypass for 60,000 cars a day. The only difference is that with a tunnel, total car travel volumes and all the associated negative impacts will be greater, and progress towards alternative transportation modes will be impeded.
Every densely populated city on earth has godawful car traffic---it's like death and taxes. That's because there's a horizontal limit to road capacity, while housing and jobs can economically stack vertically in buildings. As a city densifies, eventually there is simply not enough room for everyone to drive.
Whatever configuration of street ends up getting built along the Seattle waterfront, it will eventually fill up with cars, even if we spend billions on a bypass tunnel. That is, unless the entire waterfront is a dismal failure and nobody ever wants to go there.
That said, let's not forget that cars aren't necessarily a terrible thing in pedestrian environments---if they're sufficiently tamed. Pike Place is one example of that dynamic. I've argued that introducing cars in a controlled way would help bring new energy to Seattle Center and knit it back into the surrounding neighborhood. The streets of the world's most livable cities are crammed with cars---it's just that they are not allowed to rule.
Ultimately, the choice we have to make is how much are we willing to keep spending on the accommodation of cars when such investments undermine our sustainability goals, and in the end will gain us nothing in terms of urban livability.
In the case of the viaduct, to me that choice is a no-brainer: the I-5/Surface/Transit alternative with a low-speed, two-way, four lane boulevard along the waterfront. Yes, this will constrain car capacity. But here's the reality: Reigning in capacity is the only way we will ever make significant progress towards reducing driving, a goal that is not only aligned with basic principles of sustainable urbanism, but also happens to be an adopted goal of the State of Washington.