The Overhead Wire on the destruction wrought by downtown parking. The Houston photo is stunning.
I don't believe, though, that the interstate highways share equal blame with parking for the destruction of American downtowns.
I suppose I should be careful with my definition here: I mean "Given the construction of a large, nationwide network of interstate highways, the decision to route them through downtowns did not have a material impact on the health of downtowns."
The typical large American downtown before the 1930s or so developed around a rail, harbor or port. Manufacturing clumped together near the transportation hub to save the hefty cost of moving heavy things. Service and government workers -- the only inhabitants of today's downtowns -- located downtown because that's where their customers were.
This model was doomed once the country decided to build a national network of high-speed highways. It was just a matter of time before manufacturing decamped to cheaper, less congested greenfields. It didn't matter whether the highways ran around, through or under downtown -- manufacturers weren't going to pay a premium to be down the street from a rail terminal they no longer used. Downtowns were in for a radical restructuring.
Now downtowns are occupied mainly by service and government workers. More to the point, they are occupied by service and government workers who purport to service the entire metropolitan area. A downtown Houston law firm with a large oil-and-gas practice doesn't market itself just to downtown energy firms; it sells itself to firms in the Energy Corridor, too, even though they are a 10-mile drive down the Katy Freeway. It is easy to run a metropolitan-wide law practice from downtown Houston if it is easy to get to downtown Houston from any point in the metropolis. A rich network of highways (or of light rail or commuter rail lines -- nothing in this argument depends on the mode of trip) makes the central location more attractive, not less.
Note that this has nothing to do with the impact of highways on housing patterns. Highways of course have facilitated sprawl. By lowering the cost (in time) of commuting from land on the periphery of the metropolis, they make that land more attractive for residential development. But this isn't the dynamic that causes businesses to disperse. Service firms like to cluster near similar businesses, near their customers, and near their suppliers. They will do all three if they can. When they can't do all three, they must choose. The trade offs are more acute when a downtown has poor access to the rest of the metropolitan area. In the long run, downtown Austin will suffer for its inaccessibility.
Don't take any of this as an unqualified endorsement of slashing interstate highways through downtowns. They hurt property values in their immediate vicinity. They wreck the neighborhoods that are plowed under, barricaded or sundered to make room for them. But American downtowns would have experienced wrenching change even if highways hadn't been run right through them. And American downtowns would be more desolate than they are today without the highways.
Parking is another story . . . .