Charles Mahron at Strong Towns has the step-by-step description:
As the transformation from traditional to auto-centric continues, parking becomes more valuable for those establishments that remain. Commercial businesses that in another era would have been expanded or rebuilt at a grander scale as the community grew are now more valuable being demolished for parking. The same thing is happening to the homes throughout these neighborhoods. They are being taken down in favor of garages and "buffering". Neighborhoods originally designed to define space are now becoming space.
These changes are devastating to the tax base. Where the public has made the greatest investments in infrastructure (and has the greatest obligations for maintenance) the neighborhoods stagnate. But nobody has the job of worrying about the tax base throughout the existing neighborhoods. The traffic engineer worries about moving cars. The public works director runs the utilities and is primarily concerned with new connections. The planner administers the zoning code and is particularly zealous about parking ratios.
This all devolves into a farcical feedback loop. More people driving means that more transportation improvements are needed. There is a greater need to channel cars, to control the flow, to improve the capacity of the transportation system. The more the public realm is given over to cars, the more people must drive. The more people that drive, the more cars on the road. Etc. Etc. Etc. Nobody realizes that we're not actually adding cars. We're all just making more trips.
The whole article is worth a read, and if you like livable cities, will make you sad. There is hope, though: many cities—Chicago, for example—have avoided or reversed the spiral.