Alex Marshall writes perhaps the best synopsis I've read about urban freeways - their history, the negatives, and what can be done today. One of the most important lines is toward the end:
This is not to say that the only good highway is a dead one. But the ones that plow through neighborhoods and seal off waterfronts are truly destructive.
Too many people misunderstand the urbanist critique of freeways. I'm not at all anti-freeway. I am, however, against freeways that blast through the middle of cities and neighborhoods. Between or around cities, they've been a godsend to long-distance road-tripping and trucking.
I highly recommend the whole piece, but here are a few nuggets:
All of these features point to conceptual flaws at the heart of the original interstate program. I live within a few blocks of one of these sunken highways — the Prospect Expressway in Brooklyn that master builder Robert Moses bore right through a neighborhood of row-houses — and it's a hellish environment.
A few keen observers realized this way back in the 1950s. Soon after Congress approved the Interstate Highway Act, people began opposing the part of it that prescribed highways through cities. Swift describes this well in his excellent book, The Big Roads. The always astute Lewis Mumford, author of the prophetic 1958 article "The Highway and the City", called the inner city highways something that would create "a tomb of concrete roads and ramps covering the dead corpse of the city." San Francisco halted the Embarcadero and several other highways in 1959.
History aside, there are several reasons why it's wrong to believe that no alternatives exist to urban highways. First of all, traffic is not some sort of fixed volume. People drive cars, and if a highway isn't there, they may take a bus or bicycle to work. They may telecommute, or they may sell their suburban home and move to the city. There is no set number of driver, for which you build roads.
Secondly, big apple-corer highways decrease mobility as much as or more than they increase it. The limited access highway usually cuts across a grid-style street layout, sealing off surface avenues like a blowtorch cauterizing veins. Tearing down a big-city highway may actually improve traffic because it gives designers a chance to break open surface streets and restore overall circulation.
All this means there is no fixed limit on the number of highways we can tear down or substantially modify. It depends more on political will and specific bureaucratic factors. Almost every major city has an apple-corer highway, sometimes several, that can be torn down, decked over or boulevardized.
Emphasis mine above.