On the general subject Andrew Sullivan asks for or wonders why there is so little -- like none -- cooperation betweens liberals and conservatives. There are several reasons
(And I should add that links from Kain and Lee should be followed in Sullivan's post but I am writing my own post on an iPad and I have limited functionality so it's a hassle to add links. Sorry. So do link back from Sully.)
At any rate, why don't liberals and libertarians work together?
1. There are not a whole lot of libertarians. So liberals haven't tried to cultivate them.
2. Libertarians are the Troskyites (if I have my terminology) of the right -- they care so much about theory and getting it correct that they are not interested in real effective politics. Most businesspeople are not -- repeat, are NOT -- libertarians. Parking requirements, for example, are not imposed by government as much as mutual agreement by government (as reflected by the citizenry) _and_ (most) developers. I have a funny story about contesting pricing in Manhattan when I remember.)3. While they may agree about the desirability of walkable urbanism, they really do disagree about methods. Liberals really like government; more governmnent comes naturally; libertarians don't. Unfortunately, while libertarians would like to have an alternative, so far they have produced no sensible, practical, workable solutions to urban planning issues. The only path for a libertarian (at a personal level) who cares about building better cities is to become a developer and just do it. Some may. But the talking-libertarians have rather skimpy solutions. And it's not simply because majority urban liberalism is attuned to government solutions -- it's because management of a complex landscape by private contract (lots buried in that phrase) was found to be wanting and that's we have systems of zoning.
I'll try to write more some time.
Other than that, while both individuals of either flavor may like walkable urbanism, they don't even remotely agree about how to get there.
That's abreviated but as I have been torn about this issue for so many years I thought I'd try to set forth a few ideas. Liberals ignore the idea, for example, that there should be a "least intrusive means" test as a core value in comprehensive planning.