Cars are by now a hard to reverse environmental and urban planning disaster across the world. We are stuck with them. As a mode of transport it has grown uncontrollably at the expense of all others (except the airplane) and practically everything has been shaped to accomodate it.
Reversing that development, limiting car traffic to where its really needed is like trying to perform a complete heart and arteries transplant on a living person. Even if there was a will (which there is not) it is not clear if there is a way.
In the best scenario it will be an excruciatingly long transformation (~50 yr) as car oriented cities (or city sections) get slowly deprecated and the car-free or car-lite segments become more desirable, more livable.
In Boston there's both a will and a way. I haven't owned a car for as long as I've lived here, and the bike lanes are so, so much better now than when I first arrived. Neighboring Cambridge now has laws on the books requiring bike lanes to be added any time that a road is rebuilt. The new light-rail extension through Somerville added a bike path alongside most of its length, connecting the paths along the downtown riverside to the Minuteman bikeway that runs 15 miles out to Bedford.
It can be done. But people have to organize and give a fuck.
I live in Amsterdam which is arguably a few decades ahead in this process. It is both true that something can be done but also that we are nowhere close to actually closing this issue.
I don't mean to discourage people from switching where and when they can (or give anybody an excuse not to). There are tangible quality of life benefits that can be obtained each step along the way. So if car usage drops, say, from 90% to 60% thats hugely important.
But structural changes in the layout of urban environments are a wicked problem that will keep people busy (and procrastinating) for a long time.