As the Washington Post explained in its story on the matter: "Most suburban growth plans--including Montgomery's, until Tuesday--discourage development in congested areas, including those near public transit, and encourage construction in more sparsely populated communities, on the theory that new developments should arise where traffic is still tolerable."
Anyone who's lived in the Washington, D.C. metro area for the past 25 years can tell you how poorly that theory has turned out. Washington is a sprawling city, with developments stretching in all directions over a radius of at least 30 miles. Traffic is worse than ever, with little prospect of improving, and open farmland and forest around the city has all but disappeared.
Arlington County, which has the advantage of being small and having NO large open space to speak of, has pioneered smarter development of denser projects near mass transit hubs. We can't say it's exactly "car-free" over here, but Arlington has grown, and continues to grow, more smartly.
When we moved to the Washington area in 1980, Montgomery County--and Fairfax, too--had quite a bit of open farmland. Now the largest crop in both those counties is townhomes. Much of the development in both those counties has been done in a way that they feed into major highways, without creating the necessary connections for local residents on short trips to by-pass those thorofares, thereby contributing to congestion.
It's interesting to visit Europe, where many cities and towns have very strict zoning regulations that prevent sprawl and preserve the countryside. Those towns and cities seem to end abruptly, unlike American cities, which seem to go on forever, gradually diminishing to smaller developments that sooner or later get absorbed by larger ones. If Europe had American style development laws, there'd be no countryside left.
Montgomery's decision is a good step in the right direction. Washington's outer rim of counties need to consider some similar steps to rein in unmitigated sprawl.