It’s not news to anyone that young, urban couples sometimes have children and leave the city for the suburbs. The phenomenon seems to be so firmly embedded in our culture that an explanation of why it happens is hardly required.

Then The New York Times published a feature that explains how even so-called “hipsters,” those young, creative-class types who reside in places like Williamsburg and the East Village, eventually grow up and move their families to the suburbs, just like everybody else.

But that’s not the whole story. On the surface, the migration pattern from Brooklyn to the river towns of the Lower Hudson Valley that author Alex Williams writes about in “Creating Hipsturbia” may look  just like the flight from cities that took place a generation ago. But the next suburban migration won’t simply be about better schools and bigger backyards. The next generation of parents will expect to be able to maintain aspects of their more urban lives. And although Williams’ piece emphasizes the draw of “bearded mixologists, locavore restaurants and antler-laden boutiques,” the most crucial component they’ll hang on to is walkability.

The fact that there is a main street to stroll is a big draw for former Brooklynites who find sprawling, car-culture suburbs alienating. These pedestrian-friendly towns, filled with low-rise 19th century brick buildings and non-chain shops, offer a version of village-style living that Jane Jacobs, the Greenwich Village urbanist, would have approved of.

You might say that a move from Brooklyn to Tarrytown or Hastings-on-Hudson is an example of leaving the city for the suburbs, but that misses the point. It’s really a move from one walkable community to another. The difference has less to do with the size of the municipality and the height of the buildings than it does with the types of amenities offered and the transportation options available. If you can accomplish most of your daily tasks on foot, and a commute to midtown Manhattan still takes 30 minutes by train (like it does from many parts of Brooklyn), then no wonder these types of communities have such appeal for people who prefer walking, biking and transit over automobiles.

But is this limited to Westchester’s river towns? Are Long Island’s suburbs experiencing an influx of young families seeking “more familiar, less suburban” communities outside the city? The numbers suggest that they’re not.

Long Island’s population grew by 3.2 percent between 2000 and 2011, compared with 5.7 percent in the Hudson Valley. But what’s more concerning is that Long Island has the highest share of adults age 55 and older in the region, and that it lost 15 percent of its 25-34 year-olds — a demographic that typically includes many first time home buyers — between 2000 and 2009. If we take the same groups and add adults up to age 44, the share of the population falls from 30.1 percent in 2000 to 24.7 percent in 2010.

Long Island’s car-dependent suburban sprawl is another key factor. It’s hard to retain, let alone attract, people who want the option of walking (like the aforementioned “hipsters”) when your downtowns are built for cars. There are more than 6.5 square miles of surface parking lots around Long Island downtown areas, and more than 8,300 acres ripe for mixed-use development near the Island’s more walkable village centers.

And the importance of access to and from job centers like New York City can’t be underestimated. While Metro-North ridership is up, ridership on the Long Island Rail Road has declined in recent years, and congestion and delays on the Main Line between Floral Park and Hicksville continue to prevent ridership growth, including intra- and reverse-commuting that will bolster Long Island’s economic competitiveness.