Have we reached "peak suburbs"? In her new book, The End of the Suburbs, Fortune magazine editor Leigh Gallagher argues that powerful social, economic, environmental and demographic forces are converging to end a half-century of suburban growth in the US.

This is good news for those who believe that the US economy must become more sustainable, and that big houses, big cars and big commutes are wasteful. "No other country has such an enormous percentage of its middle class living at such low densities across such massive amounts of land," Leigh writes. Acerbic critic and author James Howard Kunstler, who's interviewed in the book, more bluntly calls suburbia "the greatest misallocation of resources in the history of the world."

But if cul-de-sac living is approaching a dead end, what's next? And what opportunities for more sustainable businesses will arise as the suburbs decline? Those are among the questions I put to Leigh in this Q&A.

Let's start with the basics. Why are suburbs in decline? Are rising energy prices – or, dare we say it, concern about the environment – playing a role?

It's a number of forces all hitting at once, really. Rising energy prices play a huge role. As we've spread ourselves further and further apart, commutes have got longer and longer: some 3.5 million people now commute more than three hours a day. As I write in the book, a couple of years ago, local TV stations started moving back their first broadcasts from 5am to 4:30am, and even 4am in recognition of just how early so many people need to leave their homes to get to work. These epic commutes take a huge toll on people in terms of their health, their relationships and, yes, especially their wallets. Many people, especially in remote exurbia, now spend a greater percentage of their income on transportation costs than on their housing costs. Concern about the environment plays a role too, and is one reason movements like minimalism and LifeEdited, which I write about in the book, have gained traction. People don't want to have as much stuff anymore.

What other forces are coming into play?

Demographics play a huge role. The birth rate has been falling rapidly, and the suburbs have been ageing. There are now more baby boomers and senior households in the suburbs than families with young children. The number of single-person households is at a record high and growing fast. So all these big houses in the suburbs were built for a family unit that once was the dominant household form, but is not the majority anymore. Households without children won't need to move to suburbs because of the schools, which is what used to pull most families to the suburbs.

Crazy as it sounds, our homes have grown bigger as families have grown smaller. "By 2006," you write, "the average home was 2,500sq ft, more than double what it was 50 years prior... The American Dream had morphed from owning a home to owning a palace." Is that about to change?

It's already changing. Homebuilders are building new kinds of floorplans that they call "right-sized", meaning they make more efficient use of space. People want less wasted space. I write about the Not So Big House movement in the book, which advocates for homes that aren't necessarily small – they might have slightly less square footage – but that are designed so that you use more of the space every day. Studies show, for instance, that most people don't use their dining room all that often. Many people say in surveys that they use it on Thanksgiving, but even then it's often used as a buffet table with the actual sitting and eating happening in other rooms. (My mother once started a business out of our house and named it DRT Enterprises, because she kept all the paperwork on the Dining Room Table.) We're already seeing much more interest in open-style great rooms that combine kitchen and dining areas.

The suburbs have been good to the auto industry and the oil companies. "We have the highest per-capita gasoline consumption rates in the world," you write. But there are signs that Americans' love affair with the car is fading. True?

True. Annual total miles driven in the US have been on the decline, which is in large part due to the recession. But one of the more striking trends I write about in the book is happening with young people, who are less enamoured of cars and driving than previous generations and who are getting their drivers' licence in decreasing numbers. I talked to one person in the book who said he asked his daughter if she wanted to get her driver's licence when she turned 16, and she looked at him, shrugged, and said "maybe next year". He couldn't believe it. There are still lots of teenagers who want to drive, of course. But behaviour is shifting. Young kids – many of whom grew up in the back seats of cars being shuttled around in remote suburbs – would rather text and access their social-media devices than be behind a wheel.

These are big changes. Presumably they create opportunities for new companies or new ways of doing business more sustainably. Can you give us an example?

There's tremendous opportunity, especially as the housing market starts to rebound. There's an oversupply of the kinds of houses and communities people don't want – big houses in loopy subdivisions – but a serious undersupply of the kinds they do want: lively communities that are closer to the things they like to do and need to do. And homebuilders are responding: nearly every major builder, and lots of municipalities, are trying to urbanise the suburbs in some way or another.

This shift is not just about all Americans racing to live in a skyscraper in a big city; it's also about communities that are built on a more human, intimate scale and located in close proximity to a pleasing, lively town centre. In a way, it's going back to the way our pre-war suburbs were built – on more urban "bones". That model is a lot more sustainable.

You write about something called Walk Scores in the book. What's that?

People want to be able to walk from their homes to stores, playgrounds or schools.

"Walkable" is the new buzzword in real-estate circles. That's created an opportunity for a startup called Walk Score, a website that, if you plug in any address, will give you a "walkability" score. More than 15,000 real estate agencies have built the Walk Score mechanism into their websites because it's something their house-hunting customers are asking for. The site also now offers a Transit Score and Bike Score mechanism. People are more interested in the location and accessibility of their houses than ever before.