City planner Jeff Speck has found the panacea for our ailing cities, something that could make even Detroit come to life again: walking.

In his new book, Walkable City: How Downtown Can Save America, One Step at a Time, Speck lays out something he calls his General Theory of Walkability. It’s not as platitudinous as one might think -- Speck does own a car -- but the book rests on the central point that cities designed for people, as opposed to those engineered for cars, will be the places of urban, demographic growth in the 21st century. If you build crosswalks, Speck’s theory goes, they will come.

“We used to call it New Urbanism -- that’s scary. We called it Neo-traditional Design -- that offended the progressives. People talk about density -- we don’t even need to discuss that,” says Speck, who lives in Washington, D.C., and co-wrote the book Suburban Nation: The Rise of Sprawl and the Decline of the American Dream. “Reframing the whole argument under the rubric of walkability seems to be changing the game.”

OK. We’ll bite. Tell us, Mr. Speck, why it’s time to abandon our treadmills.

Q. Are you familiar with a band called The Proclaimers?

With just a couple of finger taps on my smartphone, I can purchase a beer via QR code or tell the people in city hall about a downed power line on my street. It’s better living through gadgetry -- and now, a San Francisco-based business accelerator wants to put the same principles to work saving the planet.

Accelerators, common in Silicon Valley since the opening of Y Combinator in 2005, work like this: They lasso up a bunch of entrepreneurs, hand them thousands of bucks in seed funding, and ask them to grow or build companies in three months (hence the “accelerate” reference). Greenstart, founded in fall 2011, is the first accelerator program in the country that pumps dollars into cleantech -- that is, technology that expands the use of clean energy.

Its founder is 40-year-old Mitch Lowe, a guy who, at 23, bailed on a “very boring finance job” he’d landed right out of college and founded his own marketing services agency -- a proposition that turned out to be “a sort of a long failure,” he says. By 28, having watched another startup founder and sink, Lowe resolved to either get a real job or figure out how to actually launch a successful business. The result was Jumpstart Automotive Media, which handled ad sales for sites like Vehix.com and CarandDriver.com. After selling Jumpstart in 2007, he decided to bring the lessons he’d learned the hard way to companies that are committed to doing good.

“I have a passion for the environment and recognize that that is a big problem we have to face,” Lowe says. “But with a big problem comes a big opportunity.”

The Green Party gathered in Baltimore last weekend to choose a candidate who will go up against Barack Obama and (barring some strange GOP catastrophe) Mitt Romney in this fall’s presidential race.

No surprises here: Boston physician Jill Stein bested second runner (and former sitcom star) Roseanne Barr by a 41 percent margin, winning 193.5 of a total 294 delegates. (One delegate was apparently split between Stein and a third candidate.) Stein, who ran against Romney in the 2002 Massachusetts gubernatorial election and won 3 percent of the vote, is running on a platform centered on her Green New Deal, an ambitious plan that would guarantee full employment of all Americans at a living wage, develop a green economy based on renewable energy sources, tax banker bonuses at a 90-percent rate, and legalize marijuana.

In her acceptance speech Saturday afternoon, Stein railed against a two-party system that she says offers little in the way of alternatives. The U.S. is “at the breaking point, for our people, for our economy, for our democracy, and for our planet,” she said.

Stein’s vice-presidential running mate will be Cheri Honkala, who ran for sheriff in Philadelphia in 2011. In her acceptance speech, Honkala talked about being a homeless, single mom in Minnesota. After she lost her apartment, she and her son lived in her car, then, when a drunk driver totaled that car, sought refuge in an abandoned house during winter. The Green Party, with its promises of jobs and health care for all, was a natural fit for her and her values.

“We are the new and unsettling force that Martin Luther King spoke for,” Honkala said.

Jill Stein, the Green Party's presumptive presidential nominee.

The Green Party came cruising into Baltimore on Thursday -- er, wait, came riding into Baltimore. No. Had party members been able to walk and ride bicycles into Baltimore, I’m sure they would have, but even presumptive presidential nominee Jill Stein found herself riding in a jumbo jet in order to get here in a timely fashion.

But they made it nonetheless, and here they’ll stay for the next three days, holding workshops, fundraisers, and nominating their candidate for the highest office in the land. Barring any magical Roseanne Barr love-fest tomorrow at the nominating convention (the former sitcom star also tossed her hat in the ring), it will be Stein’s name on the ballot in, more than likely, 45 states by November.

In some ways it seems fitting that the Green Party chose Charm City as the location for its presidential nominating convention. Baltimore is sometimes forgotten to its bigger cousins, Washington, D.C., and New York City. It’s often seen as quirky and eccentric. And it’s easily stereotyped by the images we see in popular culture. (No, not every block is straight out of The Wire.)

Welcome to the Green Party, hanging on the heels of the Republican and Democrat parties, populated by an array of disparate interest groups, and written off by state election boards as unserious, tree-hugging, dove-releasing, organic-farming, grass-fed beef-ing … you get the point.

Don’t look now, but people in Japan are driving Hummers. By driving we mean pedaling. And by Hummers? We mean bicycles.

It’s true. An outfit called Global Innovation Company is distributing a line of bicycles bearing the names of foreign and American car manufacturers: Ferrari, Cadillac, Chevrolet, and Hummer, to name a few. The company was founded in 2002 by Katsuyoshi Ikeda, a man who thought younger cyclists would be more inclined to buy bicycles if they bore the logos of well-known, foreign car companies. Apparently no one told the Japanese that cars have lost their cool, because it seems to be working: Just last year, they bought 170,000 bikes flaunting the names of once-storied, combustion-powered four-wheelers.

For nearly 20 years, Alan Ehrenhalt served as the executive editor of Governing magazine, examining and writing about a variety of local and state-level trends and policies. In his new book, The Great Inversion and the Future of the American City, Ehrenhalt outlines at length what he dubs “a major change in American urban life” over the last decade: namely, that “living patterns are rearranging all throughout a metropolitan area,” something he calls a “demographic inversion.”

Ehrenhalt is no starry-eyed urban triumphalist (like Harvard economist Ed Glaeser), but nor is he predicting cities’ imminent demise (see Joel Kotkin). In fact, compared to the prophets of urban boom and doom, he’s a whole heap of downright boring nuance. Think of him as your teetotaling uncle at the family Christmas party -- the one who doesn’t want the eggnog spiked.

Living the dream in Copenhagen. (Photo by Mikael Colville Andersen.)

Once was that American cities competed to look more like Detroit, with gleaming lanes of highway stretching as far as the eye could see. Any more, it’s a race to imitate Copenhagen, the Danish capital where 36 percent of residents commute to work via bicycle.

So it seems, at least, when looking at today’s announcement by the League of American Bicyclists of the latest -- and largest -- round of official Bicycle Friendly Communities in the U.S. Some of the cities on the list will come as no surprise: Portland, San Francisco, and Chicago are here, as is Missoula, Mont., where 7 percent of residents bike to work, versus the 0.6 percent national average. But so are cities like Baltimore, Cincinnati, and Cottonwood, Ariz. Twenty-five more cities applied for bicycle-friendly status, but were denied.

The league hands down its Bicycle Friendly certification with a multi-tier, Olympics-like grading system: Cities can earn bronze, silver, gold, and platinum. The awards, which have been around since 1996, recognize cities that both promote cycling as a means of transportation and actively work to make cycling safer. A panel of national experts brought in by the league and local enthusiasts (bike shop owners, advocacy group leaders) assesses applications along five main criteria: engineering, education, encouragement, evaluation and planning, and enforcement.

The best cities, League of American Bicyclists President Andy Clarke says, have action plans in place to ensure that residents have opportunities to ride. They have city-sponsored bike rides, and networks of bike trails, lanes, and sharrows that connect them to where they need to go.

No guarantee that the bike you rent will be this stylin', unfortunately. (Photo by Velovotee.)

Forget riding your friend’s handlebars as he blindly navigates a crowded city street -- unless you’re into that sort of thing. Thanks to a new peer-to-peer bike-sharing website called Spinlister, you may soon be able to rent a bike almost anywhere.

The brainchild of co-founders Will Dennis and Jeff Noh, a pair of 20-somethings living in New York City, Spinlister is like peer-to-per car-sharing services such as RelayRides, only for bikes. Bike owners snap photos of their two-wheeled trophies and post them to Spinlister’s online marketplace, along with the type of bike, the price per day, and the pick-up location. For those in search of a rental, it’s as simple as punching in their location, selecting the ride they want, making an online payment/reservation via credit card, and coordinating a meet-up time with the bike owner.

Planting trees in West Philly. (Photo by Danielle Clarke.)

One Saturday in November, a few hundred volunteers descended upon parks and creek banks in and around Philadelphia to plant more than 2,000 trees. That day’s plantings were just a piece of a broader initiative to plant 300,000 trees in the City of Brotherly Love by 2015. And that initiative is but one part of a much larger program spearheaded by the Pennsylvania Horticultural Society that aims to plug 1 million trees into the ground across 13 counties in southeastern Pennsylvania, New Jersey, and Delaware. The mid-Atlantic is seriously putting the moves on Mother Nature.

As cities around the country jockey to be the King of Green, mayors and community organizations have been eager to claim their place as the next urban Johnny Appleseed. (Upon becoming mayor in 2008, Philadelphia Mayor Michael Nutter declared the city would become the greenest in America, and established an office of sustainability to show everyone he meant business.) But despite all the work days and feel-good volunteerism, urban forests are losing ground, in part because many, if not most, trees planted in cities die early deaths.

Photo by JB London.

Since gaining a foothold in the U.S. in the early 2000s, bolstered by pop culture (think: the opening scene of Casino Royale), parkour’s popularity has grown steadily, if slowly, in this country. Numerous informal organizations promote and teach this art of urban acrobatics, and a proliferation of YouTube videos show traceurs “freerunning” through the cityscape.

Still, many cities view parkour enthusiasts as lawless street ruffians, akin to skateboarders and street artists. In Manhattan’s Battery Park area, a “No Parkour” rule slaps practitioners with a $300 fine. In Hollister, Calif., anyone seen doing parkour is charged with trespassing. In November, the city of Margate, Fla., just north of Miami, banned the sport from all city parks, citing liability concerns. Mayor Pam Donovan said she thinks the sport is “dangerous and I’m never going to change my mind.”

Gradually, however, the sport seems to be gaining some acceptance -- as a tourist spectacle, if nothing else. Parkour competitions featuring professional traceurs have been around for a few years: The Red Bull-sponsored Art of Motion events are big both in Europe and the U.S. But could street parkour, like street art, come out of the shadows and become an economic boon for cities?