Among the many pieces of wisdom in Jane Jacobs’ 1961 masterpiece, The Death and Life of Great American Cities, one stands out a half-century later as a near-universal urban planning truth. It’s the idea that healthy communities are built on the face-to-face contact of their residents. Routine daily meetings of neighbors on the sidewalk foster public safety and social cohesion. “Lowly, unpurposeful and random as they may appear,” Jacobs wrote, “sidewalk contacts are the small change from which a city’s wealth of public life may grow.”

Twenty years after the publication of Jacobs’ book, the sociologist Mark Granovetter added some academic rigor to her anecdotal evidence. Granovetter studied a wide range of urban neighborhoods and found that the most successful ones were built on what he called “weak ties,” informal contacts among casual acquaintances who stop on the street to share news, gossip or simple good wishes. A robust array of weak ties gives city dwellers access to jobs, child care and practical advice, and it enhances their overall sense of well-being.

Sidewalk contacts require, at a minimum, the presence of sidewalks. But more than that, they require sidewalks to be places where people feel comfortable spending time -- gathering spots, not just thoroughfares. The best ones are visually attractive, inhabited by storefront commerce that’s fun to look at, and blessed with a diverse array of people to watch. Starting with these fundamentals, the concept of walkability has evolved.

It’s rare these days to find a city planning office that doesn’t include a commitment to walkability in its promotional literature. For the past decade, cities and neighborhoods have been able to access a website called Walk Score that tells them (and potential homebuyers) just how walkable they are in comparison to competing places nearby.

Still, walkability remains a bit of a slippery concept. Some neighborhoods seem to have most of the right physical attributes and yet lack any real vibrancy. Others don’t look like much and still manage to draw on an ample supply of local residents polishing their weak ties at all hours of the day and evening. Walkability brings to mind Justice Potter Stewart’s famous definition of pornography, “I know it when I see it.” But coming up with a precise definition of walkability is harder than you might expect. And on a more practical level, it raises a key question: How much can local governments and neighborhood activists do to create walkability through deliberate decisions?

All these issues come up in a provocative new book, Within Walking Distance, by Philip Langdon. A journalist who has spent the past three decades writing about cities, Langdon visited half a dozen American communities that seem to have achieved walkability, asking what their recipe was and whether other aspirants to walkability might be able to copy it.

The chapter that captured my attention right away was the one on Brattleboro, Vt. I happened to be there a few weeks ago, and saw most of the things that Langdon describes. It was a rainy Sunday morning, but the small downtown was full of walkers, many of them greeting each other casually in just the way Granovetter would approve. Every storefront I encountered was occupied, and passersby were darting in and out. The basement coffee shop I ducked into was crowded with customers. I checked Brattleboro out on Walk Score and found that one neighborhood that had been surveyed recorded a 90 -- basically off the charts, given that the average city gets a 48.

All of this is especially intriguing because Brattleboro doesn’t have any of the obvious features that tend to be proxies for walkability. It isn’t an affluent place; it ranks well below the national average in household income. It isn’t home to a major college or university. In fact, there aren’t any high-paying private employers of any kind. So what is Brattleboro’s secret?

Reading Langdon helps to unravel the mystery. A lot of the answer is geography. Brattleboro, population about 12,000, is an unusually narrow piece of territory nestled between the Connecticut River and a series of steep hills. There was never much room for it to spread out. Something like 90 percent of the residents live within two miles of downtown. The whole town is essentially within walking distance.

What’s more, as Langdon explains, the distinctive geography made the small downtown not only a magnet for walkers but also an engine of civic pride. When the Brattleboro Food Co-op threatened to move its headquarters to a suburban shopping center, a citizens’ group pressured it into constructing a new building on Main Street. When Home Depot opened up outside of town, it didn’t drive the downtown hardware store out of business; the opposite happened. So many locals refused to shop at Home Depot that it closed after four years.

The local government has done its share to tilt the balance in favor of pedestrians and against automobiles. There’s a public downtown garage that parks 300 cars, but it’s the only garage the downtown is going to get. There’s a city ordinance against building another one.

So layout is one thing that can make a community walkable. Ethnicity is another -- in particular, the ethnicity and culture brought to a city by a group of newly arrived immigrants. Little Village on the southwest side of Chicago is an old Czech neighborhood that became almost entirely Mexican over the course of the 1960s and 1970s. As an enclave of Spanish-speaking newcomers in the middle of a huge metropolis, Little Village suffers from many of the problems that might be expected: high unemployment, a troubling school dropout rate and sporadic gang violence. But its main commercial street is largely safe and bustling with the activities and contacts that would please Jacobs and Granovetter.

The reason isn’t difficult to find. It’s street vendors. Twenty-sixth street in Little Village is crowded from dawn until late at night with pushcarts selling a cornucopia of food: tamales in the morning, fruit a little later, and roasted corn at lunchtime and on into the dinner hour. The pushcart vendors create what Jacobs called “eyes on the street,” a benign vigilant presence sufficient to ward off troublemakers. The vendors create a corridor along which neighborhood residents feel comfortable walking their children to school.

One small irony is that most of the vendors are illegal. Until 2015, food vending on a public thoroughfare violated a Chicago ordinance. Since 2015, they have been allowed to operate if they have a license, but few of them do. They manage to stay in business anyway, and they create a walkable neighborhood that couldn’t exist without them.

Neither Brattleboro nor Little Village offers much of an object lesson to communities looking for the secrets of walkability. Geography and ethnicity are hard to duplicate. East Rock, a neighborhood in New Haven, Conn., presents a little more in the way of practical guidance.

East Rock’s walkability turns out to be, in part, a historical accident. It’s a neighborhood of homes that date, in some cases, back to the late 19th century. Most of the buildings in the neighborhood were built before the city enacted an ordinance that barred the mixing of residential and commercial uses. Pre-existing uses were grandfathered in. And East Rock’s residential blocks, unlike those in most other cities, were sprinkled with commercial businesses: schools and churches, but also taverns, hair salons, dry cleaners, convenience stores and dentists’ offices. A neighborhood pharmacy doubled as a post office. The whole arrangement already made for sidewalk vitality that few neighborhoods anywhere could equal.

In the last couple of decades, East Rock’s inhabitants have had the audacity to take this arrangement and extend it. They have turned commercial spaces into cafés with patio seating out front. The first one opened in 1991, in an old shoe repair shop. It became an informal gathering place for local politicians and a starting point for bicycle treks. The city government, skeptical at first, eventually agreed to give out grants of $20,000 to $30,000 to businesses that wanted to expand outdoors. By Langdon’s count, there are now seven of these café patios within a relatively small distance. Virtually everyone who uses them gets there on foot or on a bike.

Residents of Northern Liberties, a gritty old rowhouse section just northeast of Philadelphia’s Center City, also took things into their own hands. They turned a two-acre plot of rubble created by the demolition of a tannery -- land encumbered by $1 million in back taxes -- into a public park. They planted 60 trees in the first year, then added an herb garden and a butterfly garden. They paid for it themselves. Eventually, the city cancelled the tax liens. Two decades later, Langdon reports, half a dozen people spend five to 10 hours a week keeping the park up. The homeowners there seem convinced that the park and the enthusiasm it generated are part of the reason the area reversed several decades of population losses and became one of the city’s fastest-growing and most walkable neighborhoods over the past 15 years.

In the end, it’s sort of a nature/nurture argument. History and geography matter. The physical character of a neighborhood is probably the most important factor in creating the kind of community that Jacobs wrote about. In Philadelphia, Langdon says, “narrow, slow-traffic streets with slender rowhouses bumping up against the sidewalks helped residents get to know each other.” But creativity matters as well. So does audacity. Existing regulation and bureaucratic inertia sometimes bend to neighborhood cohesion and determination. Most of the successes in Langdon’s book are testimony to that.