On a recent episode of the NBC sitcom Parks and Recreation, Leslie Knope, the unflappably optimistic deputy parks director for the fictional city of Pawnee, Ind., enthusiastically announced her latest plans for a new public space. “I want this to be the most amazing, awe-inspiring, fun-filled park ever conceived!” said Knope, played by Amy Poehler. How big was the proposed new project? As Knope proudly announced, “It is 0.000003 square miles.”

Leslie Knope’s plan for a tiny park was funny. (She explained that the vacant space had formerly been the site of a phone booth.) But her idea actually isn’t that far-fetched. In what’s become the latest trend in urban placemaking, cities across the country are converting public parking spaces into postage stamp-sized parks called parklets.

It all started in 2005, when a San Francisco design company descended on a downtown parking space, fed the meter and created a pop-up park complete with sod, public benches and leafy trees. They called it Park(ing) Day, which eventually became an annual event. Then in 2009, when New York City began converting some street spaces into pedestrian-only plazas, urban planners started to see the appeal of pint-sized parks. Officials in different places began working with local business owners to convert parking spaces. San Francisco cut the ribbon on its first permanent parklet in March 2010; today the city boasts 27 completed parklets with another 40 in the pipeline. In the past year alone, cities from Philadelphia to Oakland and Long Beach, Calif., have launched parklet programs; others, including Chicago, Los Angeles and Roanoke, Va., are exploring the idea.

In most cases, business owners pay for the construction and maintenance of the parklets, which vary in cost but average $15,000 to $20,000. Cities may offer design help or a little extra cash -- and of course, they give up metered parking revenues -- but most of the investment is private. For businesses, it’s a way to beautify their block and help attract more foot traffic. Cities see it as a next-to-nothing investment in innovative new public spaces. “For very little or no dollars, we can change the shape of our city,” says Andrew Stober, chief of staff in the Mayor’s Office of Transportation and Utilities in Philadelphia, which opened its first parklet last summer and is hoping to add several more this year. “It’s part of a larger movement in the city as we think about how to make it a more livable place.”

Some best practices have already emerged. Parklets work best in front of businesses such as coffeehouses and pizza places, which thrive on walk-up customers. They need to be placed on streets with relatively low speed limits, and they’re too obstructive to work on corners. They should be visually distinct: San Francisco requires that any benches or chairs in a parklet look different from the seating at the adjacent business, to reinforce the idea that these are public spaces rather than an extension of a private café or coffeehouse. Similarly, those cafés aren’t allowed to serve customers sitting in the parklets. And what works in one place may not work elsewhere: Parklets in West Coast cities can stay open year-round, while in Philadelphia, for instance, the parklets will be packed up and put away in the winter. (“We don’t want them to fall victim to a snowplow,” says Stober.)

The best aspect of parklets is that, because they’re so simple and inexpensive, cities can easily experiment with what works and what doesn’t, says David Alumbaugh, the director of the city design group in the San Francisco Planning Department. “The beauty of parklets is that they’re very transformative yet not very difficult.” Alumbaugh notes that the city renews each parklet’s permit annually, although so far none has been revoked. “It’s a chance for us to say, ‘Let’s just try it. If it doesn’t work, we’ll take it out.’”