(NYTIMES) For decades, a stretch of Memorial Drive here that runs along the Charles River has been closed to cars on Sundays for the warmer half of the year. In the absence of cars on a four-lane thoroughfare beside the water, all kinds of other street uses blossom: skateboards, bicycles, hoverboards, strollers, wheelchairs and walkers, people on feet and on wheels now moving slowly enough to witness the late spring goslings, the ever-present seagulls or the rarer magic and grace of a heron feeding along the water's edge. A towering line of stately, centenarian sycamores forms an unbroken canopy over several blocks.

This section of Memorial Drive in Cambridge, Massachusetts, is formally called "Riverbend Park" during its weekend closures, but it's not a park in any physical, structural sense. It's an open public space transformed into a park without any construction. State park employees arrive in trucks in the morning and again in the evening at junctures in the road, placing gates, cones and signs that cut off traffic. By dusk, the gates disappear and traffic returns. That's it - a park that is "found" from what's already there.