"Presenting these things as trials helps people get comfortable with the ideas."

SPUR director Gabriel Metcalf said temporary projects often bypass the need for cumbersome and expensive environmental studies and outreach efforts, placing innovative urban design concepts on public display as ongoing experiments.

"They allow you to make adjustments. It's an option that exists with public spaces that you don't have with buildings," Metcalf said. "It de-escalates the fear people have over change."

Plus, as the project's director Kit Hodge notes, temporary placements let transportation planners and advocates try out new ideas instead of just endlessly studying them. As she put it, "San Francisco has a tradition of creating great plans that don't get implemented."

The prime example is Market Street, an inefficient street for automobiles and a dangerous one for bicycles and pedestrians, and one that has been subject to countless studies about how to make it more livable. Yet little changes. "You need to achieve as much consensus as possible," Peñalosa said, "but in the end, you have to take risks."

Or as Metcalf put it, "It's time to start enjoying some of the fruits of urbanity that we've been denying ourselves."