Sidewalk, curb, bike path, parking, traffic.
Of course we could never do this in Seattle. Because nothing that works anywhere else—even things that work everywhere else—works in Seattle. Our streets are too narrow (all of them), our hills are too steep (every street in Seattle goes oneway uphill), our weather is too rainy (except for the last six months), our cyclists are too crazy (totally crazy! totally crazy!), our drivers are too entitled (war on cars! war on cars!), etc., etc. Let's list all the reasons something like this couldn't work in Seattle—not even on wide streets, not even on flat streets—in the comments thread.
UPDATE: Whoops. I seem to have violated—brutally violated—Slog silence. In my defense, um, I'm in New York City and it's 1:30 PM here.
UPDATE 2: As Dominic wrote on Slog back in April, this is the kind of shit that ruined Paris.