The grade-separated pedestrian systems built in the 20th century have a variety of names: skyways, skywalks, pedways, footbridges, the +15, and the Ville Souteraine. But they have one thing in common — they have radically altered the form and spatial logic of cities around the world. North American cities like Minneapolis, St. Paul, Des Moines, and Calgary have extensive skyway systems that parallel the original streets. Montreal and Toronto have subterranean labyrinths. Hong Kong has floating three-dimensional circuits that connect transit stations, shopping malls, office towers, and parks. And multilevel urbanisms continue to expand. In the last decade, Mumbai has relieved its crowded streets by building nearly 40 pedestrian overpassess. Yet even as such infrastructures proliferate, they receive scant critical attention, despite their fundamental role in the production of urban space.
Since the 1960s, seventeen notable pedestrian systems have developed in the United States and Canada. Whereas early experiments in multilevel urbanism belonged to social utopians and the architectural avant-garde, the systems that were built at scale were advanced by pragmatists who embraced both the public and private spheres. As cities struggled to withstand the growing economic power of suburbs, civic leaders began to “pedestrianize” their urban centers. What better way to compete with the suburban shopping mall than to mimic its enclosed form? Cities began to connect and consolidate interior spaces through skybridges and arcades, in an effort to make downtown convenient, comfortable, safe, and climate controlled for office workers and shoppers.
These pedestrian systems can be conceived as a “thickening” of the street level or a “delamination” of the ground plane into a second level above or below grade. This doubling, or sometimes tripling, of the street was described by urban critic Trevor Boddy in the 1980s as an “analogous city.” The spatial ambiguity created by the stacked circulation levels — which often lack sufficient vertical connections — can render the urban layers as fully independent realms. For that reason, skyways and tunnels are seen by many theorists as deviant or untenable urban forms. […]