As San Francisco’s Bay Bridge gets a makeover, a large section of the old bridge has been abandoned - but if architects Ronald Rael and Virginia San Fratello have their way, it won’t remain in disuse. They envision this span of the bridge, which is still in great shape and able to bear a lot of weight, being turned into a futuristic neighborhood reminiscent of the “bridge city” in William Gibson’s novel, Virtual Light.

‘The Bay Line’ would feature neighborhoods hanging on the underside of the bridge, topped with a series of public parks. This proposal (PDF) would not only give the abandoned bridge an active use and prevent it from being torn down, it would also provide compact urban housing and green space. ‘The Bay Line’ is inspired by bridge neighborhoods around the world, including Florence, Italy’s Ponte Vecchio.

“The immense load capacity of rail bridges allows for the support of program beyond that of parks, suggesting the urbanization of bridges,” wrote Rael and San Fratello in their proposal. “While the current economic climate suggests a surplus of housing, the economic reality also suggests a push towards urbanization and often the “affordable” housing constructed in suburban environments, which encroaches on the rural, is not what is needed.”

“Instead, by using abandoned bridges in urban areas, we are creating opportunities for sustainable low-cost housing within the urban realm—creating the potential for creative speculation among housing developers by expounding upon the nascent potential of a layered housing-park-bridge typology.”

While critics of Rael and San Fratello’s proposal point out the immense financial implications of maintaining this portion of the bridge, particularly with the extra weight of housing added to it, a similar – if smaller – project has already been successfully completed using abandoned infrastructure. New York City’s High Line Park, which opened in June 2009, turned an old rail system into an elevated park.