The urban environment is never a 'clean slate' Transforming cities to the modernist ideal required displacing existing urban development. Still-functional urban development, dominated by subways, streetcars, elevated railways, and pedestrians, resisted encroachment and sabotaged the creation of the auto-centric utopia.
This is worth remembering--the new transit metropolis will be impeded by the old automobile metropolis as it was once impeded by the old transit city. If the pattern holds, the biggest change will be in right of way, and not in urbanized area. The built environment changes very slowly, over generations, and as long as the urban form remains, so must the transportation system to serve it. Highways will not go away or casually vanish.