I don't think this is true. I lived in SF for a while (assumedly this meets your definition of built up), and there's plenty of space. The key is that cars don

I don't think this is true. I lived in SF for a while (assumedly this meets your definition of built up), and there's plenty of space. The key is that cars don't belong in most places anyway. A residential street can easily be design as a 10mph one way cobblestone road with calming measures, freeing up the "other direction" side of the street for a 2-way bike lane and expanded sidewalks. Larger roads can be redesigned similarly for bike/transit with cars filling in particular niches like delivery of bulk items, etc.