How much can you condense a city if you have no roads at all? What I mean is that walking/biking/scootering in a city with no roads is probably twice, if not more, times more efficient.
You can compress a lot, but you can't compress away all the space currently used as roads. The space is important for allowing sunlight and fresh air to enter buildings and also acts as a noise damper in a community. Not to mention, some amount of visual privacy from neighbors. There is also the problem of emergency services (ambulances, fire trucks, police etc) being able to move around quickly. Finally, you should probably have some amount of bus/train transit in the city. Good reasons for it are bad weather, night time safety, tired or sick people, people with disabilities or long-term health problems and children.
I think the best you can do is that every building is next to one one-way car lane (shared with bikes going the same way) and one bike lane (for going in the opposite direction). The road should not be asphalt but some softer, less conductive material. No street parking. If you want to own a car, you can't externalize the cost to society but build a garage for yourself. Of course, there is a maximum size on the cars that you can buy and 30 km/h speed limit on most roads. You can have mini-buses (20ish people) and subways to take people around.
I think this is already a massive improvement on car centric cities. With the right distribution of retail and housing, you can have people enjoying a really nice life.
>you can't compress away all the space currently used as roads
You can however plant a lot of trees which would make a huge impact on the sustainability of cities by regulating the heat/cold much better. Also if you look at old European cities you can pretty much compress away all the car space. Jan Gehl's "Life Between Buildings: Using Public Space"[0] is a fantastic read if you're into thinking about this sort of thing.
How are you suppose to move goods around if you have no roads? On bike?, on foot?, by rail? Biking and on foot would be impractical for large items or many items, and if we used rail it would have to be all over the city, so your back to the same problem with roads but instead of cars and roads its trains and rail.
Design in distribution systems that are separate to human systems. There are plenty of ways you could do that from the superblock Barcelona style system to, if we're designing from scratch, hyperloop tunnels or similar sci-fi approaches.
