[electricblue]

particular small areas with extremely high pop density might improve if cars were banned but for the most part adding separate bike paths that don't interact with roads much is the way to go. The sloped apartments seem dangerous and stupid.

[ctdonath]

particular small areas with extremely high pop density might improve if cars were banned

Cars, on the whole, occupy a single flat level. Why don't buildings spread out over roads as the norm? rather than the occasional oddity of a road tunneling thru a building, why isn't that the norm? Ventilation etc problems can be solved. There's N stories of unused space over the roads; leave the first 2-3 stories for road tunneling, and fill in the building-sized empty spaces above.

[electricblue]

Probably because it's prohibitively expensive compared with just building a larger building next to the road and it's aesthetically pleasing to have space between buildings and humans like sunlight.