The problem is really that cars are too space inefficient for what they do. Cars travelling at 50km/h should have at the very least 1s of travel distance safety distance between vehicles which is about three car lengths. In other words for the often single person travelling in a car you need about one lane width times 4 car lengths of space which is probably more than your average apartment size in most cities and unlike apartments roads tend to not be stacked 5 or 6 high on top of each other. And that does not even take parking and space to enter and exit each parking space into account. Not to mention that a lot of that space is unused outside the peak usage hours at any given location.