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.
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.