IMO autonomous cars are going to massively increase car usage so car issues are going to get worse not better. Maybe we won't need parking lots but if autonomous cars happen people will use them for anything and everything. No more need for couriers just pop the package/document in a car. Want to run a business out of your house? No need to delivery people just pop your product in a car. Cook for sick relative, pop the meal in a car. Need grandma to watch the kids tonight, pop them in car.

Maybe some of that will go to drones but the point is that a device that shows up at your door and goes to any other door in the city for opens a huge number of new opportunities.

We could redesign the cars to be smaller and to drive more densely if they were fully autonomous and electric. Their footprint would then more closely resemble bikes.

Or maybe someone will invent self driving bicycles/scooters for delivery tasks.

We could, but I don't think we will. There are a lot of fixed costs in a car, so if you need a big car for just a few % of the uses it is very hard to justify having both a small car and a big car when the big car can do both.

A lot of car overhead goes into safety: they have to be big and heavy to counteract other cars that are big and heavy. But if the they are given their own routes and are controlled by the same software, a lot of that girth could easily go away.

You need a big car to haul plywood, or all of your equipment for a weekend of camping. (backpackers get by on much less, but if the car is next to the tent you tend to pack a lot of luxuries)

Safety is a factors, but that is more weight than size, 4 full grown adults take up a lot of space.

Exactly. The answer isn't public transport, but adequate public transport.

Of course if we ban cars we can't expect everyone to use a bike instead, or to rely on current public transport network. Train, tramway, metro, bus (, something new?) all need to be vastly reinforced (something between 10x and 100x maybe?) but there's room for that once cars are out of the picture (both from roads and parking space).

Cars are convenient, but not efficient at scale (and too dangerous, for now at least). We need another solution, and I think it has to be radical to really work. But then, banning or greatly reducing car use in cities will be a tough one..