Cars are very well executed machines, but I've always wondered what alternatives could happen if there wasn't the weight of backwards compatibility.
(I'm recalling from memory, Kelly's explanation was probably better. Not something I've researched beyond finding that anecdote interesting.)
If by "greenfield" you mean "eliminate all roads", then I think you'll find yourself adding them back in at some point. I've been involved in a few of those design brainstorming sessions around urban transportation and every time it comes down to emergency services. If you need to get somebody across a city as fast as possible in a situation where seconds can mean the difference between life and death, then you need roads and cars.
A greenfield design would be something like this:
Make access/collector roads slower.
Personal vehicles smaller/lighter more like golf carts and e-bikes. Limited to 25mph. For use on neighborhood access and collector roads. The lower speeds and lighter weights allow these vehicles to shed a lot of complexity/cost. Licensing can remain easy, and insurance costs are reduced. Parking requirements are reduced.
Make arterials and highways larger/faster.
Service vehicles, freight and public transportation could be less constrained by lane sizes. Your 'bus' is now two lanes wide and can carry the smaller personal vehicles as cargo (like ferry service on land). The bus has fewer stops because it can expect passengers to travel further under their own power, rather than stop every block. Arterials/highways are for 'professional' uses. Freight, Emergency Services, Public transit. Licensing requirements are more strict etc...