Re: backwards compatibility - Kevin Kelly writes in _What Technology Wants_[0] that Romans impacted the space shuttle - Roman roads were the model for most European roads, and were themselves based on the width of two horses walking next to each other, per "lane" (main use case of the time). Europeans brought this notion of roads with them to the US and built similar roads. Early cars had to fit on these roads, and early car roads were built on top of these horse roads. Eventually, when the space shuttle was designed, shipping its parts from factories across the company (on roads) had to be factored in. Therefore there were size/shape constraints on the shuttle that have their origins in Roman horse drawn carts.
(I'm recalling from memory, Kelly's explanation was probably better. Not something I've researched beyond finding that anecdote interesting.)
Space Shuttle parts manufacturing was distributed around the country for political reasons to ensure continued support from Congress. It would have been safer and cheaper to manufacture all of the large components close to the launch site, or at least at sites reachable by barges. The Challenger disaster was caused by a booster o-ring failure but there was no technical need to build the boosters in pieces joined by O-rings; they could have been assembled as a single large unit.
This has been attempted in a few places. Curitiba, Brazil comes to mind.
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.
Not eliminate roads. If anything actively acknowledge their utility and work to specialize/classify them for discrete applications. Right now mopeds and 18-wheelers share the same space. Commuters and emergency vehicles. Bus stops overlap bike lanes. Professional commercial drivers share the road with barely trained drivers who passed a test.
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...
Cars are very well executed machines, but I've always wondered what alternatives could happen if there wasn't the weight of backwards compatibility.