We don't have medieval cities, but saying cities were designed around the car is patently false.
Yes, the fifties and sixties caused huge redesigns, but we can undo those.
It's not. Cities have literally been created from nothing in the last few decades. Just because NYC, Atlanta, DC, etc. are old are have a history of redlining does not mean that every city does. Case in point: the cities around where I grew up were built in the last 3 decades. Those cities didn't have a choice but to have zoning laws and miles of road.
I live in Seattle, for instance, which used to have a network of rails. They've all been torn out and we're just now putting them back.
1. get rid of zoning laws that prevent building multi-family units.
2. invest heavily in public transit. Buses are great if they are widespread and arrive frequently.
3. in areas that have significant traffic, use trams/subways instead of buses. They carry way more people and don't sit in traffic.
4. make city centers dramatically more walkable. A 4 lane street is much harder to cross than a 2 lane street (or 2 one way 2 lane streets).
5. don't build new roads/highway exits that will encourage further sprawl.