> Cities are welcome to erect whatever barriers they like to private cars.

This is inaccurate - for liability purposes, most road design in the US conforms to the MUTCD (and other design standards), which have very car-centric requirements. For example, the minimum lane width it allows is 11 feet (3.3m). If you design and install a 10 foot lane, you're not supported by that standard, and will likely get sued for damages if someone sideswipes someone, etc. The same applies for many other features that can improve pedestrian safety. It's one of the fundamental reasons north American built environment looks so much different (even compared with a car-centric design in other countries)