The main issue is unless we ban cars from areas that benefit from pedestrian traffic, it's essentially impossible to keep pedestrians from cars. Given there will be a mixing of cars and pedestrians, who should we design primarily for: the person driving or walking? If we design for driving, then we relegate walkers to malls and parking lots. If we design for pedestrians, we relegate cars to narrow, crowded, slow streets, and possibly limit which streets cars are allowed on (or which streets through-traffic is allowed on). There isn't one "right" solution, but these are conscious decisions that communities need to be aware of.