The trouble is that, 1) cars are much more expensive, when you tally it all up, than walking and bicycles, so one must count against them the time it takes to pay for them (and if one is very exactly accounting, one might also place some portion of time spent on dedicated, otherwise-unproductive aerobic exercise against cars' account), and 2) once a city's adjusted its girth to account for cars, the points most often factoring into ones point-to-point mobility are much farther apart than they'd have been without adjustments made
due to widespread use of cars. Notably this doesn't include only people choosing to move farther apart, but everything being
pushed apart by large setbacks from unpleasant roads, enormous highway interchanges, and gigantic parking lots. The car's value, even absent its effects on health and the environment, seems unavoidably to dwindle to very little if too many of them are in use.
Sparing use? Buses, perhaps, emergency vehicles, and possibly delivery trucks? Probably a solid, significant improvement. Individual automobile use on public streets in and around urban areas, and all the adjustments necessary for that to happen or consequent of car-influenced growth patterns? Way, way murkier than one might initially expect, but now very expensive and difficult to reverse regardless, so we're stuck spending a lot more to travel farther and save, if we buy cheap cars and have a high paying job, a little time.