The central fact about cars, from a planner’s perspective, is that they take up space. Lots of space. And this matters because space in cities (a.k.a real estate) is scarce and therefore expensive.
Cars take up space when they’re moving and they take up space when they’re parked, and even though they can’t be simultaneously moving and parked, you have to plan for both states and plan for peak demand; so you have to set aside some multiple of the real estate actually occupied by the car at any given time.
That’s just a practical observation about the spatial geometry of cities that doesn’t bow to my ideology or yours. And it would still remain true even if cars ran on nothing but recycled newspapers and emitted nothing but rainbows and unicorn tears.
In the past, our policy response has been to just set aside more and more space for cars: More freeways, more roads, more lanes on existing roads, more parking garages and surface lots. This approach hasn’t worked, and there are two very practical reasons why:
First, you can never build enough. There’s a phenomenon called "induced demand" that is very well understood by now. A new lane or a new freeway never reduces congestion in the long run: People respond to new capacity by driving more or by living or working in previously remote places, and you’re very quickly back where you started and have to build still more. The same phenomenon applies to increases in the supply of parking. It’s a game you can’t win.
Second, when you do make more space for cars you quickly start to crowd out any other potential mode of transportation, especially walking. All those parking lots and freeways and roads spread everything else out so that the distances become too great for walking. And the more you optimize any given space for cars the more hostile that space is for pedestrians. Very quickly you get to the point where it becomes impossible—or prohibitively depressing—to get things done on foot.
And this last fact has huge quality-of-life implications for human beings—not just because driving to a distant strip mall for a gallon of milk is less pleasant than walking to a corner store, but also because for many people driving simply isn’t an option.