Average over what time-scale? Having a car allows you to shift all that time spent to periods when it's more convenient for you. If I want to get from Tucson to Globe, I would rather be able to do that in 3 hours and spend time at work earning the money to pay for the car, than having to do it in the 25 hours it would take me to walk the distance.
There's a similar calculus involved in hybrid or battery-powered vehicles. It may be true that there's no net pollution gain from battery-powered cars. But over small enough space scales, they can make a huge difference. Los Angeles would breathe a lot easier, yes, at the expense of having to figure out what to do with all those batteries. Didn't we have a discussion here about whether or not the third world is vastly underpolluted? :-7
-- John S Costello joxn.costello at gmail.com "All for ourselves, and nothing for other people, seems, in every age of the world, to have been the vile maxim of the masters of mankind." -- Adam Smith