/Currently, about half the petroleum used in the US is spent on gasoline and diesel for personal vehicles.  It seems that a lot of this is still being squandered, so we do have a chance to reduce consumption in a substantial way - - if people could be convinced to park their cars.
Of course, the paradox of this is that if people parked their cars, it would be the end of the "drive a car by myself anywhere, anytime" world that they had known.  As others have pointed out, the whole concept of the "end of the world as we know it" is too subjective to be meaningful.