That is only true if the costs of the return trip is free: it isn't. When the car for the guy who arrives downtown at 7am goes back to the suburbs to get the guy who wants to arrive at 8am it is burning nearly as much fuel as it did getting in, and putting just as much wear on the car. The effect is shared cars are more expensive than just a car per person. (Note, this does not account for cost of parking which is a significant factor)