Unfortunately this will make the pollution worse. For example if I drive my car to work, my car does 6 miles round trip .
If someone else does the same in total we did 12 miles. However if we share one car and the car needs to go even a mile between two places, it will have to drive significantly more.
In the situation you propose, you have would-be car owners who now share a car. That eliminates the pollution involved in the manufacturing of one car from the story. And yes, if the car had to make back and forth trips from a central location, that would be inefficient. But I think the idea is to have a network of shared infrastructure (similar to uber), so that if someone needs a car, they can have access to the closest available one.
