Do car-sharing programs reduce harm to the environment?

Where I live there are these car-sharing programs. People reserve the cars for an hour or two at a time. They are parked in spaces all over the parts of the city where people rent them.

The theory is that you use public transportation most of the time, but for those times when you have to have a car, you use the car-share. In this way, you save money—you don’t have the expense of maintaining a car that sits on the street most of the time. You don’t have to buy the car, or keep it maintained or insure it.

I suppose if everyone—or at least a significant portion of the population—did this, there would be a lot fewer cars around, so the environmental cost of building all those cars people no longer needed would disappear.

Would that reduce pollution? Or is that pollution that would happen anyway? Does it matter who owns the car if the same number of miles get driven? Is that just a feel good kind of program, or does it make sense? How or why?