In fact, I think that you can make the Web Application a bit more humanistic. For instances:

1. Everyone now is so concerned about Global Warming. Maybe you can add a "Carbon" bar on each user on how much carbon they save when they take a car pool instead of driving their own cars. Same, you can tell the people who are willing to offer the car pool on how much carbon they save by offering a carpool.

2. Make a small social circle among the users. Usually, people who take car pool are people who will go and forth from two different spots quite frequently. I think people will always go back to the same car pool with the same driver.

Anyway, just my 2 cents.