An airport is full of people who are either driving themselves home (using a car left in a nearby lot) or are being met by friends who are driving them home. Either way, the car probably has spare seating.

Suppose we had a way for the people who don't have a ride arranged to match up with people who live in the same neighborhood and do have a ride arranged. Then they could carpool home, with the spare passenger paying a few bucks to offset the cost of gas. You could do the matching-up over the internet. The tricky thing would be arranging the meeting location.

Airports could easily facilitate either this sort of thing or simply more efficient taxi-sharing by putting up some signs representing popular destination areas. "Downtown", "SOMA", etcetera. If you're going to SOMA, you go stand by the SOMA sign, meet a few other people who also are, and hail a single cab. Or carpool.