When I visited Cedar Rapids, IA, the Avis counter had a sign up saying, "No one-way rentals at this time. Sorry for the inconvenience." I assumed that anyone who already had a confirmed reservation for a one-way rental would still have it honored. This is Avis, not U-Haul.

The reason such a sellout can happen is because, since the cars randomly circulate, it is possible more people are leaving a city at a particular time than coming in. What matters is the net inventory level in the long run meeting the locations' needs.

Think of it as a related rates problem from calculus. Cars are leaving Cedar Rapids at 5 per day. Cars are coming into Cedar Rapids at 2 per day. Today, uh oh, problem. Taking the limit in the long run, however, some days 20 cars come one-way into Cedar Rapids. Now there's an overflow. I saw this happen at Westchester Airport (hpn) in the Hertz lot once. I left one night and came back the next- holy cow, no place to park the returned car! So, in the long run, the inventory level will remain approximately steady.

The problem, though, is the renter needs a car right now. But, if the inventory drops below a satisfactory level at that particular instance in time, this location can't afford to "lose" any more cars for the time being. The moral of the story: It pays to have a reservation