It seems like a simple enough concept: A one-way award flight should be available for half as many frequent-flier miles as a roundtrip flight. An airline flight is not like a car rental, after all, where the vehicle must be returned to the rental location. While it makes sense that Hertz would charge a premium for one-way rentals to cover the extra costs of restoring the car to its original outlet, no such operational considerations explain United's charging a premium for flying one leg rather than two.

ROUNDTRIPS ONLY

In fact, for most of the 26-year history of airline mileage programs, award tickets were only offered in roundtrip form. So when their plans call for a one-way flight within the U.S., for example, travelers must cash in the full 25,000 miles required for a roundtrip and simply throw away the unwanted ticket coupon.

(The roundtrip-only rule has its roots in the airlines' famously convoluted pricing scheme for paid tickets. Advance-purchase coach tickets, which account for the great majority of ticket sales, are only available for two-way travel from most airlines.)

In addition to forcing consumers to effectively overpay for one-way flights, the roundtrip-only policy has other consumer-unfriendly effects as well. All too often when cashing in miles for an award trip, there are no seats available at the restricted level for one leg of the trip. So the would-be traveler is forced to redeem twice as many miles for an unrestricted award ticket. Another source of frustration: Miles generally cannot be used to travel in coach on one leg of a trip and in first class on the other leg.

There are, then, two separate but related issues. First is the availability of one-way awards. And the second is the pricing of those one-ways. Ideally, frequent-flier program members would be able to use miles for either one-way or roundtrip flights; and a one-way would cost half as many miles as a comparable roundtrip.

While travelers' hopes remain mostly unfulfilled, several airlines have modified their award ticketing policies recently to make one-way travel a more viable option. And there may be sufficient momentum created by those changes to move the industry standard in a friendlier, more rational direction.

DELTA LOOSENS UP

Among the largest airlines, Delta late last year began allowing members of its SkyMiles program to combine restricted and unrestricted awards, and coach and first-class awards, on the same itinerary. So, for example, members can now book a domestic flight using restricted first class on the outbound and return using unrestricted coach. The price would be 47,500 miles (half the 45,000 miles normally required for a restricted first-class roundtrip, plus half the 50,000 miles for unrestricted coach).

OTHERS FOLLOW SUIT

Delta isn't alone in allowing such award mixing and matching. US Airways permits Dividend Miles members to combine Mileage Saver (restricted), Premium (unrestricted), coach and first-class segments on the same itinerary for flights booked on www.usairways.com. And Northwest allows its members to combine restricted and unrestricted award segments, but not to mix classes of service.

Note, though, that while Delta's new policy affords increased flexibility within a roundtrip itinerary — and prices the individual legs fairly, at half the roundtrip levels — it is still not possible to just book a one-way award flight.

Last month, Alaska Airlines revised its award policy to permit Mileage Plan members to book one-way award travel on Alaska Airlines and Horizon Air for half the number of miles required for a comparable roundtrip.

As with Delta's new policy, Alaska's gives members the ability to mix and match restricted and unrestricted awards, combining coach and first class on the same trip. But unlike Delta, Alaska took the next logical step, allowing its members to book single-leg trips.

If this all sounds confusing, that's because it is. The solution is an industry standard which fully comports with consumers' common-sense notions of simplicity, flexibility and fairness.

That's the policy adopted by Alaska Airlines, and should be the model for the rest of the industry.

Reach Tim Winship at questions@frequentflier.com