Most US carriers charge you if you want to check a bag, but not if you carry one on. As a result, people cram the overhead bins full of bags — a big reason it takes so long to board an airplane.

But the answer isn't letting people check as many bags as they want for free. Bag fees lower the base cost of tickets, giving people the option of packing lighter if they want to save money. They're also fair, given that fuel is a big part of the operating cost of airlines.

Instead, a better solution would be to charge passengers based on the weight of all the bags we carry aboard: our checked bags and our carry-ons.

The airlines' convoluted approach to luggage

Starting in 2008, largely in response to rising costs for fuel, most US airlines (excluding Southwest and JetBlue) began charging for checked bags. Perhaps in response to the backlash from passengers, however, most airlines still allowed people to bring carry-on bags for free (though Spirit Airlines charges for those, too). And for whatever reason, most will allow you to check bags right as you get on the plane for free, as well.

People respond to incentives, and this convoluted structure sets up some perverse ones. It means everyone has good reason to bring as many bags through security as possible, slowing down security lines. It's the reason there's a scramble for overhead-bin room during every relatively full flight. And it means that getting on and off the plane takes longer, because virtually every passenger is storing a bag in the overhead bins.

Engineers have devised a system for getting our bags into and out of the cargo holds of planes quickly, using codes, scanners, conveyor belts, vehicles, and people whose job is entirely focused on handling baggage. Yes, bags get lost sometimes. But the two-tiered charging structure means that instead of using this system, we're cramming a whole lot of luggage through a parallel system meant to put people on planes, causing all sorts of inefficiencies and delays.

Charging for all bags is fair — and will save fuel

The underlying logic behind charging for checked bags, though, is sound. Though the cost of fuel has gone down in recent years, somewhere between 20 to 30 percent of the cost of your ticket still goes toward fuel costs. And the more weight you bring aboard, the more fuel gets burned during the flight.

Paying for that fuel makes sense — and not just for the ideal of fairness. It has real effects.

In recent years, airlines have cut down on weight as much as possible, eliminating drink carts for trays carried by flight attendants, for instance. "I’ve sat in on airline fuel committee monthly meetings to discuss how we could reduce the weight on each aircraft in a fleet by a single pound," Andrew Kemmetmueller, the CEO of a company that's worked to replace heavy pilot flight manuals with iPads, told the New York Times. "Just one pound makes a big difference when you're talking about a fleet of 700 aircraft."

passengers have no incentive to save on weight

Passengers, however, don't have to pay for the weight of their carry-on luggage — so they don't have the same incentive to pack less of it.

But when you bring a few extra pairs of shoes across the country and back so you can decide on your outfit at the last minute, or you lug extra books just in case you finish the one you're reading, you're marginally driving up fuel costs for everyone on the plane. When millions of passengers do this as a matter of routine, it burns more fuel and makes tickets more expensive.

The way to solve this is to decouple the cost of tickets from the cost of bags. If all your bags were weighed before each flight and you were charged accordingly, you'd have the option of flying cheaper by packing lighter, and the people who carry a ton of baggage have to pay the full cost of it.

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