CHICAGO — Every airline passenger is entitled to overhead space, right?

Wrong. On a typical domestic flight, six passengers share luggage bins that fit four wheelie bags, at most, leaving some fliers out of luck at a time when more of them are opting to lug their bags, rather than check them, to avoid airline fees.

There are also more passengers competing for that space because planes are again filled to near-record levels, the result of carriers' capacity cuts and a rebound from last year's recession.

Boarding lines, rarely speedy, now often move in reverse when the last luggage bins fill and passengers are forced to back off a plane and return to the jet bridge to check bags.

The next obsession, at least for passengers of Spirit Airlines, may be cramming items under airplane seats. The Florida discount carrier said Tuesday that it will charge customers as much as $45 each way to place bulky items in overhead bins, in an effort to get people on and off its planes faster. Other airlines will watch Spirit's experiment.

Airline staff and passengers are still trying to figure out how best to cope with the changes in boarding and behavior resulting from new fees on checked baggage, which were widely adopted as the travel market fell into a tailspin in 2008.

Since the start of last year, the number of bags checked at the boarding gate by Chicago-based United Airlines has risen nearly 50 percent, while the volume of bags checked at ticket counters has dropped 18 percent. At American Airlines, more passengers now carry on bags than check them.

"Flying definitely has changed over the last 18 months," said Tom Parsons of BestFares.com, a low-cost-travel Web site. "It's a roller-bag derby."

A year ago, when many flights were only two-thirds full, only four people sat in the six seats that share a bin.

Now, "in effect, you have 50 percent more contention for overhead space. What's fine for four people isn't for six," said aviation consultant Robert Mann, president of R.W. Mann & Co.

"When you compare the storage space available onboard today to 20 years ago, the per-passenger number has to be double, even triple, what it used to be. And, yet, it's never enough."

Both the fees and space constraints can contribute to a breakdown in social conventions as passengers feel like they're left to fend for themselves.

"It's survival of the fittest," said Shelly Casale, a software consultant from Des Plaines, Ill., as she boarded a United Airlines flight at O'Hare International Airport last week.

Cabin baggage has been a growing inconvenience for airlines and passengers alike since the first wheeled luggage rolled onto the market in the early 1990s.

"The truth is we've never had a good handle on this," said Darryl Jenkins, founder of The Airline Zone, a Web site devoted to airline economics.

Carry-on bags didn't become the primary luggage for passengers until carriers introduced fees for infrequent fliers and then raised them to $25 to check a first bag and $35 for a second item. United, among the first to adopt the fees, has seen the volume of checked bags fall for 25 consecutive months, said a company vice president.

Every major U.S. airline except Southwest Airlines has introduced such fees since 2008, and no wonder: The 10 largest U.S. carriers collected $739.8 million in baggage charges during the third quarter of 2009, double prior-year totals, according to the Bureau of Transportation Statistics.

As planes fill and tensions rise, carriers are exploring ways to ease congestion in their aisles. American offers a valet service so travelers on its Eagle regional jets can easily hand off bags that don't fit overhead.

Airplane manufacturer Boeing Co. is finding a growing market for the new luggage compartments it created for its 787 Dreamliner and revamped for its 747 and 737 jets. The hinged bins handle far larger bags than current compartments. Dozens of airlines have purchased the new 737 interiors.

United is also assigning teams of workers to flights most prone to baggage meltdowns during peak holiday travel periods.

The gate agents and ramp workers nab boarding customers with carry-on bags as the overhead bins fill and quickly tag and cart away that luggage to the cargo hold. They were on hand last week as Casale waited among the final group of passengers to board a packed Boeing 757 headed for Boston. As she stepped down the jet bridge, there were still 27 people waiting to follow her to the few remaining empty seats, and all but a couple of them clutched black roll-aboard bags.

On board, a flight attendant hurriedly repacked overhead bins at the rear of the plane, returning coats to their owners and tilting bags on to their sides to maximize the little remaining empty space. Gate agents, meanwhile, checked about 15 bags from the final boarders.

This is how it is supposed to work. But it's not always the case.

Carry-on complaints to the U.S. Department of Transportation show people irked and sometimes victimized by the bad behavior of other passengers or airline workers: belongings jammed in overstuffed bins falling out and striking travelers on the head, jewelry stolen from bags checked at the gate, and airline workers arbitrarily enforcing bag size limits.

Overhead space typically starts to become a concern for flight attendants about halfway through boarding, said Sara Nelson, a United flight attendant and spokeswoman for its flight attendants union.

The stress builds as the plane fills, since neither flight attendants and pilots nor gate agents want to be blamed for a late departure. Injuries are likeliest in those last minutes before the door closes, as flight attendants rush to stow bags.