U.S. airlines may bump the most passengers in nine years.

Almost 220,000 passengers couldn't get on flights in the first quarter of this year even though they bought tickets, 25 percent more than a year earlier, Transportation Department data show. At that pace, so-called denied boardings in 2010 would surpass 2009's 762,400 and reach the highest total since 2001.

The increase stems from a traffic revival at carriers led by Delta Air Lines and American Airlines, which haven't yet restored reductions in capacity made during the recession. Continental Airlines and Southwest Airlines said their planes flew fuller than ever in May.

Pent-up demand for summer vacations means there won't be many empty seats in the coming months, probably resulting in more bumping, said Rick Seaney, chief executive officer of Dallas-based ticket-research firm FareCompare.com.

Airlines routinely sell more coach tickets than they have available, betting that not all passengers will show up. About 89 percent of first-quarter bumpings were voluntary, with travelers accepting inducements such as vouchers to switch flights. The rate of involuntary bumpings jumped 37 percent from a year earlier to 1.73 for each 10,000 passengers, the DOT said. The full-year 2009 rate of 1.19 was a 13-year high.

On June 2, Transportation Secretary Ray LaHood proposed raising the maximum compensation by 63 percent to $650 if passengers bumped against their wishes arrive at their destination within two hours of their intended schedule, and to $1,300 if not.

The Washington-based trade group for large U.S. carriers defends overbooking as crucial to airlines' revenue.

"Based on the disappointing recent lack of profitability in the airline industry, airlines cannot allow reserved seats to be unfilled when an aircraft leaves the gate," said Victoria Day, a spokeswoman for the Air Transport Association.

Carriers can generate $5,000 or more on some flights by selling an extra five or 10 tickets even when the plane is already booked full, said FareCompare.com's Seaney.

The extra tickets often are sold to walk-up business travelers willing to pay the highest fares, said Brett Snyder, a former analyst and manager at UAL Corporation's United and America West Airlines who is now president of the CrankyFlier travel blog.

"Airlines are deciding whether it's worth taking on one more person paying a much higher fare vs. the cost of a bump," Snyder said. "The higher fare usually wins."

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