DALLAS — Lengthy airline delays are twice as common now as in 1990 and will get worse as the economy recovers, according to a Brookings Institution report released today.

The researchers said much of the problem is because of heavy concentrations of short trips between big cities, but they also cited an "ill-equipped" air traffic control system and other factors.

They suggested increasing high-speed rail service to offer travelers alternatives to short flights. They also recommended letting busy airports charge fees on rush-hour flights to make airlines spread trips more evenly through the day.

According to Brookings, 10.1 percent of all flights now arrive at least two hours late, up from 4.3 percent in 1990. The average delay is nearly an hour, 41 minutes longer than in 1990.

On-time performance has improved lately because airlines have cut flights during the recession. But as the economy recovers, so will air travel — meaning more delays — said Adie Tomer, co-author of the report.

Tomer blamed congestion on big airlines' reliance on hub-and-spoke networks that push more flights into crowded hub airports, and on the growing number of short flights between big cities such as New York-Boston, New York-Washington and Los Angeles-San Francisco.

The report also criticized the federal government for directing only about one-fifth of airport improvement funding last year to the 26 metro areas through which most passengers travel.

The researchers recommended the creation of a federal commission to decide the best way to improve efficiency of the nation's air-travel system.