Southwest Airlines today reports its traffic fell slightly last month.
But Dallas-based Southwest says a key measure of revenue jumped -- showing that carriers have had some success in raising fees and ticket prices.
Southwest, which carries more passengers domestically than any other U.S. airline, said it flew paying passengers 6.48 billion miles in April. That's a 0.6 percent decrease from 6.52 billion miles in April of 2009.
But passenger revenue per available seat mile, which measures how much money Southwest made to fly each paying passenger one mile, increased about 19 percent compared with a year earlier.
Besides raising fares, airlines can increase how much money they make per passenger by raising or expanding fees.