Another milestone for Southwest Airlines ( LUV). The airline Tuesday reported strong March traffic figures, including a record load factor for the month. The low cost carrier said its load factor -- or the number of seats filled -- was 73.6%, up from 67.3% in the year-ago period. In March, the Dallas-based airline flew 4.7 billion revenue passenger miles, or RPMs, which is one paying customer flown one mile. That measurement of traffic was up more than 15% from last year. Available seat miles, or ASMs, increased 5.4% to 6.4 billion from a year ago. Southwest also said April and May bookings are "strong in response to recent fare sales." Southwest shares were up 4 cents at $15.02.