Northwest Airlines said its traffic in March declined 0.4 percent from a year earlier. The Eagan-based carrier flew 6.56 billion revenue passenger miles last month, down from 6.59 billion in March 2007. A revenue passenger mile is one paying passenger flown one mile. Load factor for the month was 88 percent, up 1.4 percentage point from a year ago. Capacity was 7.43 billion available seat miles, down 2.1 percent from a year ago. Year to date, traffic is up 0.7 percent from 2007, capacity is down 0.5 percent and load factor is 83.3 percent, up 1 percentage point. Figures do not include regional carriers Mesaba, Pinnacle and Compass airlines.