Ridership is up at all three Washington airports, with Baltimore-Washington International Marshall Airport leading the way on the strength of growth from Southwest Airlines.

Nearly 1.9 million people traveled through BWI in March, the airport’s highest-ever total for that month. The total is 3.3 percent higher than for March of last year and could portend the airport setting ridership records during the busy summer season.

More than 70 percent of BWI’s flights are operated by Southwest or AirTran Airways, which Southwest formally acquired last week, and BWI spokesman Jonathan Dean said that the airport’s growth showed a continued interest in lower rates.

“BWI has a number of low-fare airlines, which remain very popular for travelers,” he said.

Southwest served almost 1.1 million passengers in March, an 11.5 percent increase over March 2010. It started new routes to Charleston and Greenville-Spartanburg, S.C., that month. As of May it had 189 daily departures, and Dean said that number is scheduled to rise to 195 this summer, which could help the airline set passenger records during its busiest months.

Overall, BWI’s traffic increased for 13 straight months through March and for 21 of the previous 22 months. There is about an even split between the number of passengers who travel for business and pleasure, Dean said. Cargo shipments grew to 20.9 million pounds, up 6 percent from last year.

“There’s just been a general, steady picking up of business,” said Jim Dinegar, chief executive of the Greater Washington Board of Trade. Dinegar said the area’s strong economy led to more arrivals and that BWI was becoming a more frequent stopping point for connecting flights overseas.

He also cited the strength of airlines such as Southwest, whose operations at BWI he said gave the airport an advantage. “They did add trips into greater Washington, and, unfortunately, that might come at the expense of a Cincinnati, or a Chicago,” he said.

Ridership at airports across the country fell during the recession, but BWI, as well as Reagan National and Dulles International airports in Northern Virginia, are now showing gains.

At the two Virginia airports, ridership fell every year from 2007 to 2009. In 2010, though, ridership bounced back, with 41.8 million passengers. National served 18.1 million passengers in 2010, a 3.1 percent increase over 2009, and Dulles served 23.7 million people, up 2.3 percent from the year before.