Before the new station at the airport opened on June 22, 2003, BART officials projected that it would draw 13,900 passengers a day.
But in July of 2003, the line's first full month of operation, it only drew an average of 6,500 passengers on weekdays and ridership levels have remained well below original projections.
Johnson said ridership climbed to 10,700 per weekday in May, which represents a 65 percent jump over July of 2003.
However, he admitted that level is still about 20 percent below initial projections.
Johnson said a major reason for the recent increase in ridership to SFO is a surge in European travelers who are coming to the Bay Area to take advantage of the weak dollar.
Johnson said BART is the only American transit agency reaching out to Europeans by offering vouchers for BART tickets through travel Web sites when they book their travel online.
BART Board President Gail Murray said, "Riding BART is already a bargain for us Americans, but for Europeans our world-class train-to-plane connection proves to be an even greater value for their money."
Johnson said ridership on the entire five-station extension to the San Francisco and Millbrae stations also is at record levels.
He said ridership on the line, which also includes the Colma, South San Francisco and San Bruno stations, was 37,200 per weekday in May, a 51 percent increase over July of 2003.