Passenger traffic at Des Moines International Airport should grow 6 percent this year, logging 2.5 million departures and arrivals, said Kevin Foley, executive director and general manager. “Remember, 2.5 million is where we get busy,” Foley reminded airport board members Tuesday. He referred to studies suggesting that as the airport grows, it will have trouble providing enough restrooms, baggage handling space and checkpoints in the present terminal, at least at peak times. The airport is planning a new terminal in a broader project that could cost close to $500 million. Annual growth of 2.5 percent or so in passenger traffic has been assumed in the projections backing the project.