Columbia subject headings:
National security--Economic aspects--United States.
Tourism--Security measures--United States.
Transportation--Passenger traffic--Security measures--United States.
Harvard subject headings:
National security--Economic aspects--United States.
Tourism--Security measures--United States.
Transportation--Passenger traffic--Security measures--United States.
Princeton subject headings:
National security--Economic aspects--United States.
Tourism--Security measures--United States.
Transportation--Passenger traffic--Security measures--United States.