Speedway Field was the original name for the airfield that was to evolve into Minneapolis-St. Paul International Airport, the twelfth busiest airport in the United States; it was also the largest hub for Northwest Airlines and the third largest hub for Delta Air Lines, Northwest's successor.
There are 14 airports in the United States that are busier than the Minneapolis-St. Paul International Airport.
It was the 12th busiest, therefore there are only 11 busier airports. It's difficult because it requires complex understanding of what the "12th busiest" airport really means.