I've been told by a consultant who worked with FAA that the safe distance for landing at a US airport was back-calculated from how long the runway is at National. Probably goes back to 707s.
Ironically, perhaps, 707s couldn't take off or land at DCA except at extremely light weights. Even today, some aircraft (the 737-900 being the most prominent) don't have the runway performance for some routes, even after the runway's been extended.
