To undertake all the operational and regulatory requirements for air travel, airports have a highly organized sequence of vertical and lateral passenger flows. Each of the requirements generally generates a queue since it can process a fixed amount of passengers and since flows fluctuate during the day. More stringent security requirements and a higher prevalence of carry-on luggage have substantially expanded queues at security checkpoints.
From ground transportation, a passenger undertakes several steps and queues, some of which can be circumstantial. For instance, a domestic traveler using carry-on luggage and having checked in online with an electronic boarding pass would go directly to the security queue at the checkpoint. Security queues are often segmented according to fare (e.g. business class) and security pre-clearance programs. Inversely, an international traveler with several pieces of luggage would need to queue at the check-in/luggage drop counter, go through customs (sometimes after security), and then through security. Because of queues and delays, passengers are spending an increasing amount of time at airports, and activities taking place in waiting areas, namely restaurants, and shops, have assumed a higher prevalence as they become an important source of revenue. The last queue is the boarding process, which is often segmented by fare class. Boarding queues have also become a source of revenue for airlines selling earlier boarding access. The last queue usually involves the plane taxiing to its runway take-off slot.
On the inbound side, the first queue involves deplaning and a less complex sequence for an inbound plane. Domestic travelers go directly to baggage claim (if necessary) and ground transportation (taxi and public transit queues). International travelers queuing through customs (immigration), then baggage claim, and finally customs inspection (luggage).
With the emergence of large hub airports, a lateral array of flows has increased. The simplest lateral flows are domestic or international transits where passengers move from a deplaning to a boarding gate within an airport terminal. For instance, Atlanta is a large domestic hub, while Amsterdam is a large international hub. In both cases, passengers are moving freely within the terminal. On occasion, a security checkpoint will be required for international transit passengers. Passengers undertaking an international to domestic transit often have to go through the full customs/baggage claim procedure and then through domestic security. Domestic to international transit is simpler but can involve going through the customs exit checkpoint.