Short answer. A single-aisle aircraft has one corridor, with little room to overtake. When a passenger stops to lift a bag into the overhead bin or waits for a seated neighbour to stand up, people behind them stop too. Those stops add up across the cabin. Walking takes only a small part of the total time; the rest is waiting.
One aisle, no overtaking
On an Airbus A320 or Boeing 737, almost every passenger uses the same aisle, which is about the width of one person. An unobstructed walk from the door to the last row takes well under a minute. Boarding takes much longer because the aisle behaves like a single-lane road: when someone stops, people behind them stop too.
Simulator result: On the reference flight used across these articles (easyJet A320, 175 passengers, typical cabin luggage, forward door only, median of seeds 20260801 to 20260808), passengers boarding back-to-front spent a median of 88 seconds standing still in the aisle because somebody else was in the way. Across the flight, that amounted to about 258 person-minutes of blocked time.
The first kind of blocking: bags in the aisle
When you reach your row, you stop and put your bag in the bin. While you do that, you occupy the aisle, and people behind you wait. This is called aisle interference. It grows with every bag carried on board. Simulator result: On the reference flight, removing all overhead-bin bags cut a random boarding from 14:50 to 10:10.
The cost depends on where the people behind you are going. If the next passenger is heading for a row far behind you, they are stuck. If they are heading for a row in front of you, they have already sat down. The boarding order therefore affects how many people are trapped behind each bag.
The second kind: squeezing past a seated neighbour
If you have a window seat and the aisle passenger in your row is already sitting, they have to stand, step into the aisle and let you in. The aisle is blocked while that happens. If both the middle and aisle passengers are seated, it takes longer still. This is seat interference, counted as squeeze-bys in the simulator.
Simulator result: On the reference flight in the simulator, back-to-front boarding produced a median of 64 squeeze-bys, compared with 66 for a random queue. Orders that seated window passengers first (WILMA and the Steffen method) brought the count down to 3.
Why boarding zones often make it worse
Back-to-front zones feel logical: the people at the back sit down first, so nobody who boards later has to squeeze past them in the aisle. The problem is that everybody in the zone is heading for the same five or six rows. They arrive in the aisle at the same time and queue behind whoever is stowing a bag, while the aisle in front of them stands empty.
Simulator result: On the reference flight in the simulator, back-to-front boarding took 19:15. A random queue, with no groups at all, took 14:50. Its passengers were spread across the cabin, allowing some to stow bags at the same time.
Research finding: Published boarding studies and physical experiments with volunteers also report that zone or block boarding is not clearly better than a random queue.
What actually helps
Fewer bags in the cabin. Simulator result: In the simulator, the same random boarding took 10:10 with no overhead bags and 19:08 with heavy carry-on. See How carry-on bags slow down boarding.
Spreading each group along the cabin. Orders that send consecutive passengers to rows far apart let more people stow bags at the same time. Simulator result: The Steffen method, which spaces passengers across rows, took 8:21 on the reference flight in the simulator.
Windows before aisles. Seating window passengers first reduces squeeze-bys without requiring a strict line. Simulator result: WILMA took 12:31 on the reference flight in the simulator.
A second door. Front and rear doors give passengers two routes into the cabin. Simulator result: In the simulator, a Boeing 787-9 boarded with airline groups took 13:33 through the forward door alone and 9:38 through front and rear doors.
Frequently asked questions
Why do people stand in the aisle so long when boarding?
The person in front of them may have stopped to stow a bag or let someone into their row, leaving little room to pass. Much of the time spent in the aisle is spent stopped rather than walking.
Why is back-to-front boarding slow?
Passengers in the zone that was just called head for the same few rows, so they queue behind one another while the aisle ahead is empty. A random queue spreads people out and, in this simulator, finishes faster.
Do boarding groups make boarding faster?
It depends on how passengers are grouped. Groups that spread passengers along the cabin or seat window passengers first can reduce delays. Groups based on fare, status or cabin zone also serve other purposes; in this simulator, the conventional airline order took 19:26 on the reference flight, compared with 14:50 for a random queue.
Updated 2026-09-25. Figures labelled simulator result are outputs of this site's model at the settings stated, not airline measurements. What the model leaves out.