For decades, airlines around the world have spent untold fortunes analyzing passenger behavior and plane design to come up with the best way to squeeze all those people into a jet in a timely fashion, and yet no one has cracked the code and passengers still get stuck in long lines, often because of the same old reasons.

While there are likely a large number of causes for lengthy boarding times, here are five that we think are the biggest culprits:

1. Bullish passengers and pushover gate attendants
Unless you’ve never flown before, you know that passengers are supposed to come up according to their group numbers and that the gate attendants are supposed to turn away travelers whose groups have not yet been called.

But the only time this seems to happen with regularity is when the plane is boarding First and Business passengers. Once it gets to Economy, it often turns into a free-for-all, especially once people notice that some guy with a Group 4 ticket got through with the Group 2 passengers.

By letting these travelers jump the line, the attendants have made the boarding procedure less efficient. If it’s a front-to-back boarding plane, these people will now be blocking the aisle rows ahead of where passengers behind them are trying to go. If it’s a windows-to-aisle boarding plane, this means these line-jumping passengers will need to get out of their middle and aisle seats to make room for those who should have boarded ahead of them.

2. Early Boarding For Premium Passengers
First and Business class passengers almost always board first, and sometimes they’re in the way when the Economy travelers try to board, but usually there are so few of them and they are given such a head-start that they’re not a huge roadblock to boarding.

What does slow things down is when the airline allows passengers to pay for early Economy boarding, or gives early boarding as a reward for being a frequent flier or member of some other program. While we certainly understand the appeal of getting on the plane early, it has the same effect as when passengers jump the line.

3. “Forgot My Book/Phone/Laptop/iPad”
There are some travelers who believe that good packing means squeezing everything you need into the right bag. But there’s an additional element that separates good packing from truly great packing — putting things you’ll need during the trip in an easily accessible part of the bag so that you can get them out quickly while boarding.

Alas, far too many people (and I’m very guilty of this) don’t realize until it’s too late that something they’ll need during the flight is still inside their bag.

So when you realize you left your book/phone/tablet/laptop/snack in the bag that is now safely stowed away, you have the choice of either just waiting until after everyone is settled and then going to retrieve it, or getting up now and blocking the aisle as you rummage for your dog-eared copy of Flowers In the Attic. It has happened on every flight I’ve ever been on, and the person doing is rarely apologetic or even self-aware enough to realize he/she is slowing everyone down.

4. Rolling Suitcases and Baggage Fees
Yes, just about everyone travels with one of these suitcases these days, and that number has probably increased since airlines instituted fees for checked bags. And while most of these rolling suitcases may just fit in the overhead bin, they take up too much space, weigh too much, and are often a hassle for smaller passengers to get in or out of overhead storage, leading to all manner of traffic snarls in the aisles.

Let’s not forget the travelers who have yet to learn, even though they’ve seen a flight attendant do it a thousand times, that these rolling cases are often supposed to be stowed perpendicular to the aisle. Thus, when someone comes up later and is looking for space, they have to waste their time — and that of the people behind them — rearranging incorrectly inserted luggage.

Carry-on bags are supposed to be things you carry on to the plane, not things you roll and then need help lifting into a bin.

The L.A. Times’ Michael Hiltzik has a great column today on why he believes United’s boarding plan is the worst, and includes his feelings on the role of checked bags in boarding delays:

“Here’s another way they’ve undermined their own interests — charging fees for checked baggage has simply encouraged more people to carry on, and to stuff their roll-on bags with more stuff than they can properly hold.”

I contend that the combination of too many people, super-tight seating, luggage, and narrow aisles is a puzzle that can never be solved, at least not without removing one of those factors. But even if someone creates an airplane that can fly 150 people in comfort for an affordable price, the airline industry will just use that extra room to cram in more passengers.

The only thing we can do to speed things up is to travel with minimal luggage. Alas, I know that those of you who agree with that sentiment probably already do carry as little as possible, while those that disagree are unlikely to be swayed.

Want more consumer news? Visit our parent organization, Consumer Reports, for the latest on scams, recalls, and other consumer issues.