Why are there lines at airport security? I mean, we know why the lines form, when passenger load exceeds the capacity, with the bottleneck usually being the X-ray machines. The question is why this imbalance is allowed to happen?

The variable wait at airport security levies a high cost, because passengers must assume it will be long, just in case it is. That means every passenger gets there 15 or more minutes earlier than they would need to, even if there is no wait. Web sites listing wait times can help, but they can change quickly.

For these passengers, especially business passengers, their time is valuable, and almost surely a lot more costly than that of TSA screeners. If there are extra screeners, it costs more money to keep them idle when loads are low, but the passengers would be more than willing to pay that cost to get assuredly short airport lines.

(There are some alternatives, as Orwellian programs like Clear and TSA-PRE allow you to bypass the line if you will be fingerprinted and get a background check. But this should not be the answer.)

In some cases, the limit is the size of the screening area. After 9/11, screening got more intensive, and they needed more room for machines and more table space for people to prepare their bags for all the rules about shoes, laptops, liquids and anything in their pockets.

Here are some solutions:

Appointments at security

The TSA has considered this but it is not widely in use. Rather than a time of departure, what you care about is when you need to get to the airport. You want an appointment at security, so if you show up at that time, you get screened immediately and are on your way to the gate in time. Airlines or passengers could pay for appointments, though in theory they should be free and all should get them, with the premium passengers just paying for appointments that are closer to departure time.

Double-decker X-ray machines

There may not be enough floor space, but X-ray machines could be made double decker, with two conveyor belts. No hand luggage is allowed to be more than a foot high, though you need a little more headroom to arrange your things. Taller people could be asked to use the upper belt, though by lowering the lower belt a little you can get enough room for all and easy access to the upper belt for all but children and very short folks.

A double width deck is also possible, if people are able to reach over, or use the other side to load. (See below.)

This might be overkill, as I doubt the existing X-ray machines run at half their capacity. It is the screener's deliberation that takes the time, and thus the next step is key...

Remote X-ray screeners

The X-ray screener's job is to look at the X-ray image and flag suspect items. There is no need for them to be at the security station. There is no need for them to even be in the airport or the city, come to that. With redundant, reliable bandwidth, screeners could work in central screening stations, and be sent virtually to whatever security station has the highest load.

Each airport would have some local screeners, though they could work in a central facility so they can virtually move from station to station as needed, and even go there physically in the event of some major equipment failure. They would be enough to handle the airport's base-load, but peak loads would call in screeners from other locations in the city, state or country.

Using truly remote screeners creates a risk that a network outage could greatly slow processing. This would mean delayed flights until text messages can go out to all passengers to expect longer lines and temporary workers can come in -- or the outage can be repaired. To avoid this, you want reliable, redundant bandwidth, multiple screener centers and the ability to even use LTE cell phones as a backup. And, perhaps, an ability to quickly move screeners from airport to airport to handle downtimes at a particular airport. (Fortunately, there happens to be a handy technology for moving people from airport to airport!)

Screeners need not be working a specific line. Screeners could be allocated by item. Ie. one bag is looked at by screener 12 and the next bag is looked at by screener 24, just giving each item or set of items to the next available screener, which means an X-ray could actually constantly run at full speed if there are available staff. Each screener would, if they saw an issue, get to look at the other bags of the same passenger, and any bag flagged as suspect could immediately be presented to one or more other screeners for re-evaluation. In addition, as capacity is available, a random subset of bags could be looked at by 2 or more screeners.

It can also make sense to just skip having a human look at some bags at random to reduce wait and cost. It might even make sense to let some bags go unviewed in order to have other bags be viewed by 2 screeners. Software triage of how many screeners should look at a bag (0, 1, 2, etc.) is also possible though random might be better because attackers might figure out how to fool the software. With the screeners being remote and the belts operating at a fixed speed, passengers won't learn who was randomly selected for inspection or not.

Some screeners need to be there -- the one who swabs your bag, or does an extra search on it, the one who does the overly-intimate patdown and the one with the gun who tries to stop you if you try to run. But the ones who just give advice can be remote, and the one who inspects your boarding pass can be remote for passengers able to hold those things up to the scanners. I suspect remote inspection of ID is also possible though I can see people resisting that. The scanner who looks at your nude photo can certainly be remote -- currently they are out of view so you don't feel as bothered.

This remote approach, instead of costing more, might actually save money, especially on the national level. That's because the different time zones have different peak times, and remote workers can quickly move to follow the traffic loads.

It's also easier with remote screeners for passengers to use both sides of the belt to load and get their stuff. Agents would have to go in among them to pull bags for special inspection, though.

Of course it could be even better

Don't misunderstand -- the whole system should be scrapped and replaced with something that is more flyer-friendly as well as more capable of catching actual hijacker techniques. But if it's going to exist, it should be possible to remove the line for everybody, not just those who go through background checks and fingerprinting just to travel.

After 2001, a company developed bomb proof luggage containers and now there is a new bag approach which would reduce the need to x-ray and delay checked luggage as much as they do. They were never widely deployed, because they cost more and weigh more.

I have 3 things I carry on planes:

1. The things I need on the plane (like my computer, books and other items.)
2. The vital and fragile things which I insist not leave my control, such as my camera gear and medicines.
3. When I am not checking a bag, everything else for short trips.

I'm open to having all but #1 being put into a bomb-proof container by me and removed by me in a manner similar to gate check, so I can assure it's always on the plane with me. Of course if I'm to do that then security (for just me and the items of type one) must be close to the plane -- which it is for many international flights to the USA. That would speed up that security a lot. The use of remote screeners could make it easier to have security at the gate, too.

Personally, once the problem of taking over the cockpit was solved by new cockpit doors and access policies, I think there was an argument that you need not screen passengers at all. Sure, they could bring on guns, but would be no longer able to hijack the aircraft, so it's no different from a bus or a train. Kept to small items, they would not be able to cause as much damage as they could do with a suitcase sized bomb in the security line. The security line is, by definition, unsecured, and anybody can bring a large uninspected roll-aboard up to it, amidst a very large crowd -- similar to what happened in Moscow in 2011.

Instead, you would have gates where a portal in the wall would have a bomb-proof luggage container into which you could put your personal bags and coats. Most people would then just get on, but a random sampling would be directed to extra security. Those wishing to bring larger things on-board (medical gear, super-fragiles, mega-laptops) would need to arrive earlier and go through security too. A forklift would quickly move the bombproof container into the hold and the plane would take off.