travelators

http://www.bbc.co.uk/news/magazine-11417063
I was set thinking by this article, and I realised that these are actually enormous annoyances for me.
They might be efficient in terms of output for the power input, but they're not efficient investments or uses of space. I have no idea how much they cost, but I can still say that they're poor investments since they are detrimental. I'm not going to concern myself with comparisons to other systems, such as the pod system mentioned, since I don't know those.
I can simply say that travelators are worse than empty floor. To start with, they go slowly. I can walk faster than they move. In an empty hallway that'snot an issue: I can either walk on a moving floor or a still one, and I'll go faster on the moving one.

However, airports are not empty places. Anywhere that one of these might be installed is a bustling centre of travelling people. People travelling come in many shapes and sizes, but some relevant characteristics are that we get lots of people idling around, people in a hurry and people who take their time, people who don't know where they're going and people who have luggage. The uniting factor between every type of person is that they all get in the way.
Because they get in the way, we need as much space as we can get. The more space we have, even in a corridor, the better, because it allows for streams of movement: people who move fast can move round people who are slow.
Now we don't get single-file avenues of movement, as we do on a running track, with people staying in one lane. People tend to walk in the centre of any thoroughfare (up to a certain width, but including corridors of many sizes). People tend to veer from side to side without a care for people behind them, and with very little attention to people walking at them. Finally, people take up more than the width of one person. It is rude to brush past people or push them out of the way, and therefore there needs to be a little bit more space than a person's worth for someone to go by.
Therefore a corridor as wide as two people standing side by side will only accommodate one file of traffic: someone will walk in the middle, leaving half a space on each side. If there is two-way traffic, it will be very slow because people will feel the need to edge past each other, and might struggle if there's someone with luggage.
As the corridor gets wider we get expanding returns. A three shoulder-width corridor can accomodate two streams of single-file traffic, although there will always be someone who walks in the middle and slows everything down by leaving a space each side, which isn't quite enough for full-speed passing.
For multiple streams of traffic we need wide corridors, especially as we get varying sizes of people (luggage makes people much, much wider and less manoeuverable). Larger widths are better able to accommodate different patterns of sizes, giving less wasted space at the sides. If I took two two-person corridors and compared the flow of people to within one four-person corridor it would be less. A four-person corridor can accommodate three single-file flows.

Now I've laid out the basic intuitions about this, we find that comparing travelators to open spaces isn't even that kind to the travelators, because they take up some of the space as walls. So a six-person corridor might become two two-person corridors. This might look very snazzy, and be great for the executives who test it before it's opened to the public, but once people get involved we can get blockages much more easily.
I would rather be able to walk around a collapsed trolley at an airport, or overtake a dawdling idler, rather than have an option of resting my legs and going more slowly. If I wanted that option, I could just stop moving!