This leads to a bottleneck of people going in/out because there's only a single point to enter or leave. Not to mention potential stampede situation in the event of a fire. Is this just something we all see others doing so we figure, "Why not"?
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Here are some potential reasons:
Regardless, having one locked door in a set of double doors is not user friendly. The building and its entrances should be designed so that all general use doors can be used. One thing a better building or door design won't help is forgetfulness on the part of the building attendant/staff.
Here is an example of a double door design that I think affords approrpirate usage:
By making one of the two doors larger, it communicates to users that this is a utility door, and that you probably shouldn't try and open it. Another way of helping users understand that one of two double doors isn't intended for regular use is to remove the door's handle and push plates.
There are many other other work-arounds, but the list could go on and on.
This makes several things easier for the business:
Businesses are especially leery of the closing time procedure. They don't want to leave a door unlocked all night accidentally. As such, many business owners instruct that the extra door just remain locked.
Here is an article that complains about this from a usability standpoint in a situation where pull and push is not clear as well:
Business owners often don't think that the time and security risk to deal with the extra door is worth it compared to small frustration of the occasional customer. This could be an example of the business owner and employees not experiencing this frustration themselves because they get used to which door to use.
Many businesses try to lessen the impact of this on their customers by posting signs about which door to use. For example here is a humorous one from Funny Den:
From a usability standpoint, maybe double door locks could be redesigned. Ideally, the turn of a single key would lock or unlock both doors simultaneously.
My girlfriend works in a shop where they have both double door open all the time, regardless of the weather. It's pretty much all to do with sales. There have been a number of studies done over the years, and my gf has done some studies of her own, that shut doors provide an obstacle for people. And weirdly - as much as it's only a door - that obstacle is enough for people to not casually wander in.
My gf shut the doors on a day she knows is normally a busy day - her sales almost halved.
The amount of casual 'wander in and lets just have a look' shoppers dropped dramatically, as did people with 'extra baggage' (e.g. prams, buggies, strollers etc), or weirdly, people in groups of 2 or more.
There may be other reasons cited, security, air flow etc - and they may all be valid reasons, but by far and away - it's a sales thing.
Think of the phrase "My door is always open", or "He's got an open door policy". Everything meant to be welcoming is where the door is always open for you, no obstacle. Lazy, but true.