The need to periodically urinate during the day is a defining annoyance of being human. If you spend any time, say, moving about a city, then you are familiar with addressing this need in coordination with, maybe, coffee or snacks. Where’s my trusty Starbucks, aka public bathroom with a latte stand attached?
Every public single-user restroom features one of many styles of locking door. Only one design is good, and the rest are terrible. This is the correct design:
It is the deadbolt style used on aircraft lavatories, and like so many things designed for challenging (cramped, noisy) aerospace use it works great elsewhere. If you are outside the door, it tells you by just glancing whether this restroom is VACANT, or if it is IN USE. There’s even a color code so you don’t need to get near the door! You don’t need to knock or jiggle the handle, freaking out the occupant inside. You don’t need to test the knob extra hard when there’s a line of people behind you, just in case you messed up and thought the restroom was occupied but actually the knob is just kind of hard to turn. Oops.
Because the outside indication is mechanically connected to the locking mechanism, and the lock is manipulated from inside, its outside indicator is never wrong.
Once inside the restroom, you use the simple deadbolt handle to lock the door. You know that it’s locked because you feel that solid deadbolt clunk over, and if you try the handle, you will find that indeed the door is not openable. You don’t have to guess, like on the knobs that have a little clicky button (or worse, the twisty twiddler) in the middle of them — wait, if I twiddled it, am I sure it’s locked? Let me try the knob, oh, it opened, but was that because I didn’t lock it or because turning the knob disengaged the lock?? Who knows. Those knobs are garbage for public restroom use.
Only the airplane-style deadbolt is good. You can tell by looking at it from the inside, too, if it’s in the locked position. You can reassure yourself from a distance (i.e. while positioned upon the toilet) that it’s still locked. And nobody will knock, and nobody will frantically turn the knob while your pants are pulled down, because they will already know from the indicator outside that it is IN USE.
So why isn’t this kind of lock set literally ubiquitous in public single-occupancy restrooms? I have no idea. Maybe they are expensive or hard to find at hardware stores that mom-n-pop café owners buy their hardware from.
What I do know is that these indicator deadbolts are worth their weight in happy-customer gold if you own and operate a publicly available restroom. Install them. Your customers will (quietly, subconsciously, unwittingly) thank you and you will sell more cappuccinos.