1. must be able to determine if lock worked from the inside
    if you lock the door and then trying to open it unlocks the door, there needs to be a visual indicator that the lock was successful. it is better to just let the person attempt to open it and verify to their satisfaction they the locking was successful!
  2. there must be a clear indicator from the outside that the bathroom is occupied
    a green / red sign preferably with the words vacant / occupied is ideal. should be impossible to miss. I do not want to have to answer someone's question about whether the bathroom is occupied while I am pooping
  3. that's it.
    this is so easy how does everyone get it wrong?