A workshop owner argues that cutting a second doorway at the far end of the shed will let his workers get out faster if there is a fire, since at present everyone has to reach the single door at the near end. Which of the following must hold if the owner's argument is to stand?

A second exit is worth something only when it is usable at the moment the first is not. If smoke or flame would block the far end as well, or the new doorway would be kept bolted, the escape it promises never exists and the argument fails. The shed's history, the size of the workforce and the cost of other precautions leave that requirement untouched.