Unless the user actually adds green available time at regular intervals throughout the day, people learn that they have to ignore the red busy light and ask.
There's no clear, easy protocol that works for everyone, unfortunately. Some people are always going to operate on the 'better to ask forgiveness than permission' model. And I say this as someone who is often in the 'always busy' camp.
Most employees probably have enough credibility to explain away one or two missed deadlines, but not 5 or 6 in a row without providing actual proof.