Best way to handle a late night knock on your door

Some people are way too paranoid. :E

A late night nock on your door, especially in a rest area, could be a state trooper telling you to move on, if the rest area is posted with "No Overnight Parking" signs, though rarely enforced. Or it could be somebody in urgent need of help.

Caution is prudent, of course! I would not open the door without knowing who was outside, but then neither would I stick the barrel of a shotgun or handgun in the nose of a policeman.