Under what circumstance is backing up on an entrance or exit ramp permissible?

• Only if there's heavy traffic block in the ramp
• Never, it is not allowed
  Explanation:

  "Backing up on an entrance or exit ramp is never permissible, no matter what. Do not back up even if you have come the wrong way as doing so can create a hazardous situation for other drivers. Instead, continue forward to find a safe place to turn."
• Only if you see a WRONG WAY or DO NOT ENTER sign or red reflectors on lane lines
• Only if you get on a wrong exit or entrance ramp