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

• Only if there's heavy traffic block in the ramp
• Only if you see a slow moving vehicle in the ramp
• Only if you see a WRONG WAY or DO NOT ENTER sign or red reflectors on lane lines
  Explanation:

  "Never back up on an entrance ramp or exit ramp. The ONLY exception to this is if you see a WRONG WAY or DO NOT ENTER sign or red reflectors on lane lines. You must backup or turn around."
• Only if you get on a wrong exit or entrance ramp