When can you use your turn signals to tell drivers behind you that they can pass?

• Never, it is against the law
  Explanation:

  "You should never use your turn signals to tell drivers behind you that they can pass. Turn signals are meant to indicate your own intentions to turn or change lanes, not to communicate with other drivers about passing. Using them for this purpose can create confusion and lead to accidents."
• Only when driving on a highway
• When the road has multiple lanes
• Whenever you want, as a courtesy signal to other drivers