How is oncoming traffic warned of a blocked or narrow lane ahead?

• The people nearby will tell the oncoming traffic.
• A flashing light from emergency vehicles will warn you of a problem ahead.
  Explanation:

  "Oncoming traffic is typically warned of a blocked or narrow lane ahead by flashing lights from emergency vehicles. These lights alert drivers to the potential hazard, allowing them to take appropriate action."
• Both A and B
• You will not be warned, you need to go and see for yourself.