What is the law regarding riding in a trailer when it is being towed?

• It is allowed to have a seasoned driver riding in a trailer when it is being towed.
• It is against the law to allow only an underage person ride in the trailer when it is being towed.
• It is against the law to let any person ride in a trailer when it is being towed.
  Explanation:

  "It is against the law to let a person ride in a trailer when it is being towed."
• Anyone can ride in a trailer when it is being towed.