Final answer:
The emergency contact number, UN ID number, and signal word must be included on a pipeline warning sign.
Step-by-step explanation:
One item that must be included on a pipeline warning sign is the emergency contact number. This allows for immediate communication in case of any emergencies or accidents related to the pipeline.
Other important items to include on a pipeline warning sign are the UN ID number and the signal word.
The UN ID number is a unique identifier assigned to hazardous materials, while the signal word indicates the level of severity of the hazardous material. Both of these elements help to provide essential information for individuals handling or coming into contact with the pipeline.