Final answer:
Placards indicating hazardous materials must be placed on all four sides of transport vehicles to communicate the nature of the hazard to emergency services and the public.
Step-by-step explanation:
Placards are a critical safety feature on transport vehicles that carry hazardous materials. According to regulations, placards must be placed on all four sides of a transport vehicle. This requirement is essential for the safety of the driver, emergency response personnel, and the public. The placards provide important information about the type and nature of the hazard of the materials being transported, ensuring that in the event of an accident, proper precautions can be taken to minimize risks.