Which of the following statements about transporting hazardous materials is true?

The statement that placards with four-digit identification numbers allow the contents of the container to be accurately identified is true because this practice is crucial for ensuring safety during the transportation of hazardous materials. The use of placards helps first responders and those in the vicinity to quickly recognize the type of hazardous materials being transported, which is essential for implementing appropriate safety measures in the event of a leak, spill, or accident. This identification system is standardized under various regulations, such as those set by the U.S. Department of Transportation, which aim to minimize risk and promote public safety.

While other statements about hazardous material transport hold some elements of truth, they do not encapsulate the central importance of the placarding system in emergency situations and regulatory compliance as effectively. For example, while special vehicle registration may be required depending on the type of hazardous materials being transported, it is not universally applicable to all transport vehicles. The requirement for trained personnel is important; however, this does not focus on the immediate identification of materials in the field. Sealing containers is also critical for safety but is more about containment rather than initial identification and response. Thus, the significance of placards for accurate identification sets this statement apart as the most true and relevant in the context of hazardous materials transportation.