Why are placards used on vehicles?

Placards are used on vehicles primarily to warn others of hazardous materials being transported. This is crucial for safety, as identifying the type of hazardous cargo allows emergency responders, other drivers, and the general public to understand the potential risks involved during incidents such as accidents or spills. By displaying specific placards, drivers comply with regulations that aim to ensure not only their own safety but also that of others on the road. These markings signal to everyone encountering the vehicle that special caution is required due to the nature of the materials being carried.

The other options might suggest alternative uses for signage on vehicles, but they do not align with the primary purpose of placards, which is centered around safety and hazard identification.