When fueling a placarded vehicle, what must someone always do?

When fueling a placarded vehicle, it is essential to control the fuel flow at the nozzle. This practice is crucial for several reasons. First, controlling the fuel flow helps to prevent spills, which can create hazardous conditions, especially when dealing with materials that are dangerous if leaked. It allows the person fueling the vehicle to monitor the process actively, ensuring that the fuel does not overflow and cause environmental contamination or pose a fire hazard.

Additionally, actively managing the fuel flow ensures that the task is completed safely and efficiently. This control reduces the risks associated with static electricity discharge and the potential for ignition of flammable vapors that could escape if the nozzle were unattended or improperly handled.

In contrast, remaining in the driver's seat, keeping the vehicle running, and using a fire extinguisher are not standard practices during this fueling process and do not enhance safety; rather, they could potentially increase hazards. Therefore, controlling the fuel flow at the nozzle is the best practice when fueling a placarded vehicle.