In what situation should workers exercise caution around concrete mixer trucks?

Workers should exercise caution around concrete mixer trucks primarily when the mixer is operating because this is when the equipment is active and poses the highest risk of injury. During operation, moving parts, high mixing speeds, and potential overspills of concrete or other materials can create hazardous conditions for nearby personnel. The risk of injury can arise from multiple factors, such as being struck by the truck, getting caught in the machinery, or encountering an unexpected movement of the drum.

While caution may also be wise when the mixer is parked—due to possible allergens, hydraulic leaks, or tripping hazards—these situations generally present lower immediate hazards compared to when the mixer is actively mixing. Similarly, the absence of cement in the mixer does not significantly lessen risks, as other hazardous elements associated with the mixer’s operation could still be present.

In summary, the highest level of caution is warranted when the mixer is operating, which is why that specific choice is highlighted.