What event activates the stop arms and warning lights on a school bus?

The activation of the stop arms and warning lights on a school bus is directly linked to the bus coming to a complete stop. This mechanism is designed to protect students as they board or disembark from the bus. When the driver halts the vehicle, the warning lights will start to flash, and the stop sign arm extends out from the side of the bus.

This protocol serves to alert other drivers that they must stop their vehicles to ensure the safety of children who may be crossing the street or approaching the bus. It is crucial that all road users respect these signals to maintain safety around school buses, especially during loading and unloading times.

The other options, such as pulling into a bus stop, opening the window, or flashing the headlights, do not trigger the safety measures that the bus employs when children are present. Those actions do not indicate an immediate risk or need for other vehicles to stop, which is why the complete stop of the bus is the necessary event for activating the warning systems.