What must never occur when the red stop bar lights are illuminated?

When the red stop bar lights are illuminated, it indicates that an area, typically near a runway, is not safe for vehicular movement. Therefore, entering or crossing a runway while these lights are illuminated is strictly prohibited. This rule is in place to ensure the safety of both aircraft and vehicles on the airside, as the presence of illuminated red lights is a signal that an aircraft may be on approach or landing, or that other critical operations are in progress. Crossings at this time could lead to dangerous situations, including potential collisions between vehicles and aircraft.

Taxiing without clearance, loading passengers, and maintaining distance from other aircraft are important operational considerations but do not relate to the specific safety hazard indicated by the stop bar lights. Only the act of crossing a runway when the stop bar lights are lit poses immediate risk according to the established safety protocols.