School bus guidance names conditions under which the driver has to get everyone off the bus rather than weigh the decision. Which of these is one of them?

Explanation

A bus brought to a halt on a railroad-highway crossing, or right beside one, is an evacuation the driver does not get to debate. The manual treats five situations that way: fire aboard or the threat of one, a bus stopped on or beside a crossing, a bus whose position could shift and make things worse, a collision about to happen, and a hazardous materials spill that forces people out quickly. What unites them is that the danger is travelling toward the children where they sit and will arrive before any help does, which leaves nothing to weigh. The wrong options are genuine emergencies, but each is the sort children ride out more safely in their seats than standing next to a live traffic lane. A flat tire, a sick child, and a dead heater do not deteriorate because everyone stayed put, while tipping a busload onto a narrow shoulder or out into freezing air manufactures a hazard that was not there a moment earlier. 49 CFR 383.123(a)(2)(ii) makes emergency exits and the safe evacuation of passengers a required knowledge area, a direct subject match for this topic, but sets out no condition and no procedure, so the list itself is attributed to the manual.

Source: 49 CFR 383.123(a)(2)(ii)