What rule of thumb does the AAMVA CDL Manual give for judging whether the containment or storage area beyond a railroad crossing is big enough for a school bus to fully clear the tracks?

AAMVA CDL Manual, Section 10.4.4 (Special Situations), p. 10-8

A simple, memorable rule like "bus length plus 15 feet" lets a driver make a fast, reliable judgment call in the seconds before committing to a crossing, without measuring anything. Getting this number right is directly tied to the "if it won't fit, don't commit" principle that prevents a bus from stalling across live tracks.