At a passive railroad-highway crossing, who is responsible for deciding whether it is safe to cross the tracks?

AAMVA CDL Manual, Section 10.4.1 'Types of Crossings,' p. 10-7; 49 CFR Part 380, Appendix D, Unit D1.6 (Railroad-Highway Grade Crossings)

Knowing that a passive crossing puts 100% of the safety decision on the driver, with no light, bell, or gate to warn of an approaching train, is why school bus drivers are trained to always stop and visually/audibly check at every crossing rather than assume the absence of a signal means the absence of a train.