Approximately how far before a railroad crossing should a school bus driver activate the hazard warning lights?

AAMVA CDL Manual Section 10.4.3, p. 10-8; 49 CFR 383.123(a)(2)(iii) (general knowledge-test topic only; does not itself specify a 200-foot distance); 49 CFR Part 380, Appendix D, Unit D1.6 (Railroad-Highway Grade Crossings)

A school bus slowing well ahead of a crossing can surprise drivers behind it who are not expecting a stop. Activating hazard lights at the specified distance gives trailing traffic enough warning to react safely instead of rear-ending the bus.