According to the AAMVA CDL Manual, when should a driver activate the alternating flashing amber warning lights (if the bus is so equipped) when approaching a school bus stop?

AAMVA CDL Manual, Section 10.2.1 'Approaching the Stop,' p. 10-3; 49 CFR 383.123(a)(2)(i) (general topic only, no specific figures stated); 49 CFR Part 380, Appendix D, Unit D1.2

Early, consistent warning lights give drivers behind and approaching the bus enough time to slow down before students are anywhere near the roadway. Activating them too late, or confusing this step with the turn signal or red lights, removes the margin that keeps drivers from closing distance on a bus that's about to stop.