When should a school bus driver activate the alternating red warning lights once stopped at a designated school bus stop?

AAMVA CDL Manual, Section 10.2.1 'When stopping,' p. 10-3; 49 CFR 383.123(a)(2)(i) (general topic only); 49 CFR Part 380, Appendix D, Unit D1.2 (Loading and Unloading)

Red lights and the extended stop arm are the strongest legal signal to other traffic that they must stop. Timing their activation to when traffic is already a safe distance away, rather than reflexively, helps avoid triggering them too late to actually stop oncoming vehicles.