While driving a bus, the manual says the driver should scan the road ahead, the sides, and the rear. What else should the driver be scanning at the same time?

49 CFR 383.117(a)(6), operating practices and procedures not otherwise specified; AAMVA CDL Manual Section 4.3.1 (p.4-2/4-3)

A bus driver is responsible for passengers behind them, not just traffic ahead. Splitting attention between the cabin and the road is a core passenger-endorsement skill the knowledge test expects, since rider behavior can turn into a safety hazard just as fast as something happening outside the bus.