A rider has their arm hanging out the bus window while the vehicle is moving. Per the manual, what may the driver need to do?

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

Arms or heads outside a moving bus can be struck by passing objects or vehicles. Catching this during routine interior scanning, and correcting it with a simple reminder, is the kind of everyday in-cab supervision the passenger-endorsement test checks for.