Occupational Safety and Health Administration (OSHA)
The federal workplace safety regulator. For fleets its reach is mainly the shop rather than the road.
OSHA governs the working environment, which for a fleet means the maintenance facility: lifts, jacks and stands, ventilation, hazardous materials, personal protective equipment, lockout procedures and the training records behind them.
State plans matter here. Many states run their own approved programs, and some cover public employees where federal OSHA does not, so what applies to a municipal shop varies by state in a way that catches people out.