One sentence in § 392.2 decides something every earlier chapter has had to work around, and it decides it cleanly. Every commercial motor vehicle must be operated in accordance with the laws, ordinances and regulations of the jurisdiction in which it is being operated — the State you are in, the county, the city, whoever writes the traffic law where your wheels are. However, if a regulation of the Federal Motor Carrier Safety Administration imposes a HIGHER STANDARD OF CARE than that law, ordinance or regulation, the federal regulation must be complied with. So the default is local and the override is federal, and the override runs in one direction only: upward.

Set that beside what § 2.1 established and the two halves of the picture finally meet. § 390.9 says the federal rules do not preclude a State from establishing or enforcing its own safety law, provided compliance with it would not prevent full federal compliance — that is about whether State law may EXIST. § 392.2 is about which one you obey when they differ at the wheel, and it answers with the stricter. A local speed limit below what you were expecting is simply the law where you are. A local rule laxer than a federal one is not a permission, because the federal standard of care is higher and § 392.2 says the federal one must be complied with. The practical form is short: obey where you are, unless the federal rule asks more, and then obey that.

§ 392.1 then sets who is bound, and the answer is not just drivers. Every motor carrier, its officers, agents, representatives and employees responsible for the management, maintenance, operation or driving of commercial motor vehicles — or for the hiring, supervising, training, assigning or dispatching of drivers — shall be instructed in and comply with the rules in this part. Read that list against § 2.5's roles and it is the same design: a duty written about driving reaches the dispatcher who plans the run and the supervisor who assigns it. The part carries one narrow exclusion, in § 392.1(b): its rules do not apply to drivers of pipeline welding trucks as 49 CFR 390.38(b) defines them.