The federal rules are not the only rules that reach a commercial vehicle, and § 390.9 says so directly. Except as otherwise specifically indicated, subchapter B is not intended to preclude States, or subdivisions of States, from establishing or enforcing State or local laws relating to safety — provided that complying with those laws would not prevent full compliance with these regulations by the person subject to them. Two things follow, and they pull in opposite directions.

The first is permissive: a State may add safety requirements on top of the federal ones, and it may enforce them. Nothing in the federal rules displaces a State’s own safety law simply because the subject overlaps. The second is the limit, and it is the whole of the protection § 390.9 gives you: the State law must be one you can obey WHILE fully complying with the federal regulations. A State requirement that made full federal compliance impossible is the one thing this section does not preserve. In practice the two almost always stack rather than collide, which means the working answer is usually that you must satisfy both.

Chapter 1 deferred applicability to this chapter, and this is where state law is taken up. Where a State applies rules of this kind to purely intrastate work, that obligation comes from the State’s own law — with its own text, its own scope and its own variations — and not from these regulations reaching further than § 390.3T says they do. The distinction matters when you go looking for the rule: a federal requirement is in this rulebook and reads the same in every State, while a State requirement has to be checked where that State publishes it. Do not assume a rule you learned here is the rule that applies to intrastate work, and do not assume it is not.