Which federal regulation makes it the motor carrier's own responsibility to route a hazmat shipment around tunnels, heavily populated areas, and narrow streets whenever a practicable alternative exists?

49 CFR 397.67(b)-(c); cross-referenced in AAMVA CDL Manual Section 9

Two separate rules work together here: if an existing state/tribal NRHM designation applies, the carrier must follow it (397.67(a)); otherwise the carrier's own routing duty under 397.67(b) kicks in. Knowing which rule is which matters because 'the dispatcher told me to' or 'it's faster' is explicitly not a legal excuse under 397.67(c).