What makes a shipment of radioactive material a "Highway Route Controlled Quantity" (HRCQ) under federal hazmat regulations?

49 CFR 173.403; 49 CFR 397.101

HRCQ loads, such as spent nuclear fuel, carry a radiation and public-exposure risk serious enough that federal rules impose mandatory routing and route-plan requirements most hazmat shipments never face. Recognizing when a load crosses into HRCQ territory determines whether a driver can run a normal route or must follow a pre-approved, written route plan.