What states don't participate in UCR?
Currently 9 states + D.C. don't participate: Arizona, Florida, Hawaii, Maryland, Nevada, New Jersey, Oregon, Vermont, Wyoming, plus the District of Columbia. Carriers based in non-participating states still register through the nearest UCR state.
UCR is a 41-state compact, not a 50-state federal program. The 41 participating states each get a share of the collected UCR fees to fund state safety enforcement. The 10 non-participating jurisdictions (9 states + D.C.) opted out of the compact when it was first formed.
Carriers based in non-participating states are still required to register with UCR — they just register through the nearest participating state. California serves AZ/NV; Idaho or Washington serves OR; Colorado serves WY; New York or Pennsylvania serves NJ/VT; Virginia serves MD/D.C.; Georgia serves FL; Texas or Mississippi serves HI (with a fairly long mailing route).
Once registered, the UCR receipt is valid in all 41 participating states. Operating in a non-participating state doesn't trigger a UCR-specific roadside check, but operating in a participating state without a current UCR receipt can.
A common edge case: a carrier moves their base state mid-year from a participating to a non-participating state. The UCR registration follows the base state at the time of registration; the next renewal would route through the new (or nearest) participating state.