On his website, interstate motor carrier operations expert witness Lew Grill provides links to the Federal Motor Carrier Safety Regulations, Single State Registration System:
§367.3 Selection of registration State.
(a) Each motor carrier required to register and pay filing fees must select a single participating State as its registration State. The carrier must select the State in which it maintains its principal place of business, if such State is a participating State. A carrier that maintains its principal place of business outside of a participating State must select the State in which it will operate the largest number of motor vehicles during the next registration year. In the event a carrier will operate the same largest number of vehicles in more than one State, it must select one of those States.


