"Apportioned Registration" is the nickname for International Registration Plan, or what most people call IRP. Normally, or at least before the IRP program came into being, interstate truckers had to pay for trip permits for each state they traveled in or through, because you have to be registered in each state to operate there. The miles they drove in each state determined how much they have to pay to operate in that state. Because it's a hassle to stop in every state and get a permit, it discouraged many interstate trips and thus was a barrier to interstate commerce. It's basically a registration reciprocity agreement among the contiguous United States and Canadian profinces which provides payment of license fees on the basis of total distance operated in all jurisdictions.
I think it was back in the early 70's when the IRP program began, but it took a lot of years to finally get everyone on board.
The IRP program was introduced
to promote and encourage the fullest possible use of the national highway system, by authorizing the apportioned registration of fleets of motor vehicles and the recognition of apportioned registered vehicles in other states, thus contributing to the economic and social development and growth of the states.
Basically, you register your vehicle in your "base" state, the state in which you live, or in the case of a carrier-owned fleet, the state in which their business is located, and
the registration fee paid is determined by the percentage of miles traveled in each state. The registrant then reports the mileage for each state to the base state, and the base state calculates the fees due and collects the fees for each state. The base state then distributes the monies to the appropriate states.
Vehicles are eligible for apportionment under the provisions of the International Registration Plan if they meet the following criteria:
- Intended for use in two or more jurisdictions
- Is a power unit having two axles and a gross vehicle weight in excess of 26,000 pounds (11,794 kg)
- Is used in combination, when the weight exceeds 26,000 pounds (11,794 kg) gross vehicle weight
- Is a power unit having three or more axles
Trailers also used to be apportioned, but as 2002 they no longer are, despite some states still issuing them. Old habits, and revenue streams, die hard.