All States currently use this classification scheme or some variation of it for classifying vehicles, although few use it exclusively. Many States separate one or more of the FHWA categories into two or more additional classifications to track vehicles of specific interest to them. They then aggregate the categories back together when reporting to the FHWA. This allows each State to meet both its own needs and the FHWA's needs. In addition, many States use other classification schemes in places where axle sensors do not work effectively (e.g., congested urban conditions) or where non-intrusive sensors are needed.

Since the earliest work done by Maine DOT (Wyman, Braley and Stevens 1985) on classification algorithms, it has been apparent that different States have trucking fleets with slightly different axle spacing characteristics. Thus, even when States use the same FHWA classification scheme, the algorithm they use to convert axle-sensor information into vehicle counts by category differs. In most cases, the vehicle classification algorithm provided by each manufacturer needs to be "fine tuned" to accurately convert that State's truck fleet axle spacing characteristics into an accurate measure of truck volumes for the FHWA categories.