haul freight of various shapes, sizes, and densities and are often loaded below capacity. While van trailers present a convenient and predictable space envelope for aerodynamic evaluation, other trailers such as flat-decks have no such fixed outer shell, and therefore the shape of flat deck trailers will change with each load transported, which in turn influences aerodynamic drag and thus fuel consumption.
Given the complexity and challenge of establishing a new regulatory policy for medium- and heavy-duty vehicles, there could be a tendency to narrow the scope of a regulation, or at least to focus early implementation, on the largest fuel users in the fleet. This would suggest focusing on Class 8, Class 6, and Class 2b vehicles to cover roughly 90 percent of the fuel use in the medium- and heavy-duty vehicle fleet.
While starting with a subset of heavy-duty vehicles is tempting, there are several drawbacks that should be considered. Uneven policy application will cause disruptions in the marketplace and create the potential for reclassifying various classes of vehicles, as has been done in light-duty vehicles (LDVs), in which cars were reclassified as light trucks to achieve less stringent regulatory standards. Related to “reclassification” is a potential for change in market behavior to avoid higher prices due to regulation (i.e., if Class 2b is regulated but not Class 3 then buyers might buy more of the larger Class 3 trucks because they would become less expensive relative to 2b trucks).
Second, while regulation of medium- and heavy-duty vehicles is complicated, there could be further complications created by seeking to draw artificial lines between various segments. For example, Class 8 straight/vocational trucks typically have two drive axles and Class 7 trucks have one. Class 5 and Class 6 trucks are similar to each other, and Classes 2b and 3 tend to be more similar to each other.
Third, when considering a regulatory framework, there are a number of important parameters beyond fuel use, such as cost effectiveness, equity among manufacturers, potential for gaming, minimizing unintended outcomes, and technology potential. Finally, Congress instructed NHTSA to estab-