What is the National Network (NN)?

The National Network is the federally-designated system on which certain federal truck size standards pre-empt state rules. Its width rule is unusual because it is a floor and a ceiling at once.

Width: "On the NN, no State may impose a width limit of more than or less than 102 inches. Safety devices (e.g., mirrors, handholds) necessary for the safe and efficient operation of motor vehicles may not be included in the calculation of width."

Trailer length: "no State may impose a length limit of less than 48 feet … on a semitrailer … Similarly … no State may impose a length limit of less than 28 feet on a semitrailer or trailer operating in a truck tractor-semitrailer-trailer (twin-trailer) combination."

Height: "No Federal vehicle height limit exists. State standards range from 13.6 feet to 14.6 feet."

Exhibit 2, Compilation of Existing State Truck Size and Weight Limit Laws — FHWA, May 2015. Public domain.

That combination explains most of what a state legal-limit table looks like: width is 102 inches almost everywhere because federal law fixes it there, while height varies by a full foot across the country because federal law says nothing about it at all.

What federal law fixes, and what it leaves to the states →