In the United States, vehicle weight limits are set by laws and regulations enacted at the state and federal levels. Weight restrictions are imposed to protect highway infrastructure from excessive damage and to ensure the integrity of the nation’s bridges.

To maintain full eligibility for federal highway funding, states must abide by federal weight limits on the interstate highway system. However, states can establish weight limits for other roads under their authority. These limits are typically based on the maximum gross vehicle weight (GVW) or weight per vehicle axle. States also establish statutes, regulations, and policies that allow some vehicles to exceed legal limits, either through special overweight permits or statutory exemptions. 

Since 1942, Kentucky has enacted many statutes and regulations related to vehicle weight limits. The earliest of these – KRS 189.210 – limited vehicles other than motor trucks and semitrailer trucks to no more than 15 tons (with some exceptions). Later statutes and regulations established a road classification system that set varying weight limits on different roads based on the number and spacing of vehicle axles. The GVW for county roads was capped at 18 tons, while state-maintained roads were designated as either A, AA, or AAA, with higher maximum limits. Table 1 lists standard operational weight limits for different road classifications.

The road classification system provided a relatively straightforward method to determine the maximum weight allowed on a highway for a given type of vehicle. By requiring more axles for higher weights and limiting the weight that could be carried on any single axle, Kentucky’s regulatory framework helped ensure the forces from heavy vehicles would not excessively damage pavements and bridges.

The system also facilitated the design of pavements and bridges able to meet traffic demands expected for different road classifications. Specifically, practitioners could determine if it was necessary to post weight limits for bridges that had deteriorated due to aging or damage given the maximum legal weight limit allowed for the given road classification. If a bridge load rating indicated a structure could carry more weight than the road’s maximum legal load, no posting was necessary.

Several laws now authorize some vehicles to exceed the standard operational weight limits found in Table 1. These exemptions — described in the next three sections — allow specified vehicles to carry up to 40 tons on local roads and 44 tons on most state roads depending on the type of material being hauled.

Today, when designers make decisions related to road design, bridge design, and bridge load rating analysis they should assume a road will handle loads classified as AAA and proceed on that basis.

Several industries are exempt from regular operational weight limits defined by the road classification system. How exemptions are applied varies significantly based on the type of material being hauled and the road segment(s) on which a load travels. The types of exemptions allowed include:

1. Increased allowable weights. This exemption allows carriers of certain cargo types to treat all state-maintained roadways as if they are AAA – which allows for up to 40 tons GVW. KRS 189.222(1)(f) also allows for weights up to 40 tons for all cargo types on any state highway that is within 15-miles of an interstate or parkway exit.

2. 10% tolerance on the gross weight. Allows carriers to exceed the allowable GVW by 10% before any penalties are assessed. When combined with exemption 1 above, this allows for a GVW of up to 44 tons on any state-maintained roadway – excluding interstates.

3. Axle weight exemption. Typically, loads must be distributed on a truck so that the weight of any axle or group of axles does not exceed specific tolerances. This rule helps to ensure that excessive point stresses aren’t applied to bridges and pavements. The exemption allows certain carriers to ignore this requirement.

4. 10% axle weight tolerance. Like the axle weight exemption, this exemption allows certain carriers to exceed axle weight limits, but only by 10%.

Table 2 lists the various types of cargo to which these exceptions apply.

Haulers, vehicle enforcement officers, and other commercial vehicle industry stakeholders often find it difficult to keep track of every aspect of Kentucky’s complex regulatory framework. The flowchart in Figure 1 illustrates this framework by showing — from the perspective of motor vehicle carriers travelling a given route — how weight limits are applied and enforced.

For a detailed summary of all statutes and regulations pertaining to weight limits, readers can download Table 3 .

In 1986, KRS 177.9771 established Kentucky’s Extended Weight Coal Haul or Coal By-Products Road System (EWCHRS). This system includes:

• All state-maintained Parkways or routes previously part of the Parkway system
• All public highways on which 50,000 tons or more coal or coal by-products were hauled in the previous calendar year.

Except for interstate routes grandfathered into the system based on prior inclusion in the Parkway system, the EWCHRS excludes interstate highways.

On the EWCHRS most vehicles are capped at a GVW of 80,000 pounds. Under KRS 177.9771, however, vehicles using tractor-semitrailer combinations with five or more axles and transporting coal or coal by-products on public highways on the ECWHRS may operate at a GVW of up to 120,000 pounds with a tolerance of five percent (5%).

Each year KYTC’s Secretary certifies public highways or highway segments included in the EWCHRS. Routes change each year because the system is designated based on tonnages reported by companies that haul coal or coal by-products. The Secretary can also add or remove roads from the EWCHRS after consulting with stakeholders.

• Parkway and former Parkway routes always remain on the EWCHRS regardless of reported coal or coal by-product hauling tonnages.
• Other routes that do not meet the 50,000-ton threshold during a calendar year are dropped from the next year’s EWCHRS.

Coal companies must accurately report tonnages to KYTC to ensure routes are not inadvertently omitted from the system.

KRS 189.2713 allows annual and single-trip permits for transporting loads of metal commodities weighing up to 120,000 lbs. on specified state-maintained routes. This includes motor carriers transporting products from metal-producing industries in their most basic and original form from a mill or storage facility to market for processing.