​Unrestricted Movement

Maximizing the efficiency of the highway network requires that vehicles and vehicular combinations be capable of unrestricted movement. This movement is possible if vehicles or vehicular combinations do not exceed legal size and weight limits imposed by the States and the Federal Government.

Highway Permits

Vehicles and vehicular combinations that exceed the legal highway limits may require permits for highway transport. The difficulty in obtaining these permits depends on the State's ploicy and the amount that the legal permit is exceeded. Permits for vehicles that exceed the legal width and length limits are not as difficult to obtain as those for vehicles that exceed the legal height and weight limits. Circuitous routing or time of day restrictions may be required as a condition of the permit. Throughput delays may result. In general, states may not issue permits for reducible or divisible loads; however this may not apply to military vehicles on the Interstate System.

DOD 4500.9-R Defense Transportation Regulation Part III Mobility Appendix AV, explains the procedures for obtaining highway permits.

The States and local government own and control all the roads within their boundaries, including local, state, U.S., and Interstate routes. They do not have to grant a permit if they feel the load is too large or too heavy for safe transport.

U.S. Highway Bridge Gross Weight Formula

Axle spacing is as important as axle weight in bridge design. A useful metaphor is thin ice on a pond. Walking on the ice concentrates a person's weight on the small area covered by the person's feet, therefore increasing the chance of the ice breaking and a very cold dunking. Lying down on the ice, on the other hand, spreads the same weight over a much larger area and decreases the chance of the ice breaking.

In 1982, Federal highway law was amended to accept the following bridge formula limits:

W = 500 (LN/(N-1) + 12N + 36)

Where: W = the maximum weight in pounds that can be carried on a group of two or more axles to the nearest 500 pounds L = the distance in feet between the outer axles of any two or more consecutive axles, and N = the number of axles being considered. A vehicle that exceeds the legal weight limits but meets the bridge formula is more likely to obtain permits than a vehicle that exceeds both. The bridge formula is incorporated into table 23 CFR 658. A sample problem for determining bridge formula requirements can be found in Appendix A of MIL-STD-1366 Transportability Criteria (1.58 MB).

Nondivisible Loads

Many states are reluctant to grant highway permits for overweight cargo vehicles with divisible loads since these vehicles can be brought within normal legal limits simply by reducing the payload. In some cases, a vehicle with a high empty weight may have a very limited legal payload.

SDDCTEA has been very successful in easing Federal permit limitations for movement of heavy DOD vehicle systems on public highways. The definition of a nondivisible load was modified to allow "marked military equipment or materiel" to be transported on the Interstate System without the need for disassembly as long as State bridge restrictions are not violated. Federal nondivisible restrictions, which had been more limiting in many instances, are no longer applicable to marked military loads and equipment. The actual rule change can be found at the Federal Register (28 K).

As an example of the potential significance of the change -- previously, State transportation departments had to restrict movement of the Palletized Load System (PLS) truck-trailer combination system to a total weight of 80,000 pounds due to Federal nondivisible load rules. This only allowed the combination to carry a total legal load of 4.23 tons. With the change, States may now allow up to an 11.48 ton payload, which is determined by State bridge limits (this is an increase of over 14,000 pounds of payload). Other vehicle configurations will also benefit from the change.

However, one system that will not be able to take advantage of this change is the Army's Heavy Equipment Transporter System (HETS), which far exceeds all Federal and State gross weight and axle load limits and has a much more complex movement process.

Certification as Essential to National Defense

Highway movement essential to national defense applies to essential materiel that cannot be reduced in size or weight to meet permit limits or cannot be moved by another transportation mode. Training, maintenance, public relations mission, and transportation savings are not valid justifications. The Major Army Command (MACOM) Commander must certify that the load is essential to national defense. The states have full authority to refuse transport permission since they have absolute authority over their public highways in both peacetime and wartime.