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REPORT
This report is an archived publication and may contain dated technical, contact, and link information
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Publication Number:  FHWA-HRT-16-024     Date:  June 2016
Publication Number: FHWA-HRT-16-024
Date: June 2016

 

LTBP Program's Literature Review on Weigh-In-Motion Systems

Chapter 2. Regulations on Truck Weight Limits

History of Federal TS&W Regulations and Related Studies

The Comprehensive Truck Size and Weight Study, published by the U.S. Department of Transportation (USDOT) in 2000, summarizes the chronology of developments in Federal TS&W limits.(7) The following section presents an abbreviated description of that chronology as it helps to frame the evolution of the Federal TS&W limits currently in place for the U.S. Interstate System.

The first limits on TS&W for vehicles operating on the Interstate System were imposed by the Federal Government in 1956.(7) The Federal-Aid Highway Act of 1956 (Public Law 84-627) authorized construction of the National System of Interstate and Defense Highways (Interstate System) and established size and weight limits for commercial vehicles operating on this system.(8) The maximum weight limits were set at 73,280 lb for gross weight, 18,000 lb for single axles, and 32,000 lb on tandem axles. States that already had stricter weight or size regulations in place when the Federal limits went into effect were permitted to keep using them.

Both the allowable gross weight and axle weight limits for interstate highways were subsequently increased by Congress in 1975 when it enacted the Federal-Aid Highway Amendments of 1974 (Public Law 93-643).(9) The legislation amended Title 23 United States Code (U.S.C.) § 127, increasing the gross weight limit to 80,000 lb, the single axle limit to 20,000 lb, and the tandem axle limit to 34,000 lb.(10) It also enacted the Federal Bridge Formula, limiting the weight-to-length ratio of any vehicle crossing a bridge.(11) The law still permitted States to adopt lower, stricter weight limits if they already had such regulations in effect in 1956.(7)

Congress later required States to adopt the Federal length and weight limits on the Interstate System with the passage of the Surface Transportation Assistance Act of 1982 (STAA) (PublicLaw 97-424).(12) This legislation also required States to permit commercial vehicles with STAA-defined dimensions to operate on the Interstate System and other qualifying Federal-aid primary system highways. The last significant changes to Federal TS&W limits were made by the passage of the Intermodal Surface Transportation Efficiency Act of 1991 (Public Law 102-240), which prohibited States from allowing expansion of longer combination vehicle (LCV) operations, and the Transportation Equity Act for the 21st Century (Public Law 105-178), which extended the prohibition of expanded LCV operations.(13,14)

In 1990, the Transportation Research Board (TRB) conducted a comprehensive study that quantified the potential impacts of 10 proposed truck weight regulations.(15) It was found that a 10 percent increase in the number of equivalent single-axle loads (ESALs) on the Nation’s highways would trigger an annual need for additional $25 million for new and reconstructed pavements and $350 million for the resurfacing of existing pavements. It was also found when designing new pavements that the required pavement thickness slightly increases as a result of the increased traffic loadings.

In 1990, another TRB study assessed the costs of fatigue life reduction and substandard load ratings on bridges caused by a variety of proposed new truck weight limits.(16) The study covered bridges constructed according to current and older design standards. It was estimated that the proposed new truck weight limits and vehicle configurations would require an additional investment of several billion dollars per year for bridges depending on the scenario of the proposed truck weight limit. The cost for replacing substandard bridges as a result of strength rating was dominant among all the bridge cost impacts considered. This was the first effort made to include new bridges as a cost impact category.

In the Comprehensive Truck Size and Weight Study conducted by USDOT, several vehicle scenarios of truck weight limit changes were considered and compared with a base case to estimate the cost impacts.(7) Each scenario included seven or eight truck configurations. The factors considered in the study include infrastructure costs, safety, productivity, traffic operations, and intermodal competition. It was found that the impacts of TS&W depend on several factors, including GVW, axle weight, the distance between axle groups, pavement type, and bridge type and length. The analytical framework developed from this study is flexible and can be adjusted to assess specific proposals.

In 2002, TRB conducted Special Study 267 concerning the regulation of weights and sizes of commercial motor vehicles.(17) This study suggested that methods used in past studies had not received adequate estimates of the effect of changes in truck weights regarding bridge costs. It found that the efficiency of the highway system may be improved by reforming Federal TS&W regulations, which may involve allowing larger trucks to operate. The committee recommended a federally supervised permit program to allow the operation of heavier vehicles, provided that the changes applied only to vehicles with a maximum weight of 90,000 lb, double trailer configurations with each trailer up to 33 ft, and an overall weight limit governed by the Federal Bridge Formula.

A new comprehensive TS&W study is currently being conducted by USDOT under the Moving Ahead for Progress in the 21st Century Act.(18) The objectives of this study include evaluating safety risks, impact on infrastructure (pavements and bridges), levels of compliance and enforcement caused by trucks operating at different TS&W limits, and potential modal shift. Alternative configurations (including configurations that exceed current Federal TS&W limits) are being compared with the current Federal TS&W regulations, and the effects on freight diversion resulting from these alternative configurations will be analyzed. The results of this study were not available at the time this report was written.

Current Federal Truck Weight Regulations

Truck weight regulations significantly affect the design of transportation infrastructure and the efficiency of truck freight transportation operations. Truck weight regulations have evolved continually through fluctuations of the economy, vehicle technology advancements, pavement structural capacity changes, and emerging bridge structural designs.

Federal law currently limits single axles to 20,000 lb and tandem axles (axles closer than 96inches apart) to 34,000 lb.(7) GVW is limited to 80,000 lb. Federal law also regulates that States cannot impose stricter weight limits than the Federal limits on interstate highways. In addition, Federal law regulates bridge formula weight limits, which control vehicle weights, to protect the Nation’s bridges.(7) In particular, it limits the weight on groups of axles depending on the distance between those axles. The limits are determined by using the Federal Bridge Formula (see figure 1).(11)

Figure 1. Equation. Federal Bridge Formula. W equals 500 times open bracket LN divided by N minus 1 plus 12 times N plus 36 closed bracket.

Figure 1. Equation. Federal Bridge Formula.

Where:

W = Gross weight on any group of two or more consecutive axles, to the nearest 500 lb.
L = Distance between the outer axles of any group of two or more consecutive axles, in ft.
N = Number of axles in the group under consideration.

For example, the weight limit on a tandem axle with an axle spacing of 9 ft is 39,000 lb (see figure 2).

Figure 2. Equation. Example calculation of the weight limit for a tandem axle by using the Federal Bridge Formula.  W equals 500 times open bracket open parenthesis 9 closed parenthesis times open parenthesis 2 closed parenthesis all divided by open parenthesis 2 minus 1 closed parenthesis plus 12 times open parenthesis 2 closed parenthesis plus 36 closed bracket which equals 39,000.

Figure 2. Equation. Example calculation of the weight limit for a tandem axle using the Federal Bridge Formula.

State Regulations on Truck Loads

Generally, each State has a State-specific version of weight limits and regulations governing trucking activities. Detailed regulations may impose separate limits for certain classes of roads and bridges. Although basic Federal TS&W limits have not changed since 1982 (with the exception of the LCV freeze), several States have been granted exceptions by Federal legislations to GVW or axle weight limits.(15) In addition, States are granting an increasing number of permits for oversize/overweight (OS/OW) trucks.(15) The Comprehensive Truck Size and Weight Study conducted by USDOT summarizes the following general State weight limits: single axle, tandem axle, bridge formula, and GVW.(7) These limits usually apply both on and off the Interstate System.

The following is a summary extracted from the National Cooperative Highway Research Program (NCHRP) Synthesis 453 on the States’ legal loads:(19)

Overload Permits

Trucking accounts for approximately 80 percent of freight transportation expenditures in the United States. An OS/OW permit is required for trucks traveling with a size or weight exceeding the legal limits for dimension and weight regulated by State agencies. The state-of-practice of OS/OW vehicle permitting systems varies at different State and local agencies in terms of permit type, fee structure, and operation process. The trucking industry has expressed its concern over the multiple permit application processes and permits fee structures available across the States in an interstate trip, which may cause truck fleets to change their vehicle configurations or pay loads when transporting the same goods through different States.(20)

According to FHWA, any vehicle with a total GVW exceeding the legal weight of 80,000 lb (Title 23 Code of Federal Regulations (CFR) Part 658.17) requires an overweight permit.(7,21) It has been noted that an 80,000-lb truck does as much damage to the road as 9,600 cars do.(22) The stress level for pavement is mainly determined by truck wheel loads. However, bridge stress levels are controlled by weight distribution. Hence, the weight per axle and axle spacing must also be monitored. FHWA limits the allowable single axle weight to 20,000 lb for overweight vehicles. In addition, a bridge weight formula was developed and applied to commercial vehicles to determine the total gross weight.(11)

In some cases, a permit cannot be issued if a bridge is determined to be overstressed when carrying the excessive weight. Therefore, the load capacity of bridges might control the issuance of permits.(20) For example, an old bridge may need to be improved in case of repeated overweight loading.

Many State agencies issue superload and oversize permits in addition to annual or routine permits. Threshold dimensions or threshold weights for the extra-legal loads are specified by each State agency. For example, Indiana issues oversize and/or overweight permits for a load that exceeds legal dimensions but does not exceed the following upper threshold dimensions: 16 ft wide, 110 ft long, 15 ft high, and 120,000 lb.(23) The resulting fee for an oversize and overweight permit is the larger of the calculated overweight or oversize fees, and a superload permit is required if the load exceeds the upper threshold dimensions and does not fall under any other permit type.(23) In certain States, the number of axles is considered in conjunction with superload thresholds. In Illinois, for example, the threshold of 120,000 lb is valid only for trucks with six or more axles.(24) However, it is unknown how the thresholds were established among the various States; they might have been primarily based on expert judgments that considered the load capacity of pavements and bridges in that State.

Permit Fee Structure

Typically, permits can be divided into several categories based on cargo, load, truck configuration, trip, and time. Based on loads, permits can be divided into divisible load-related permits and non-divisible load-related permits. According to Title 23 CFR Part 658.5, non-divisible means any load or vehicle exceeding applicable length or weight limits which, if separated into smaller loads or vehicles, would compromise the intended use of the vehicle, affect the value of the load or vehicle, or require more than 8 work hours to dismantle using appropriate equipment.(25) It is noted that the number of non-divisible trip permits issued in each State significantly exceeds the number of divisible trip permits.

Based on trips, permits can be divided into single-trip permits and multi-trip (annual) permits for a given time period. Single-trip permits are valid from one point of origin to one specific destination, and the hauler is allowed to make the move during the times specified on the permit (usually 4 to 6 consecutive days). Recently, annual or multiple trip permits have become more commonplace, which has become a concern for FHWA, presumably because the larger loads and their increased frequency may not be adequately represented by the notional load model used for bridge design.(26)

Though there are a variety of permits in each State, the fee structures in overweight single-trip permits, oversize single-trip permits, and annual permits can be representative of the fee structure in each State agency. An extensive review of the permit fee structure of each State shows five major fee considerations when issuing single-trip permits: number of axles, distance and weight (combined), distance, weight, and flat rate.

The number of States in each fee structure is shown in table 1. Most States in the West, including California, Arizona, Oregon, Washington, Utah, and Nevada, use a flat fee for their single-trip permit structure. Single-trip permits are very attractive for truckers in the West because flat-rate fees are generally much less expensive than distance-based fees.

Table 1. Factors considered in single-trip permits in 50 States.

Factors Number of States
Number of axles
4
Distance and weight
13
Distance
2
Weight
9
Flat rate
22

In terms of the difference between single-trip permit fees and annual permit fees, it can be concluded that fees associated with multi-trip permits are only slightly more expensive than those associated with single-trip permits. Most States have adopted single-trip permit fees that scale by weight or distance but still assign flat-rate fees to annual permits.

Agencies issue permits for the purpose of ensuring the safe travel of loads on State highways. It is difficult to effectively assess the damage done to the infrastructure as a result of permitted vehicle travel. Currently, in Oregon, a value is periodically assigned to the damage caused by trucks to both pavements and bridges, and that cost is used to establish a weight-mile tax structure.(27) Another trend in permit issuance is that some highway agencies switched from single-trip permit systems to annual, blanket flat-fee permit systems after the early 1990s. It is reported that while these agencies benefited from the convenience of reduced monitoring of single trips, they lost significant revenue because there was no limit for the number of trips they made in 1 year on an annual permit.(28)

 

 

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