Comprehensive Truck Size and Weight Limits Study: Pavement Comparative Analysis Technical Report

Executive Summary

Background

This report documents analyses conducted as part of the U.S. Department of Transportation (USDOT) 2014 Comprehensive Truck Size and Weight Limits Study (2014 CTSW Study). As required by Section 32801 of MAP-21 [Moving Ahead for Progress in the 21st Century Act (P.L. 112-141)], Volumes I and II of the 2014 CTSW Study have been designed to meet the following legislative requirements:

• Subsection 32801 (a)(1): Analyze accident frequency and evaluate factors related to accident risk of vehicles to conduct a crash-based analyses, using data from states and limited data from fleets;
• Subsection 32801 (a)(2): Evaluate the impacts to the infrastructure in each State including the cost and benefits of the impacts in dollars; the percentage of trucks operating in excess of the Federal size and weight limits; and the ability of each state to recover impact costs;
• Subsection 32801 (a)(3): Evaluate the frequency of violations in excess of the Federal size and weight law and regulations, the cost of the enforcement of the law and regulations, and the effectiveness of the enforcement methods; Delivery of effective enforcement programs;
• Subsection 32801 (a)(4): Assess the impacts that vehicles have on bridges, including the impacts resulting from the number of bridge loadings; and
• Subsections 32801 (a)(5) and (6): Compare and contrast the potential safety and infrastructure impacts of the current Federal law and regulations regarding truck size and weight limits in relation to six-axle and other alternative configurations of tractor-trailers; and where available, safety records of foreign nations with truck size and weight limits and tractor-trailer configurations that differ from the Federal law and regulations. As part of this component of the study, estimate:
  1. the extent to which freight would likely be diverted from other surface transportation modes to principal arterial routes and National Highway System intermodal connectors if alternative truck configuration is allowed to operate and the effect that any such diversion would have on other modes of transportation;
  2. the effect that any such diversion would have on public safety, infrastructure, cost responsibilities, fuel efficiency, freight transportation costs, and the environment;
  3. the effect on the transportation network of the United States that allowing alternative truck configuration to operate would have; and
  4. the extent to which allowing alternative truck configuration to operate would result in an increase or decrease in the total number of trucks operating on principal arterial routes and National Highway System intermodal connectors.

To conduct the study, the USDOT, in conjunction with a group of independent stakeholders, identified six different vehicle configurations involving six-axle and other alternative configurations of tractor-trailer as specified in Subsection 32801 (a)(5), to assess the likely results of allowing widespread alternative truck configurations to operate on different highway networks. The six vehicle configurations were then used to develop the analytical scenarios for each of the five comparative analyses mandated by MAP-21. The use of these scenarios for each of the analyses in turn enabled the consistent comparison of analytical results for each of the six vehicle configurations identified for the overall study.

The results of this 2014 Comprehensive Truck Size and Weight Limits Study (2014 CTSW Study) are presented in a series of technical reports. These include:

• Volume I: Comprehensive Truck Size and Weight Limits Study – Technical Summary Report. This document gives an overview of the legislation and the study project itself, provides background on the scenarios selected, explains the scope and general methodology used to obtain the results, and gives a summary of the findings.
• Volume II: Comprehensive Truck Size and Weight Limits Study. This volume comprises a set of the five comparative assessment documents that meet the technical requirements of the legislation as noted:
  • Modal Shift Comparative Analysis (Subsections 32801 (a)(5) and (6)).
  • Pavement Comparative Analysis (Section 32801 (a)(2)).
  • Highway Safety and Truck Crash Comparative Analysis (Subsection 32801 (a)(1)).
  • Compliance Comparative Analysis (Subsection 32801 (a)3)).
  • Bridge Structure Comparative Analysis (Subsection 32801 (a)(4)).

Purpose of Pavement Technical Report

The purpose of the Volume II: Pavement Comparative Analysis is to use the six vehicle configuration scenarios identified by USDOT to address two major items:

1. How the full spectrum of axle weights and types may change as a result of modal and configuration shifts in each scenario, and
2. How these changes affect pavement performance and expected pavement costs.

The first three scenarios asses the impacts of heavier tractor semitrailers than are generally allowed under current Federal law. Scenario 1 would allow five-axle (3-S2) tractor semitrailer to operate at a maximum gross vehicle weight (GVW) of 88,000 lb. while Scenarios 2 and 3 would allow six-axle (3-S3) semitrailers to operate at maximum GVWs of 91,000 lb. and 97,000 lb. respectively.

Scenarios 4, 5 and 6 examine vehicles that would serve primarily lower density cargoes commonly associated with those trucks that carry cargo from more than one shipper (known as less-than-truckload traffic or LTL). Scenario 4 examines twin trailer combination with 33-foot trailers (2-S1-2) with a maximum GVW of 80,000 lbs. Scenarios 5 and 6 examine triple trailer combinations with 28 or 28.5-foot trailers having maximum GVWs of 105,500 lb. (2-S1-2-2) and 129,000 lb. (3-S2-2-2), respectively.

At this point it is important to note that while the control double has an approved GVW of 80,000 pounds, the GVW used for the control double in the study is 71,700 pounds based on data collected from weigh-in motion (WIM)-equipped weight and inspection facilities and is a more accurate representation of actual vehicle weights than the STAA authorized GVW. Using the WIM-derived GVW also allows for a more accurate representation of the impacts generated through the six scenarios.

Table ES-1 on the following page depicts the vehicles assessed under each scenario as well as the current vehicle configurations from which most traffic would likely shift (the control vehicles).

Approach and Methodology

The comparative analysis for the study consisted of a step-wise approach to determining the effects of the various truck traffic configuration scenarios, also accounting for potential freight traffic shifts to or from the rail mode as a result of each scenario, on performance and life-cycle costs. The process started with the selection of representative pavement sections (flexible and rigid along with their local materials and design inputs) within each of the four primary geographic locations in the United States.

The USDOT study team used the AASHTOW are Pavement ME Design® software to evaluate each of these sections to determine a base case of the expected pavement life prior to any needed rehabilitation under representative base case traffic conditions (e.g., representative of the mix of vehicle types and operating weights that might be expected). The Pavement ME Design® software tool uses a procedure that directly applies axle load spectra to calculate the amount of damage produced by the estimated range of traffic loads. The axle load spectra data were obtained from processing weigh-in-motion (WIM) data and include axle-load distributions (e.g., single, tandem, tridem, quads) and axle-load configurations (e.g., axle spacing and wheelbase). The study team performed an initial analysis of climatic variability within the vicinity of each geographic location to ensure that the sites selected were representative of typical weather effects and inclusive of typical subgrade soils in that area. The data from Long-Term Pavement Performance (LTPP) sections were used as a starting point for each sample section.

The analysis then considered the four pavement types (new flexible pavement, flexible overlay of existing flexible pavement, new jointed plain concrete pavement (JPCP), and composite (flexible overlay of existing JPCP) pavement) which represent the overwhelming majority of pavements used on the Interstate and National Highway System (NHS) in the United States. The Study does not attempt to evaluate the impact of the scenarios on the performance of overlaid pavements.

Table ES-1: Truck Configurations and Weights Scenarios Analyzed in the 2014 CTSWL Study

Scenario	Configuration	Depiction of Vehicle	# Trailers or Semi-trailers	# Axles	Gross Vehicle Weight (pounds)	Roadway Networks
Control Single	5-axle vehicle tractor,53 foot semitrailer (3-S2)		1	5	80,000	STAA 1 vehicle; has broad mobility rights on entire Interstate System and National Net­work including a significant portion of the NHS
1	5-axle vehicle tractor, 53 foot semitrailer (3-S2)		1	5	88,000	Same as Above
2	6-axle vehicle tractor, 53 foot semitrailer (3-S3)		1	6	91,000	Same as Above
3	6-axle vehicle tractor, 53 foot semitrailer (3-S3)		1	6	97,000	Same as Above
Control Double	Tractor plus two 28 or 28 ½ foot trailers (2-S1-2)		2	5	80,000 maximum allowable weight 71,700 actual weight used for analysis 2	Same as Above
4	Tractor plus twin 33 foot trailers (2-S1-2)		2	5	80,000	Same as Above
5	Tractor plus three 28 or 28 ½ foot trailers (2-S1-2-2)		3	7	105,500	74,500 mile roadway system made up of the Interstate System, approved routes in 17 western states allowing triples under ISTEA Freeze and certain four-lane PAS roads on east coast 3
6	Tractor plus three 28 or 28 ½ foot trailers (3-S2-2-2)		3	9	129,000	Same as Scenario 5 3

The data used in the pavement analyses of this Study came from several FHWA sources, namely the Highway Performance Monitoring System (HPMS), vehicle classification and weight data reported by the States to FHWA, the Long-Term Pavement Performance (LTPP) database, and from MEPDG calibration data from four State departments of transportation (DOT). The models used for the analysis are those that are in Version 2.0 of the Pavement ME Design® software.

After compiling the input data required for each of the sections, the USDOT study team analyzed the base case traffic volumes were analyzed for each geographic location and pavement type and ran a set of analyses for each of the six modal shift scenarios in order to estimate the change in initial service interval.

A number of key assumptions and limitations apply to this study. The main limitation is that this Study considers only initial service lives predicted by Pavement ME Design® software,version 2.0, and only for the distresses and pavement types that the software could suitably model. Deterioration caused by the interaction of loads and construction deficiencies or decreased materials durability (e.g., deterioration of HMA transverse cracks caused by low temperatures, deterioration of concrete pavement “D” cracking) are outside the scope of this study, although it should be recognized that these elements can significantly impact the performance of pavements. In addition, the impacts of truck tire types (e.g., wide-based radial) and tire-pavement interaction (e.g., braking, torquing, and other physical responses) are not considered.

Pavement distress levels and ride quality were reported based on average predicted performance.

Truck Size and Weight Scenarios

The first three scenarios assess tractor semitrailers that are heavier than generally allowed under currently Federal law. Scenario 1 assesses a five-axle (3-S2) tractor-semitrailer operating at a GVW of 88,000 pound, while Scenarios 2 and 3 assess six-axle (3-S3) tractor semitrailers operating at GVWs of 91,000 and 97,000 pounds, respectively. The control vehicle for these scenario vehicles is the five-axle tractor-semitrailer with a maximum GVW of 80,000 pounds. This is the most common vehicle configuration used in long-haul over-the-road operations and carries the same kinds of commodities expected to be carried in the scenario vehicles.

Scenarios 4, 5, and 6 examine vehicles that would serve primarily less-than-truckload (LTL) traffic that currently is carried predominantly in five-axle (3-S2) tractor-semitrailers and five-axle (2-S1-2) twin trailer combinations with 28 or 28.5-foot trailers and a maximum GVW of 80,000 pounds. Scenario 4 examines a five-axle (2-S1-2) double trailer combination with 33-foot trailers with a maximum GVW of 80,000 pounds. Scenarios 5 and 6 examine triple trailer combinations with 28.5-foot trailer lengths and maximum GVWs of 105,500 (2-S1-2-2) and 129,000 (3-S2-2-2) pounds, respectively. The five-axle twin trailer with 28.5-foot trailers is the control vehicle for Scenarios 4, 5, and 6 since it operates in much the same way as the scenario vehicles are expected to operate.

At this point it is important to note that while the applicable Federal gross vehicle weight limit is 80,000 lbs., a GVW of 71,700 lbs. was used for the control double configuration in the study based on empirical findings generated through an inspection of the weigh-in-motion data used in the study.

Analysis of the relative impacts of one group of vehicles compared with another at the national system level requires some simplification of assumptions about the vehicles themselves. For each scenario in this study, freight was shifted either from one vehicle to another, or to a vehicle of the same type but with a different weight. The approach used in the study assumed that both the before and after vehicles in each scenario had the same temporal use patterns, the same tire and suspension characteristics, were traveling at the same speeds, and behaved in ways that were similar; thus, the only variable considered for pavement analysis was the change in axle weights and types.

The scenarios assessed in the Study used the Interstate and National Highway System (NHS) roadways. More than 80% of total annual truck miles travelled occurs on the NHS. There are more than four million center line miles of public roadways in the United States with most of those miles located off of the NHS. There is generally little quantitative information available regarding travel, by facility, occurring on this non-NHS roadway network and on how pavements on the local road system are designed, built, and maintained. Except in rare cases, there is minimal to no history of travel or pavement characteristic data on local roads. These data limitations have made it prohibitive to perform an accurate and representative study on the impacts of loading scenarios on local roads at this time. The lack of pavement structure characteristics, pavement surface type and typical travel levels for local system roadways yields it impossible to develop sampling based approaches that would produce results supported with adequate statistical confidence. A review of the low-volume NHS sample section results very generally point in the direction of impacts that scenario configurations may have on local roads but, it must be understood, local roads are, overall, built to lower design standards than roadways on the higher functionally classified roadway networks. It is also understood that daily travel demand levels and daily truck travel on local roads are typically low, hence the lower design standards they were built to.

Desk Scan

The purpose of the Volume II: Pavement Comparative Analysis desk scan was to review the most relevant previous studies comparing pavement impacts from vehicle use. These studies included international, national and state cost allocation and truck size and weight studies as well as any other studies that included estimates of vehicle-induced pavement costs on either an absolute or relative basis. The USDOT study team also searched for pavement analyses or design studies intended to assist in the selection of an appropriate analytical tool and support application of the selected tool – the Pavement ME Design® software.

The principal objective of the search was to gain a thorough understanding of the current state of research and practice concerning pavement performance and cost analysis related to heavy vehicle use. The literature search included a variety of information sources: (1) engineering and scientific periodicals and journals; (2) conference proceedings; (3) Federal, State, international, and university reports that show up in library search engines (such as Compendex) based on key word searches; and (4) studies identified during the May 29, 2013, stakeholder public hearing for the study or by USDOT officials.

The results of the Desk Scan can be found in Appendix A.

Summary of Results

This Volume II: Pavement Comparative Analysis analyzed the effect of overweight axles in current operations, defining overweight as single axles weighing more than 20,500 lbs. and tandem axles weighing more than 35,000 lbs. to be consistent with the axle weight group boundaries used in the vehicle weight analysis. Initial service intervals were found to increase significantly for both flexible and rigid pavement sections (except in the case of one rigid pavement section that did not reach the end of its initial service interval during the analysis period). Flexible pavement initial service intervals increased by between and 19 percent and 34 percent, and rigid pavement initial service intervals increased by between 0 percent and 10 percent when overweight axles were removed from the traffic mix.

The estimated impact of the truck size and weight scenarios varies among the scenarios as well as the pavement type and service conditions considered in the analysis. Scenario 1 (allowing 3-S2 tractor semitrailers to operate at a GVW of 88,000 lbs.) results in a heavier array of tandem axle loads, while by contrast, Scenarios 2 and 3 (allowing 3-S3 semitrailers to operate at GVWs of 91,000 and 97,000 lbs., respectively) transferred loads from some of the heavier tandem axles to tridem axles. Scenario 4 (allowing 2-S1-2 twin trailer combinations with 33-foot trailers) results in an increase in the weight distributions of single-load axles. Scenario 5 (allowing 2-S1-2-2 triple trailer combinations with GVWs of 105,500 lbs.) transfers some tandem axle loads to lighter single axle vehicles. Scenario 6 (allowing 3-S2-2-2 triple trailer combinations with two extra axles with GVWs of 129,000 lbs.) results in lower tandem axle weights as well as a similar shift in freight movements from tandems to lighter single-axle vehicles.

Average impacts of each scenario in terms of both time to first rehabilitation and life cycle cost are summarized in Table ES-2. Notably, flexible pavements exhibited more accelerated deterioration with Scenarios 1 and 4, whereas rigid pavements were more negatively impacted by Scenarios 4, 5, and 6.

The more significant impacts are predicted to occur on lower volume facilities, specifically on low-volume other NHS arterials, which are typically constructed with thinner cross-sections. The estimated impacts of the scenarios are relatively minor for those thicker pavement sections that were built to handle higher truck volumes. The range of impacts for each scenario results from varying pavement conditions, climatic conditions, and highway types.

The life cycle cost (LCC) implications of the scenarios also varied. Table ES-2 summarizes these differences averaged over all pavement types, geographic locations, and types of facilities under two alternative discount rates, a rate for estimating conservative or lower bound values and a typical rate to estimate upper bound values. The 1.9% discount rate was provided in guidance to federal agencies issued in 2014 by OMB in the annual update to Circular No. A-94 and 7.0% provided in Circular A-94 that was used in the FHWA 2013 Status of the Nation’s Highways, Bridges, and Transit: Conditions & Performance Report. Note that the table shows the range of the results of applying each discount rate. On average, Scenario 4 resulted in the largest LCC overall increase of 1.8% to 2.7% from the base case, whereas Scenario 2 and 3 resulted in 2.4% to 4.2% decreases in predicted LCC from the base case. Scenarios 1, 5, and 6 showed only slight increases in LCC. LCC is defined herein as agency cost for pavement rehabilitation (e.g., overlays, retexturing) over a 50-year analysis period, not including initial construction costs or user costs.

Study results, being national in nature, must be reviewed with the understanding that truck travel demand across the country is not evenly distributed. Impacts on pavements will vary by region, not only due to stress applied to pavement structures due to differences in geotechnical and climatic conditions and situations, but due to the type of truck travel demand occurring in various regions of the country. Recently, the emergence of a strategic change in the US energy model saw new and significant pressures put on transportation modes in very specific regions of the country and on very specific travel corridors. The travel demand and impacts assessed for each of the scenarios in this Study must be reviewed in this light.

Table ES-2: Impacts of Study Scenario (Compared to Base Case) on Pavement Performance and Costs

Scenario		Weighted Average Change in Service Intervals	Weighted Average Change in Life Cycle Costs
1	88,000-lb 5-Axle Single-Semitrailer Combinations	- 0.3%	+0.4% to +0.7%
2	91,000-lb 6-Axle Single-Semitrailer Combinations	+2.7%	-2.4% to -4.2%
3	97,000-lb 6-Axle Single-Semitrailer Combinations	+2.7%	-2.6% to -4.1%
4	5-Axle Double-Trailer Combinations with 33-Foot Trailers	-1.6%	+1.8% to +2.7%
5	105,500-lb 7-Axle Triple-Trailer Combinations	0.0%	+0.1% to +0.2%
6	129,000-lb 9-Axle Triple-Trailer Combinations	-0.1%	+0.1% to +0.2%

Note: Individual pavement sections were weighted based on the number of lane-miles of pavement of each type, thickness range, and highway type.

References

Chatti, K. “Effect of Michigan Multi-Axle Trucks on Pavement Distress.” Michigan DOT and Michigan State University, Final Report, Executive Summary, Project RC-1504. February 2009. http://www.michigan.gov/documents/mdot/MDOT_Research_Report_RC-1504__ExecSum_272183_7.pdf

Chatti, K., Salama, H., and C. Mohtar. “Effect of Heavy Trucks with Large Axle Groups on Asphalt Pavement Damage.” Presented at 8th International Symposium on Heavy Vehicle Weights and Dimensions, Johannesburg, South Africa, March 2004. http://road-transport-technology.org/Proceedings/8%20-%20ISHVWD/EFFECT%20OF%20HEAVY%20TRUCKS%20WITH%20LARGE%20AXLE%20GROUPS%20ON%20ASPHALT%20PAVEMENT%20DAMAGE%20-%20Chatti.pdf

Timm, D., Turochy, R., and K. Peters. Correlation between Truck Weight, Highway Infrastructure Damage and Cost. Auburn College of Engineering for FHWA, DTFH61-05-Q-00317, Subject No 70-71-5048. October 2007. http://www.eng.auburn.edu/files/centers/hrc/DTFH61-05-P-00301.pdf

Tirado, C., Carrasco, C., Mares, J., Gharaibeh, N., Nazarian, S., and J. Bendaña. “Process to Estimate Permit Costs for Movement of Heavy Trucks on Flexible Pavements.” Transportation Research Record: Journal of the Transportation Research Board, 2154, National Research Council. Washington, D.C., pp. 187-196, 2010. http://pustaka.pu.go.id/files/pdf/BALITBANG-03-C000066-610032011103843-process_to_estimate_permit_cost.pdf

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