CTS&W Limits Study: Volume 1 Technical Reports Summary - Chapter 2: Scope and General Methodology
Chapter 2: Scope and General Methodology
This section discusses the overall scope of the analysis conducted under the study to address the directives identified in MAP-21. (See Box 1 for the specific provisions in the law.) Section 2.1 identifies the six alternative truck configurations examined and how they were selected and also presents the six highway network scenarios studied. Section 2.2 presents the general methodology underlying the analysis, including the study process set in place, steps to ensure public input, and the peer review at all critical junctures in the study process.
More specific and detailed information on the scope and methods used in the study and a summary of results of the technical analysis for each of the project focus areas are provided in Chapter 3 of this Technical Reports Summary.
Scope
Congress directed USDOT to compare the impacts of vehicles currently operating above Federal truck size and weight limits to those operating at or below these limits as well as to assess the safety, enforcement and pavement and bridge infrastructure impacts of "six-axle and other alternative configurations of tractor-trailers" if they were allowed to operate above current weight regulations. FHWA did not intend to develop or support a position on changes to current Federal truck size and weight limits in this study; rather, the agency intended to assess the impacts that any such changes might have in the various areas included in the study to better understand the impacts that trucks operating above current Federal truck size and weight limits have today. The study was set up to provide the results of the assessments that were completed and to provide a summary of this analysis to Congress.
The provisions in MAP-21 require the USDOT to conduct a study that:
- Addresses differences in safety risks, infrastructure impacts, and the effect on levels of enforcement between trucks operating at or within Federal truck size and weight limits and trucks legally operating in excess of Federal limits;
- Estimates changes in freight movements by truck types and by various modes caused by the introduction on alternative truck configurations;
- Assesses the impacts that alternative configurations examined in the study may have on highway safety, infrastructure service life, fuel consumption, the environment, traffic operations and costs; and
- Identifies all Federal rules and regulations impacted by changes in Federal size and weight limits.
To answer these questions, USDOT structured the analysis around the following five major study areas:
- Modal Shift Analysis
- Safety Analysis
- Pavement Analysis
- Bridge Analysis
- Compliance Analysis
Each of the five study focus areas was the subject of a technical report, the results of which are summarized later in Chapter 3. Box 7 provides an overview of the subjects addressed in these technical reports. The modal shift analysis is listed first not because the issue was given a higher priority, but because it produced a common vehicle miles of travel dataset that was the foundation for analyzing safety, pavement, bridge, and compliance topics, in addition to the modal shifts topic.
At the outset of the study, USDOT identified the roadway networks on which to conduct the comparative analysis. These networks are the Interstate System, the National Highway System (NHS), 10 and the National Truck Network (NN). The evaluation also included "Reasonable access roadways," which are those roadways connecting the Interstates or other National Network roads with freight terminals or distribution centers.
More than 80 percent of total annual truck miles traveled occurs on the NHS. There are more than 4 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 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 make it prohibitive to perform an accurate and representative study on the impacts of loading scenarios on local roads. The lack of pavement structure characteristics, pavement surface type, and typical travel levels for local system roadways prevent sampling-based approaches that would produce results supported with adequate statistical confidence. Data limitations also made it impossible to perform an accurate and representative study on the impacts of loading scenarios on local roads.
A review of the low-volume NHS sample section results suggests the impacts that scenario configurations may have on local roads and local roads are generally built to lower design standards than roadways on the higher functionally classified roadway networks. However, it is also understood that daily travel demand levels and daily truck travel on local roads is typically low, consistent with the lower design standards to which they are built, so it is not possible in this study to draw definitive conclusions about the impacts of any potential changes in truck sizes or weights on local roads.
Box 7. The Five Focus Areas for the Study
The results of the study were produced from extensive comparative analyses in five focus areas. These analyses have been combined into a Technical Summary Report, summarized in more detail in Chapter 3.
The Modal Shift Analysis assesses how the use of alternative truck configurations might shift freight commodities among truck types and between modes and examines how such shifts could affect traffic operations, fuel consumption, distribution of cost responsibilities among the different types of vehicles, cost recovery, and the overall impacts on the U.S. freight system and the environment.
The Highway Safety and Truck Crash Comparative Analysis compares the impacts that trucks operating at and below Federal truck size and weight limits have on highway safety and crash frequency and severity compared to trucks operating above those limits. It also assesses the impacts that the six alternative truck configuration vehicles as compared to the control vehicles used in the study in terms of their highway safety records, safety risk factors, and vehicle stability and control under six network scenarios.
The Pavement Comparative Analysis compares the impacts that trucks operating at and below Federal truck size and weight limits have on highway pavement infrastructure compared to trucks operating above those limits. It also compares the costs and benefits for pavement performance, pavement maintenance, and rehabilitation under the relevant network scenarios of the six alternative truck configurations with trucks operating at current Federal size and weight limits terms for a range of paving materials, climatic, geographic and environmental conditions.
The Bridge Structure Comparative Assessment compares the impacts that trucks operating at and below Federal truck size and weight limits have on highway bridge infrastructure compared to trucks operating above those limits. It also compares the structural effects of the six alternative truck configurations with trucks operating within Federal limits (the control vehicles) for a representative sample of bridges selected from the National Bridge Inventory.
The Compliance Comparative Analysis evaluates violation rates for commercial motor vehicles not complying with Federal truck size and weight limits by type (for example, Federal bridge formula, gross vehicle weight, single-axle-weight, and tandem-axle weight violations), and examines enforcement costs. In addition, as required by MAP-21, this analysis identifies all Federal laws and regulations that would be affected by a change in Federal truck size and/or weight limits.
To conduct the structural bridge impact comparative analysis, a representative sample of 490 bridges from the National Bridge Inventory was selected as the number of bridges that could be analyzed within the project period. These bridges are located on the Interstate System, the NHS, or the NN. The sample database of bridges provides a diverse representation of the bridges that make up the NHS inventory. The bridge types selected for the study were determined based on the material of construction, distinct structural behavior, and span configurations. Bridge selection was further refined to include additional considerations including year built, maximum span length, and live load capacity to get a diverse sample space.
Selecting a representative mix of alternative truck configurations for examination was critical, as MAP-21 only specified that the six-axle combination be included among the mix of alternatives. As discussed in Chapter 1, many countries, including Canada and Mexico, allow trucks to operate on their roadways in excess of U.S. truck size and weight limits. Even in the United States, “grandfather clauses” in Federal law allow some States to permit heavier or longer trucks to operate on some sections of the Interstates, the NN or the NHS than would be allowed under the Federal limitations. Table 3 summarizes current U.S. combination truck traffic operating at weights within and above the 80,000 pound Federal gross vehicle weight limit on the Interstate System, other National Highway System (NHS) routes, and highways off the NHS. For purposes of this study, truck configurations are defined in terms of the number of trailers and the number of axles on the vehicle.
Source: Study, Modal Shift Comparative Analysis Technical Report.
The table shows that there is appreciable truck traffic above the 80,000-lb. Federal GVW limit that applies to the Interstate System on the Nation’s roads. Much of this travel in vehicles weighing more than 80,000 lbs. is off the Interstate System where State weight limits apply, but some also is on the Interstate System. Some Interstate System travel in vehicles weighing more than 80,000 lbs. occurs in States with grandfathered weight limits over 80,000 lbs., some travel is under non-divisible load permits, and some travel reflects illegal overloads.
Early in the study, USDOT determined that up to six alternative truck configurations could be examined as part of the comparative analysis in the timeframe established in MAP-21. Also, to be selected for study, USDOT proposed that alternative truck configurations needed to be currently in use in the United States, Canada, or elsewhere, and practical for use in the United States. USDOT then identified three candidate truck configurations and solicited input from stakeholders regarding the selection of the additional configurations to include in the mix.
After extensive public and stakeholder input, USDOT identified the six alternative truck configurations to compare with control or baseline vehicles meeting current Federal size and weight limitations. In addition, two truck configurations that now meet Federal size and weight limitations were selected to serve as “baseline” or “control” vehicles. The comparisons would be conducted over six illustrative network scenarios, using data analysis, modeling, and other state-of-the-art methods to derive technical results in each of the five study focus areas. All but one of the vehicles selected for analysis are currently in common use on some U.S. highways, providing some baseline data and experience with these vehicles. The outlier, Scenario 4, had strong support for inclusion from stakeholder input and is in limited use in one State.
Box 8 shows the reasons why each alternative configuration was selected for inclusion in this study.
Box 8. Reasons for Selecting the Alternative Truck Configurations and Control Truck Configurations
The rationale for selecting the six alternative truck configurations to compare with control or baseline vehicles is discussed here.
Control Vehicle for Comparison with Single Trailer Combinations: A five-axle, tractor-semitrailer combination (3-S2), 80,000 lbs.; this is the standard configuration of a three-axle tractor (the “3” in 3-S2) with a 53-ft.-long, two-axle semitrailer (the "S2" in 3-S2) and a gross vehicle weight (GVW) of 80,000 lbs. that operates on U.S. Interstates and other roadways. This combination is used in the study to compare with alternative truck configurations 1 through 3 below. It is an STAA vehicle meeting current Federal size and weight limitations.
Alternative Truck Configurations with One 53-Foot Semitrailer
- Five-Axle, Tractor-Semitrailer Combination (3-S2), 88,000 lbs.: The same vehicle as the Control but loaded to the gross manufacturers weight rating (GMWR) of 88,000 lbs. This configuration was identified at the outset of the study to understand the performance implications of trucks operating at the manufacturers’ gross vehicle weight rating;
- Six-axle, Tractor-Semitrailer Combination (3-S3), 91,000 lbs.: This six-axle, 91,000-lb. configuration was selected to evaluate a six-axle truck that complies with the Federal Bridge Formula. (See Box 2 for description of the Bridge Formula.)
- Six-axle, Tractor-Semitrailer Combination (3-S3), 97,000 lbs.: A tractor-semitrailer configuration with a three-axle tractor and a three-axle semitrailer (hence 3-S3) and a GVW of 97,000 lbs. This configuration was selected because of the reference to analyzing the impacts of a six-axle truck in Section 32801, and the 97,000-lb. weight was identified due to Congressional interest (U.S. House of Representatives Bill HR 612, as introduced in the 113th Congress in 2013).
Control Vehicle for Combinations with More Than One Trailer: A tractor with twin 28.5-ft. trailers weighing 80,000-lbs. This standard configuration is in widespread use. Like the control vehicle for single trailer combinations described above, this vehicle is used to provide “baseline” data in the comparative analyses and is defined as an STAA vehicle that meets current Federal size and weight limitations.
Alternative Configurations with More than One Semitrailer/Trailer
- Twin 33-ft. trailers, 80,000-lbs. (2-S1-2): A configuration with two twin trailers, each 33-foot long and a GVW of 80,000 lbs. This combination was selected because of the strong interest expressed by carriers specializing in Less-Than-Truckload (LTL) shipments. This is the only alternative configuration not currently in general use in the United States. (During the analysis phase of the study it was learned that FedEx is piloting this configuration on the Florida Turnpike.)
- Triple 28.5-ft. trailers, 105,000 lbs. (2-S1-2-2): A triple-trailer configuration with three 28.5-foot trailers, seven axles, and a GVW of 105,000 lbs. This combination was selected because of the high level of interest from diverse Stakeholders.
- Triple 28.5-foot trailers, 129,000 lbs. (3-S2-2-2): The triple-trailer configuration with three 28.5-foot trailers and a GVW of 129,000 lbs. It was selected to evaluate the upper GVW limit allowed to operate under the “ISTEA Freeze” discussed in Box 2.
Table 4 shows the vehicles that would be allowed under each scenario as well as the current vehicle configuration from which most traffic would likely shift (the control vehicle).
| 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 |
1 The network is the 1982 Surface Transportation Assistance Act (STAA) Network (National Network or NN) for the 3-S2, semitrailer (53’), 80,000 pound gross vehicle weight (GVW) and the 2-S1-2, semitrailer/trailer (28.5’), 80,000 pound. GVW vehicles. The alternative truck configurations have the same access off the network as its control vehicle. return to Footnote 1
2 The 80,000 pound weight reflects the applicable Federal gross vehicle weight limit; a 71,700 gross vehicle weight was used in the study based on empirical findings generated through an inspection of the weigh-in-motion data used in the study. return to Footnote 2
3 The triple network starts with the network used in the 2000 Comprehensive Truck Size and Weight (CTSW) Study and overlays the 2004 Western Uniformity Scenario Analysis. The LCV frozen network for triples in the Western States was then added to the network. The triple configurations would not have the same off network access as its control vehicle, the 2-S1-2, semitrailer/trailer (28.5’), 80,000 pound GVW. Use of the triple configurations beyond the triple network would be limited to that necessary to reach terminals that are immediately adjacent to the triple network. It is assumed that the triple configurations would be used in Less-Than-Truck Load (LTL) line-haul operations (terminal to terminal). As a result, the 74,454 mile triple network used in this Study includes: 23,993 mile network in the Western States (per the 2004 Western Uniformity Scenario Analysis, Triple Network), 34,802 miles in the Eastern States, and 15,659 miles in Western States that were not on the 2004 Western Uniformity Scenario Analysis, and the Triple Network used in the 2000 Comprehensive Truck Size and Weight Study (2000 CTSW Study). return to Footnote 3
Six different scenarios, each involving one of the alternative truck configurations, were developed to see what the likely results would be if an alternative truck configuration were allowed to operate on a specified highway network. The study focus areas used these scenarios as a framework for simulating or modeling the performance of an alternative truck configuration in comparison to a control vehicle that meets the current Federal size and weight standards. Each scenario includes one alternative truck configuration, the network on which it would operate if allowed, and the access assumptions off that network. In general, the scenarios’ alternative truck configuration uses its control vehicle’s nationwide network and access rules with the exception of the triple truck configuration, which has a restricted network and access rules.
As shown in Table 4, the Network and access off the Network are the same for both control vehicle and the specific alternative vehicle configuration examined in Scenarios 1 through 4. However, the network for the two triple combinations in Scenarios 5 and 6 is more limited than for its control vehicle, as the existing triple network used for modeling is limited to the Interstate System, the current Western State highways allowing triple trailers, and some four-lane non-Interstate highways in the East. In addition, it is assumed that the triple-trailer configurations would have limited off-network access to terminals located just off the highways.
The first three scenarios would allow heavier tractor semitrailers than are generally allowed currently under Federal law. Scenario 1 would allow a (3-S2) five-axle tractor-semitrailer to operate at a GVW of 88,000 lbs. while Scenarios 2 and 3 would allow (3-S3) six-axle tractor semitrailers to operate at GVWs of 91,000 and 97,000 lbs., respectively. The control vehicle for these scenario vehicles is the (3-S2) five-axle tractor-semitrailer with a maximum GVW of 80,000 lbs. 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 (3-S2) five-axle tractor-semitrailers and (2-S1-2) five-axle twin trailer combinations with 28 or 28.5-ft. trailers with a maximum GVW of 80,000 lbs. Scenario 4 examines a (2-S1-2) five-axle double trailer combination with 33-ft. trailers that have a maximum GVW of 80,000 lbs. Scenarios 5 and 6 examine triple-trailer combinations with 28.5-ft. trailer lengths and maximum GVWs of 105,500 lbs. (2-S1-2-2) and 129,000 lbs. (3-S2-2-2), respectively. The five-axle twin trailer with 28-ft. trailers is the control vehicle for Scenarios 4, 5, and 6.
Once the scenarios were identified, and in order to assure consistency and uniformity across the five study areas, the task of preparing estimates of potential freight diversions from one truck configuration to another or truck/other modes diversions as compared to the base case was undertaken for each scenario. For example, there could be freight diversions from one truck configuration to a different truck configuration. There might be changes in the distribution of operating weights for different truck configurations, and there might also be diversions from rail to truck. These scenario freight diversion estimates were used in evaluating impacts for each of the five task areas.
For analytical purposes, the triple trailer combinations in Scenarios 5 and 6 are assumed to be restricted to about 74,500 miles of Interstate and other principal arterial highways. Access off this network to terminals and facilities for food, fuel, rest, and repairs is assumed to be restricted to a maximum of two miles. These restrictions recognize that the length, stability, and control properties of triples may make them unsuitable for travel on roads with narrow lanes or restrictive geometry.
Analyses from the 2000 Comprehensive Truck Size and Weight (CTSW) Study and the 2004 Western Uniformity Scenario Analysis report, combined with the vehicle stability and control analysis for slow-speed off-tracking in the study, provide the basis for the Scenarios 5 and 6 triple network and access off the network, as shown in Table 4.
Study Process and Methodology
USDOT determined early on that the study would be conducted as an objective, transparent, and data-driven initiative using the most current, best suited analytical methods, tools, and models. To this end, the study Process described below was set in place to ensure that these characteristics were applied to the study technical work. Plans and procedures were established and applied across the project, in terms of public input, peer review, guidelines to apply when necessary to use commercial or proprietary data, and project planning. The purpose was to ensure that the best available data, models, and analytical tools were used to answer relevant questions.
To provide overall direction, a USDOT Policy Oversight Group (POG), with representatives from USDOT operating administrations with relevant jurisdiction, was established to guide the overall process on an on-going basis from the beginning, including the technical work discussed here. Representatives from each of the following USDOT Operating Administrations serve on the POG:
- Federal Highway Administration (FHWA) has the lead responsibility for the study
- Federal Motor Carrier Safety Administration (FMCSA)
- National Highway Traffic Safety Administration (NHTSA)
- Maritime Administration (MARAD)
- Federal Railroad Administration (FRA)
- Bureau of Transportation Statistics (BTS)
- The Office of the Secretary of Transportation (OST)
USDOT’s approach to managing the technical aspects of the study was to form a Technical Oversight Committee (TOC), a group of subject-matter-experts with expertise directly relevant to the work being conducted to complete the study. On a day-by-day basis, the TOC oversaw the technical work, and on-going reviews of study products. The TOC also helped craft statements of Work to procure contractor services. In addition to FHWA, FMCSA, FRA, and NHTSA have representatives on the TOC.
Public Involvement and Transparency of Information
Engaging stakeholders and other interested members of the public has been a key part of the study process. Public outreach efforts have been guided from the early days of the project by a stakeholder outreach and engagement plan aimed at ensuring that diverse communities with a view on Federal truck size and weight limits had opportunities to register their positions at key junctures during the study. Goals stated for public involvement included:
- Interpreting and understanding critical issues and elements desired by stakeholders;
- Offering stakeholders the opportunity to recommend models and data that would beneficially contribute to the study as well as prior work relevant to the work being undertaken to complete the study;
- Providing stakeholders with opportunities to participate in the study as appropriate, including defining scenario configurations for evaluation and helping stakeholders understand the potential impacts and opportunities of changes to TSW limits (study, project management plan).
USDOT held three outreach sessions with interactive public access available through the Internet or on the telephone. In May 2013, prior to the commencement of the technical analysis and modeling work activities, a "listening session" was conducted at the USDOT Headquarters building for people wishing to attend in person. This session was also made available to the public as an interactive webinar. As indicated above, this gave participants an opportunity to share their thoughts on alternative configuration vehicles that should be included in the study, and at least two of the proposed configurations were selected based on this input.
In addition to the alternative configuration discussion, a breakout session on data, modeling and methodology prompted 120 comments from people attending in person as well as 45 additional comments from the webinar attendees on which project staff followed up. The session was divided into several rooms each covering the one of the following topics: pavement impacts, bridge impacts, modal shift, safety, and compliance and enforcement.
Two additional public input sessions were conducted during the course of the technical work: in December 2013 and in May 2014. At the December 2013 webinar session, the public was briefed about the rationale behind decisions for selecting the alternative configurations; the networks to examine; and the methods, data and modeling approaches to be used, all of which were influenced by the public comment process. Desk scan activities were also discussed. At the May 2014 meeting, the USDOT study team provided stakeholders and the public with an update on the status of the study, focusing on issues of concern to the public on the basis of prior input sessions, and presented a review of progress in each study area. USDOT solicited and received comments at each of these sessions and afterwards. For example, approximately 154 comments, questions, and recommendations were made by stakeholders at the December 2013 public input session and were subsequently evaluated for inclusion in the technical work. Similarly, the May 2014 session received 38 comments that were evaluated for inclusion in the study.
Transparency was a key objective in the study process; for example, all contract and subcontract personnel working on the project in support of USDOT were identified by name and affiliation on the study page on the FHWA Web site: (https://ops.fhwa.dot.gov/freight/sw/map21tswstudy/outreach/publicimput052913.htm).
Project plans and desk scans for each of the five study focus areas also were posted on the Web site, and public comment was encouraged throughout the study. A docket was created and made publicly available hosting written comments submitted by stakeholders and interested parties. Comments were invited throughout the project with an e-mail account set up for receiving input at any time from stakeholders and interested parties.
FHWA also contracted with the National Academy of Sciences to form a Peer Review Panel to conduct an independent review of the Desk Scan Reports and the Technical Reports.
Technical Analysis Approach
USDOT identified the five study focus areas, specified in Box 3, at the outset of the project. The topics were selected to ensure that all seven topic areas identified in MAP-21 would be addressed in the study. One of the topic areas, modal shift, encompasses more than one of the topic areas in the law, and the safety topic includes the international safety comparisons required in the law.
USDOT procured contractor services to assist in completing the data collection and technical work needed to conduct the modeling and analysis required to develop the technical results to complete the study. A request for technical proposals was issued to pre-qualified indefinite demand/indefinite quantity contract holders asking firms qualifying in this area to specify in detail how they would conduct the work for each of the five study focus areas. 11
CDM Smith successfully competed for the contract to provide the technical and analytical support Under the contract with FHWA, CDM Smith committed to:
- Conduct the data collection and technical and analytical work required to complete the study;
- Generate results from the completion of the technical work in each of the study’s five areas of investigation;
- Produce a Compiled Technical Report (of which this report is a Volume I) for USDOT to consider as a basis for its Report to Congress; and
- Support and assist in the delivery of public input sessions.
Project teams, consisting of subject matter experts in areas germane to the specific study topics, were proposed for each analysis area. The contractor also provided information on previous work by these experts so USDOT could determine the team’s qualifications and ability to complete the work. Over 45 subject matter experts carried out the study, which was conducted by the successful bidder with additional expertise secured by subcontracts with individuals at universities, research institutions, and other consulting firms.
The USDOT study team made extensive efforts to determine that each project team member was unbiased. All project team members committed to forego any other truck size and weight work during the course of the project. The contractor established a review process to evaluate each team member for indicators of bias.
Desk Scans
The USDOT study team conducted a review of relevant worldwide research pertinent to the specific subject area. The purpose of these "desk scans" was to identify the most relevant and current data, useful methodologies, and most important studies available. The desk scans were also intended to identify the most important past studies, such as prior USDOT truck size and weight studies, and analyses carried out by the Transportation Research Board (TRB) of the National Academies of Sciences pertinent to the specific topic. Relevant materials from the desk scans helped to shape the project plans for each of the five study areas. Once desk scan reports for each of the five study areas were completed, they were posted to the study Web site and input was sought. In addition, a panel of experts from the TRB reviewed these desk scans for relevancy and inclusiveness (see Peer Review on page 30 below for further discussion).
The initial desk scan reports were updated after their initial release to the public to address comments shared by the NAS Peer Review Panel and to reflect the information and reports shared by Stakeholders and the public throughout the course of the project. Based on recommendations provided in the Peer Review Panel’s Report #1, several actions were taken to address these recommendations (see Peer Review section for a more thorough discussion of the Peer Panel and how recommendations were addressed).
Study Focus Area Project Plans
Relying in part on the results from the desk scans, a detailed individual project plan and schedule was developed for each of the five study areas to guide the work. In some study areas, the technical work was divided into subtasks carried out concurrently, with sub-topic plans prepared and incorporated into the overall project plan. These plans identified the questions to be addressed, the general approach to be taken, the specific methodology to be employed, and the schedule for completion of tasks. The draft project plans were posted on the study Web site, and public comments were received and considered. The individual project plans for each study task are discussed in greater detail in Chapter 3 below.
Role of Data and Modeling in the Technical Analysis
As noted earlier, USDOT determined at the outset that the study would be data driven and would use modeling and other technical analytical approaches best suited to a particular topic. The study team used a wide range of data sets, most of which are publicly available from USDOT or State agencies, including, among others:
- State-submitted data through FHWA’s Highway Performance Monitoring System (HPMS) and Traffic Monitoring Program;
- FHWA’s Freight Analysis Framework (FAF);
- Rail-based data available on the Surface Transportation Board’s Carload Waybill Sample to measure the impact that "alternative truck configurations" have on rail operations;
- FHWA’s Long-Term Pavement Performance (LTPP) data, supplemented by data needed to meet the input requirements of the Mechanistic-Empirical Pavement Design Guide (M-EPDG) software of the American Association of State Highway and Transportation Officials (AASHTO);
- Data models previously constructed by State DOTs for 490 bridges in order to operate the analysis using the AASHTO’s Bridge Rating Program;
- Compliance and enforcement data submitted by the States through annual certifications and state enforcement plans submitted to FHWA;
- Data needed to operate the TruckSims™ software for vehicle stability and control analysis of the "alternative truck configurations"; and
- Violation and Inspection information maintained by FMCSA’s Motor Carrier Management Information System (MCMIS) database.
Several limitations in available data sets exist. For example, weigh-in-motion (WIM) data availability is an important aspect of completing truck size and weight comparative assessments. WIM data coverage for the Interstate System was adequate to complete the analyses included in the various areas of study. WIM data availability on the non-Instate National Highway System was insufficient. In certain areas of the study, the lack of sufficient WIM data for the NHS presented itself as a significant limitation on the work completed. In the safety assessment area of the study, State crash reports do not include information on the GVW or vehicle configuration (for example, number of axles or number of cargo carrying units) information. This precluded development of an adequate comparative assessment of the various scenario vehicles which are defined by gross weight and configuration. In conducting the comparative assessment of violations and citations, GVW provided on the Federal Motor Carrier Safety Administration’s Motor Carrier Management Information System (MCMIS) could not be used due to variations on how the States reported this data element.
Data Acquisition Technical Analysis Plan
A data acquisition and technical analysis plan was developed providing common guidelines for data/model accessibility and data custody, and also a generic data agreement to use where proprietary or confidential data were needed to conduct the study (Table 5 shows these guidelines). The data plan also describes the key data sets to be used and the technical analysis methods (including analytical models) in each study task or subtask, and the sources for each data type.
While each of the five study areas used subject-area specific datasets, the entire study analysis for the five technical reports (safety, pavement, bridge, compliance, and modal shift) shared a common set of data for the base case as well as for the six scenarios. The common data is vehicle miles of travel (VMT) for the base case in 2011. The base case VMT (reflecting the current fleet’s use of the highway network system) was used to estimate the change in VMT for each of the alternative truck configurations (six scenarios) introduced into the existing fleet. Modal shift analysis included both shifts between the truck and rail modes, and shifts in vehicles and operating weights within the truck mode.
Box 9. Data Used Involving a Non-Disclosure Agreement (NDA) or Usage Fee
While most data used in the study are readily available for examination at no cost to the public or third parties, there are a few exceptions.
Safety Carrier Data – Some individual carriers made proprietary safety data available to the Study under an NDA. Under this NDA, the names of the individual carriers and the data will not be available to the USDOT and third parties. As discussed in Section 3, the individual carrier data was blended for use in the safety analysis. This blended database is available to the USDOT and third parties, as allowed under the NDA/DA requirements.
Truck Flow Data – The truck flow data used by the study will be a county-to-county disaggregation of USDOT’s Freight Analysis Framework database that will be available to third parties.
Vehicle Stability and Control Model – The vehicle stability and control (VSC) analysis used the commercially available TruckSIM® model. The TruckSIM® model is available to third parties for a fee with a NDA/DA.
Pavement Analysis Model – The pavement cost analysis task used the AASHTOWare Pavement ME Design® model, which is commercially available from the American Association of State Highway and Transportation Officials for an annual license fee.
Confidential Waybill Sample – In cases where rail flow data from the USDOT Surface Transportation Board (STB) confidential waybill sample is used for the rail traffic impact analysis, STB’s standard NDA governing the restricted use of the data will apply. These data were acquired with USDOT Federal Railroad Administration’s cooperation. Third parties wishing to see this data will have to request the data directly from the STB.
Peer Review
USDOT engaged the National Academy of Sciences (NAS) to provide an independent, objective peer review of the desk scan reports (literature reviews) and the compiled technical report. The peer review process was an important element in developing and completing the study. The National Academy of Sciences selected a well-balanced peer review panel, chaired by Dr. James Winebrake of Rochester’s Institute of Technology and 15 experts from both the private and public sectors and from academia to conduct the reviews. The USDOT Technical Oversight Committee (TOC) met with the panel in December, 2013, to brief the panel on the contents of the desk scan reports. In April 2014, the TOC presented the data, models, and approach for completing the work needed to finalize the compiled technical report. Assumptions applied in each study area and limitations imposed on the technical work due to data availability or modeling capacity were also presented to the Panel at that meeting.
The peer review panel released its Report #1 on the desk scan reports for each of the five study areas in early April 2014. The Report reviewed the thoroughness of the literature search, analysis of existing models and data for conducting the study, and the overall synthesis of previous research as it relates to the present study. The report noted:
- Desk scans are a logical step in conducting a study where significant prior work has been completed.
- No superior models and data sets were omitted.
- A synthesis of models and data used in previous research needs to be prepared to strengthen the case for models and data used in the study.
- The linkage between the desk scan reports and the project plans needs to be strengthened.
As a result of the findings, a comparison of results report and linkage report were prepared. The comparison of results report identifies models, data, and results produced in prior related studies and evaluates the relevancy of the results to the results produced under this study. The Linkage Report shows how previously completed similar work was considered and, in some cases, used at a starting point for the work conducted under this study. Also, the Desk Scan Reports were updated to show how studies not included in the original Reports were considered during the operation of this study. The Peer Review Panel’s recommendations on studies that should be included in the various Desk Scans are included in the revised Reports. More detailed information on the NAS Peer Review Panel and its work is available at http://www8.nationalacademies.org/cp/projectview.aspx?key=49568
The peer review panel will review the compiled technical report, including this summary, shortly. USDOT will schedule a meeting to present the report to the panel.
Changes in Methodology from Prior Studies
Several methodological changes characterize this study in comparison to prior truck size and weight studies. It uses the FHWA’s FAF, a more refined data set for assigning vehicles to freight corridors, more advanced models to assess truck impacts on pavements and on bridges, and a careful examination of modal shift impacts on short-line and regional railroad operations caused by the introduction of the scenario vehicles. Also, a unique feature in the approach for completing highway safety and truck crash analysis was to simultaneously analyze truck safety on three tracks: State crash data analysis, corridor-based crash analysis, and fleet-based crash data analysis.
FAF: FHWA’s Freight Analysis Framework data set was not available for use in prior comprehensive truck size and weight studies. FAF integrates data from several sources to provide more detailed estimates of freight movement among States and major metropolitan areas by all modes of transportation. For the study, the FAF was disaggregated to the county level, which allows analysis of certain configurations on limited highway networks. The FAF data were used in the Intermodal Transportation and Inventory Costing (ITIC) Model to estimate modal shifts that were then used to estimate changes in truck VMT by configuration and weight group.
Modeling of Pavement Impacts: The pavement analysis used new pavement impact modeling software not available when prior nationwide truck size and weight studies were conducted. The software reflects an 8-year effort by the National Cooperative Highway Research Program (NCHRP) to develop a mechanistic-empirical pavement design guide. The software, available commercially as AASHTOWare™ Pavement Mechanistic-Empirical Pavement Design Guidelines (M-EPDG) software, is considered superior to prior software because it incorporates material mechanics, geo-technic considerations, stress due to hydraulic conditions, climate data, axle-load spectra, and other advances that allow more precise prediction of pavement performance.
Modeling the Impacts on Bridges: TheAASHTOWare Bridge Rating® (ABrR) software was employed to complete the structural analysis of load bearing capacity of 490 representative bridges on the Interstate Highway System (IHS) and non-IHS National Highway System roadway systems. Bridge models previously prepared by 11 different State DOTs were used in completing the structural analysis work. The ABrRanalytical software enabled a more precise estimate of the load bearing capacity of the bridges selected for analysis to be made and to evaluate the ability of the structures to accommodate the alternative scenario trucks versus the 80,000-lb. control vehicles.
Modal Shift Analysis on Regional and Short-line Railroads (Class II and III): It was decided that the rail component of the modal shift analysis must include impacts on Regional and Short-line Railroads. Members of USDOT and the study team met with the American Short Line and Regional Railroad Association (ASLRRA) and solicited input on establishing an analytical framework for evaluating potential mode shifts in freight traffic caused by the operation of the scenario vehicles. Access to freight pricing needed by the modal shift model is highly proprietary and the importance of the confidentiality of that data is understood by USDOT; as a result, this data was not able to be used in this study. However, consultation with ASLRRA was very instrumental in developing the estimates of mode shift of short line and regional railroad freight from the introduction of heavier, (3-S3) six-axle trucks.
10 MAP-21 designates the entire Principal Arterial System (PAS) as part of the National Highway System. return to footnote 10
11 The indefinite demand/indefinite quantity (IDIQ) contracting approach was used. Such IDIQ contracts are competed as full and open under Federal Acquisition Regulations (FAR). Firms selected for IDIQ awards compete for subsequent proposals in the areas in which the Government has found them to be qualified. return to footnote 11
