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3CHAPTER 1 INTRODUCTION BACKGROUND Trucks constitute a large and growing segment of the traf- fic on American highways. On many rural Interstate high- ways, trucks now constitute more than one-third of the total traffic stream. The increase in truck traffic is related to a strong and growing economy, shifts in manufacturing pat- terns and inventory reduction through just-in-time delivery, and changing trade patterns resulting from the North Ameri- can Free Trade Agreement (NAFTA). In addition to the growth in truck volumes, there have also been shifts in the mix of truck types on U.S. highways toward larger vehicles. The Surface Transportation Assistance Act (STAA) of 1982 established the tractor-semitrailer combina- tion with a 14.6-m [48-ft] trailer as a standard vehicle on the U.S. highways. The 1982 STAA required all states to permit trucks with single 14.6-m [48-ft] trailers and twin 8.7-m [28.5-ft] trailers to operate on the National Truck Network. Since 1982, combination trucks with single 16.2-m [53-ft] trailers have become common on the National Network (NN) in many states and a few states permit combinations with trailers as long as 18.1 m [59.5 ft]. Trucks are an important consideration in geometric design of highways. Many geometric design criteria, as presented in the AASHTO Policy on Geometric Design of Highways and Streets (1), commonly known as the Green Book, and in the policies of individual state highway agencies, are based on vehicle characteristics. Truck characteristics are often a key consideration in determining the recommended values of such criteria. Every roadway and intersection is designed to accom- modate a specific design vehicle, selected from among those presented in the Green Book and other design policies, and for many projects the appropriate design vehicle is a truck. The design vehicles in the 1994 edition of the Green Book did not adequately represent the truck fleet currently on the road. Extensive changes in the design vehicles and their dimensions have been made in the new 2001 edition of the Green Book, but further review has been conducted to determine whether these design vehicles are consistent with the current truck fleet. Furthermore, truck considerations are not addressed con- sistently throughout the Green Book. For some geometric design criteria, the Green Book shows how a designer can accommodate a truck as the design vehicle, while other design criteria are based solely on passenger car characteristics, with little or no mention of trucks. Research is clearly needed to recommend a more consis- tent treatment of trucks in the Green Book and in other related design policies. Midwest Research Institute (MRI) and Penn- sylvania Transportation Institute (PTI) undertook similar research for the FHWA in a project completed in 1990 (2,3). The objectives and scope of that project were to do as follows: ⢠Determine the dimensions and performance character- istics of trucks. ⢠Identify all geometric design criteria in the Green Book and all traffic control device criteria in the FHWA Man- ual on Uniform Traffic Control Devices (MUTCD) (4) that are based on a vehicle characteristic. ⢠Determine what models are used in setting the design and traffic control criteria and what vehicle characteris- tics are used as parameter values in those models. ⢠If a specific design or traffic control criterion is based on passenger car characteristics, conduct a sensitivity analysis to determine how that criterion would need to be changed to accommodate trucks. ⢠Recommend whether changes in either the models used to establish design and traffic control criteria or in the parameter values used in those models would be desir- able and cost-effective to better accommodate trucks. The results of this previous study have been published in Reports No. FHWA-RD-89-226 and -227, entitled Truck Char- acteristics for Use in Highway Design and Operation (2,3). While this previous study was comprehensive in scope, it is in need of updating because both the truck fleet and geomet- ric design policies have changed considerably in the inter- vening years. This previous work provides a firm starting point to meet the current need for updating the treatment of trucks in the Green Book. RESEARCH OBJECTIVES AND SCOPE The objective of the research is to ensure that geometric design criteria for highways and streets can reasonably accom- modate the dimensions and performance characteristics of the current and future truck fleet using the U.S. highway system. The main product of the research is a set of recom- mendations on modifications and/or additions that should be made to the AASHTO Green Book (1). The scope of the
research addresses geometric design issues, but not structural or pavement issues. The scope of the research has included all truck-related geo- metric design issues currently addressed in the Green Book. The approach used by the Green Book to address each of these issues has been evaluated and any appropriate modifications have been proposed. Modifications considered included both use of (1) different parameter values in a model used in the Green Book to determine design criteria for passenger cars and (2) revised models that might be more suitable for trucks. In addition to looking at design criteria that currently con- sider trucks, the research also included a review to determine whether design criteria that do not currently address trucks should do so or whether new design criteria that address trucks should be added to the Green Book. It is vital that the review of Green Book design criteria be based on the most up-to-date information available about the composition and characteristics of the truck fleet. Therefore, the research team sought to characterize the current truck fleet on U.S. highways and to make reasonable projections of changes that may occur in the years ahead. The project scope addresses what are often referred to as heavy trucks (i.e., not including light trucks like pickups and vans). A key aspect of the research objectives is to ensure that highway design criteria can reasonably accommodate current and future trucks. Reasonable accommodation does not mean that all roads should be designed for the largest vehicles that use them or that every design criterion should be based on a large truck. Rather, it means that roads should be designed to accommodate the vehicles likely to use them with reasonable frequency and that both the potential safety benefits and the expected costs to highway agencies should be considered before any proposed change in design policy is adopted. ORGANIZATION OF THIS REPORT This report presents an overview of the size and character- istics of the current truck fleet, a review of geometric design 4 issues related to trucks, and recommendations for potential future changes to geometric design policy to better accom- modate trucks. The remainder of this report is organized as follows. Chapter 2 summarizes current size and weight lim- its for U.S. trucks, as well as comparable data for trucks in Canada and Mexico. The size, composition, and characteris- tics of the U.S. truck fleet are presented in Chapter 3. The current truck design vehicles used in the AASHTO Green Book are reviewed in Chapter 4, and recommendations for changes in these design vehicles are presented. Chapter 5 summarizes the characteristics of trucks that are related to highway geometric design. Chapter 6 reviews highway geo- metric design criteria and their relationship to truck charac- teristics. Chapter 7 presents recommendations for potential future changes in geometric design policy to better accom- modate trucks. Appendix A summarizes truck characteristics based on data from 1992 and 1997. Appendix B presents the results of field studies conducted at truck weigh stations to estimate selected truck characteristics. Appendix C assesses the turn- ing performance of selected design vehicles, including their offtracking and swept path width. Appendix D presents the results of field studies to estimate weight/power ratios for the current truck population. Appendix E describes a spread- sheet program developed to estimate truck speed profiles on upgrades. Appendix F presents recommendations for future revisions to the Green Book to better accommodate trucks. The text of this report uses both metric and U.S. custom- ary units of measure. For consistency with the Green Book, which is the key reference addressed by this report, the quan- tity in metric units appears first, followed by the quantity in U.S. customary units in brackets. Some tables and figures show quantities in both units of measure, but others present only one system of units, when the data being presented were collected or published or the legal requirement being pre- sented was adopted in that system of units. In addition, the abbreviation for miles per hour used in this report is mph, rather than mi/h, for consistency with the Green Book.
