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11 The software package is supported by detailed documen- applied separately in any case where a study may seriously tation and a set of guidelines for preparation of all required consider a major change in tax rates that would significantly inputs to the software (see chapter five). As a result of the affect highway use. States considering conducting an HCAS recent Vermont HCAS, a few minor problems with this have a range of options, including using existing software HCAS model have been identified and should be corrected. such as the State HCAS Model prepared by the FHWA or There is also a growing need to update the default database. developing a more simplified model similar to the Arizona Appendix C contains a letter from Bart Selle of VTrans that SMHCAS. documents the technical problems with the FHWA State HCAS Model. VEHICLE CLASSES AND HOW THEY ARE DIFFERENTIATED The Simplified Model for Highway Cost Allocation Studies in Arizona (Arizona SMHCAS) was designed to One key issue that is addressed in all HCASs is the determi- enable the Arizona DOT to update the state's HCAS report nation of vehicle classes for the study. Highway cost alloca- simply and in a cost effective manner. As noted in its com- tion is strongly influenced by the weight and configuration of pleted survey, Arizona DOT representatives believe that if a a vehicle. Damage caused to road systems is strongly influ- state cannot find the budget to complete a comprehensive enced by vehicle weights and axle loadings. Ideally, HCASs HCAS on a frequent basis (at least once every five years) it would be designed to examine an extensive set of vehicle is better to use a simplified methodology rather than not configurations and weight classes. In practice, vehicle cate- doing a comprehensive study, because older HCASs are gories, axle configurations, and weight classes are limited by often "criticized or dismissed as obsolete given new traffic data constraints (when selecting vehicle classes for analysis, and new construction programs" (J. Semmens, personal com- the examiner must at a minimum acquire data that can effec- munication, Jan. 2007). tively be used to attribute revenue, to estimate VMT, and to identify gross weights and axle loadings to each vehicle The Arizona SMHCAS breaks highway expenditures into class), research budget limitations, and the inability of many three categories: capacity-driven expenditures, strength- transportation tax structures to effectively implement HCAS driven expenditures, and common costs. Both capacity-related findings at a detailed level. and common costs under the Arizona SMHCAS are allocated to vehicle and weight classes based on relative shares of It is important to note that unless those performing an VMT. Capacity-related expenditures, however, are allocated HCAS are considering recommendations for making changes based on urban VMT only, whereas common costs are allo- in the tax structure, the selection of vehicle classes to be used cated based on total VMT shares. One key element of the in the study is often primarily driven by the state's existing tax Arizona SMHCAS is that it treats urban and rural programs structure. For example, the Oregon HCAS uses 2,000-lb gross differently, with urban expenditures being allocated based on weight classes primarily because the state's weight-mile tax the view that these expenditures are driven by congestion rates are graduated in 2,000-lb increments. The availability of and, thus, should be allocated based on relative shares of data and the state's tax structure are the two principal criteria VMT. Expenditures on rural roadways are assumed to be used in determining vehicle classes. It is also important to driven by the need to provide pavements that are wide, thick, note, however, that HCASs that include more detail in terms and strong enough to accommodate heavy-truck traffic. of vehicle class differentiation can aid in policy analysis and Based on this premise, rural costs are allocated based on consideration of future changes to the existing tax structure vehicle axle loads driven per mile. even if modifications are not currently recommended. In 2001, contractors hired by the Oregon Department of After reviewing what various recent HCASs have done in Administrative Services converted the state's HCAS model, defining vehicle classes, we will conclude this subsection which was based on the State HCAS Model developed by with some additional elaboration of the issues relevant to FHWA, from an Excel-based spreadsheet model to a data- establishing vehicle classes. base model programmed in Access. The Access model was built with a dynamic structure that incorporated feedback The 1997 Federal HCAS examined a broad spectrum of loops enabling it to capture the impact that alternative tax vehicle types and weight classes. Table 3 identifies the 20 ve- rates would have on travel, vehicle ownership, and tax eva- hicle types included in the 1997 Federal HCAS. In addition, sion (Jack Faucett Associates with ECONorthwest 2001). these vehicle types were further examined according to weight Although the model was constructed with this capability, categories in 5,000-lb increments. With the vehicle types evasion rate data along with price elasticity of demand data and weight classes identified, the Federal HCAS could have for vehicle purchases and highway travel were not developed examined vehicles in 600 categories or classes; however, or input into the model. there were many categories with no vehicles registered within them. For example, there are no 100,000-lb automo- A model as complex as the one deployed in Oregon is not biles or 20,000-lb combination trucks registered in the necessary to conduct an HCAS, because the feedback can be United States. Ultimately, the Federal HCAS examined

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12 TABLE 3 1997 FEDERAL HCAS VEHICLE TYPES VC Acronym Description 1 Auto Automobiles and motorcycles 2 LT4 Light trucks with 2-axles and 4 tires 3 SU2 Single unit, 2-axle, 6 tire trucks 4 SU3 Single unit, 3-axle trucks 5 SU4+ Single unit trucks with 4 or more axles 6 CS3 Tractor-semitrailer combinations with 3-axles 7 CS4 Tractor-semitrailer combinations with 4-axles 8 CS5T Tractor-semitrailer combinations with 5-axles, two rear tandem axles 9 CS5S Tractor-semitrailer combinations with 5-axles, two split (>8 feet) rear axles 10 CS6 Tractor-semitrailer combinations with 6-axles 11 CS7+ Tractor-semitrailer combinations with 7 or more axles 12 CT34 Truck-trailer combinations with 3 or 4-axles 13 CT5 Truck-trailer combinations with 5-axles 14 CT6+ Truck-trailer combinations with 6 or more axles 15 DS5 Tractor-double semitrailer combinations with 5-axles 16 DS6 Tractor-double semitrailer combinations with 6-axles 17 DS7 Tractor-double semitrailer combinations with 7-axles 18 DS8+ Tractor-double semitrailer combinations with 8 or more axles 19 TRPL Tractor-triple semitrailer or truck-double semitrailer combinations 20 Bus Buses (all types) vehicles in 212 vehicle classes. The Federal HCAS could cars, buses, and 14 truck classes differentiated solely by explore vehicle cost responsibility at such depth because the registered or declared weight. U.S.DOT was completing a comprehensive trucks size and The 2002 Idaho HCAS covered five vehicle types weight study. (autos, pickups, buses, single-unit trucks, and combina- tions) spread over 9 weight classes. States have historically examined far fewer vehicle classes than what was considered in the 1997 Federal HCAS. Studies can also differentiate between vehicle classes Most but not all state HCASs differentiate vehicle classes based on fuel type (e.g., gasoline, diesel, alternative fuels or based on both weight and configuration. For example, the hybrids) and on treatment in the tax code (e.g., full-fee pay- 2006 Vermont HCAS examined 20 broad vehicle classes ing, partial-fee paying, and exempt). without consideration of weight: passenger cars, pickups and vans, 3 single-unit truck configurations, 14 combination- To elaborate and summarize, vehicle classes are generally truck configurations, and buses. The 1999 Nevada HCAS defined in any HCAS with the following considerations: classified vehicles only according to broad weight cate- gories: basic vehicles weighing 10,000 lb or less and heavy From a perspective of distinguishing vehicles by cost vehicles weighing in excess of 10,000 lb. responsibility: (1) operating axle weights of vehicles, (2) gross weights of vehicles, (3) miles operated, and Other states have established vehicle classes based on (4) differences in the streets and highways on which both axle configuration and registered vehicle weight: vehicles operate. From a perspective of distinguishing vehicles by user The 2007 Oregon HCAS modeled vehicle classes based payments made: (1) fuel economy, (2) registered weight on vehicle weights and number of axles in 2,000-lb class and other vehicle class differences, and (3) fee ex- increments. emptions and reduced fee classes of special vehicles, The 1999 Arizona HCAS considered five broad vehicle such as publicly owned vehicles, out-of-state-based types and 23 weight classes. vehicles, and vehicles used in particular industries or ve- The 2000 Kentucky HCAS used both vehicle and weight hicles providing special services. categories to establish 17 vehicle classes: motorcycles, Existing vehicle registration classes.