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13 The possible need to subdivide any vehicle registration should be allocated based on relative shares of VMT, whereas classes into two or more subclasses because of any of expenditures on rural roadway systems are driven by the the aforementioned cost responsibility considerations. strength requirements caused by heavy truck traffic and, there- Defining vehicle classes in one way for analysis of fore, should be allocated based on vehicle axle loads and cost responsibility and in another way for revenue attri- mileage. bution: it is important to have a good way of applying conversions from one class to the other or to summarize categories of vehicle classes for reporting results and SELECTING APPROPRIATE COST ALLOCATORS estimating equity ratios. Each element of a state HCAS relies on some measure that can be quantified and used to allocate costs to various classes FUNCTIONAL CLASSES OF ROAD SYSTEMS EXAMINED IN HIGHWAY COST ALLOCATION of highway users. Under the Incremental Method, the recog- STUDIES nition that roads are built wider and thicker to withstand the loading of heavy trucks led to the allocation of a certain por- The determination of the functional classes of road systems tion of roadway width and depth solely to heavy trucks. In re- examined within an HCAS is important because higher order cent years, however, more comprehensive models, including systems (e.g., Interstates, other freeways and expressways, NAPCOM, have been developed to assign cost responsibil- and other principal arterials) are designed to higher standards ity to vehicle users based on a more complete understanding to withstand the punishment of heavy axle loadings and high of the influence of vehicle traffic, environment, and other traffic levels. Therefore, the attribution of cost responsibility factors on pavement deterioration. These models predict the is inextricably linked to the design standards of the roadway impact that highway use will have on pavement damage systems where both the miles of travel occur and the con- based on empirically established relationships between axle struction and maintenance expenditures are made. The weights and pavement damage, and assigns cost responsibil- following is a list of the standard 12 functional classes desig- ity based on these established allocation factors. nated by AASHTO in cooperation with FHWA. In the absence of a more comprehensive pavement model, Rural some states have historically used more straightforward mea- Interstate sures that are designed to vary in proportion to the damage Other Principal Arterials caused to the roadway system by vehicle classes. These allo- Minor Arterials cators include: Major Collectors Minor Collectors Axle Miles of Travel (AMT)--VMT multiplied by the Local number of axles. Because trucks generally have more Urban axles than cars, sports utility vehicles (SUVs), or pick- Interstate ups, their share of the total AMT on any given highway Other Freeways and Expressways system will be about double their share of the VMT on Other Principal Arterials that system. Minor Arterials Axle Weight or Axle Load--The gross load carried by Collectors an axle. Local Ton-Miles--VMT multiplied by tonnage. Equivalent Single-Axle Loads and Equivalent Single- Historically, these 12 functional classes have served as the Axle Load Miles--The pavement stress imposed by a standard in terms of the treatment of functional classes of single axle with an 18,000-lb axle load is termed one road systems in HCASs. The 12 functional class system was ESAL. ESAL-miles are equivalent single-axle loads used in the 1997 Federal HCAS, 1999 Arizona HCAS, 2000 times miles traveled. Kentucky HCAS, and the 2007 Oregon HCAS, although some other recent HCASs have compressed these functional These allocators have been used extensively at the state class road systems into a smaller number of categories for level to assign specific wear-related costs to highway-user reporting purposes (1999 Oregon HCAS and 1999 Nevada classes. For example, the 2007 Oregon HCAS, while using a HCAS). comprehensive HCAS model, assigns striping costs based on axle-miles of travel (ECONorthwest 2007). Roadway strip- The designation of highway functional class between ing deteriorates as a result of friction of tires wearing away the rural and urban is another important distinction. The dis- paint on roadways. Thus, the number of axle-miles is used tinction of rural versus urban has taken on additional signif- as a proxy for the number of times contact is made between icance in recent Arizona HCASs. The Arizona SMHCAS vehicle tires and roadway striping. The 1999 Arizona HCAS simplifies the cost allocation procedure by assuming that allocates the costs associated with the extra roadway thick- expenditures on urban roads are driven by congestion and ness required to accommodate heavy-truck traffic based on