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29 projections will be broken down into agency programs or for- Fuel type, and mula allocations--for example, MVA, state police, and state Functional class of highway or other user-specified aid to local governments. If not, the first task is usually to highway classes. estimate such broad breakdowns based on trends and inter- views with program administrators. The primary source for the basic set of VMT data in most states is the data compiled for the Highway Performance A critical next step is usually to break down administrative Monitoring System, which is reported each year by each state expenditure projections into broad cost allocation categories to the FHWA. These data include VMT by 12 vehicle con- (e.g., construction, maintenance, motor vehicle, police, tran- figurations (13 if motorcycles are separated out) and 12 high- sit, and other modes). The critical question to be raised in this way functional classes. context is how such administrative costs should be allocated to vehicle classes. In some HCASs, most or all administrative Some states may not have all the breakdowns required, costs have been allocated as common costs, often using VMT such as splits of VMT for single-unit trucks broken down as the allocator. However, most practitioners prefer the more into two, three, or four or more axles, or splits of combina- refined approach that has been used in many recent HCASs-- tions into all of the standard seven classes of combinations. that is, treating each major component of administrative cost In such cases, a state's VMT for larger classes can be disag- as an overhead on the specific program category. For exam- gregated into the more detailed classes using FHWA data. ple, construction engineering and construction management FHWA may have developed estimates for more detailed costs are allocated to vehicle classes in proportion to the breakdowns for that state and can supply data from other results of the cost allocation for direct construction expendi- neighboring states or states with similar economies and other tures. Similarly, administrative and management costs for characteristics. highway construction and maintenance as a whole are allo- cated based on the combined cost allocation results for direct The developers of the FHWA State HCAS Model recom- construction and maintenance expenditures. mended that each state analyze trends in VMT by vehicle configuration and functional highway class for the most re- Motor vehicle administrative costs are usually treated cent several years. Unless a state routinely performs such somewhat differently. They are first broken down into func- analysis in the process of preparing each year's estimates, tions relating to different classes of vehicles that are admin- there are usually irregular trend lines for some of the break- istered in a different manner--usually two or more classes: downs, particularly for the breakdowns that have very small (1) heavy and larger vehicles whose registration fees are pro- shares of total VMT (e.g., three- and four-axle combinations, rated among the states, and (2) all other vehicles, sometimes and six- or seven-axle doubles). further divided into light- and heavy-vehicle classes if administrative costs per vehicle are significantly different be- The analysis of trends in the breakdowns of VMT will tween these two classes. Police costs are also usually broken usually result in the need for a few small adjustments in the down into vehicle classes covered by the various functions percentage splits for one or more of the several years of data. (e.g., vehicle inspection, weight enforcement, and general Once this is done, a decision should be made as to how to traffic service and enforcement). project these percentage splits, either by (1) using the most recent splits for the future year(s), (2) using average splits for The final major step in analyzing expenditure data is to the several years, or (3) projecting trends in changes in the link all expenditure projections to cost allocation factors. In splits. Care should be taken in choosing the third option, many cases, this requires significant further analysis to pro- however, because such trends may reflect short-term fluctu- vide a basis for splitting expenditures into cost components, ations that are not likely to be sustained for long. Thus, a such as the breakdown of pavement expenditures into broad compromise might be made between the first or second classes of pavement work, the cost of pavements of mini- option and the third option. mum thickness versus the cost of added thickness necessary for supporting axle-load repetitions, and the expenditures for Annual mileage per vehicle and gallons per mile are vari- different types of maintenance activities. ables for which good estimates have been developed for each state in the default data contained in FHWA's 2001 VEHICLE-MILES OF TRAVEL AND State HCAS Model, based on analysis of the 1992 Truck In- RELATED DATA ventory and Use Survey (TIUS). Some refinements in these estimates might be made, however, if a state desires, using The travel and related data requirements for the FHWA State more detailed analysis of that state's 1997 and 2002 VIUS HCAS Model include annual statewide VMT broken down data. into the following categories: Estimates of new power unit prices were provided in the Vehicle configuration, FHWA State HCAS Model's default database in the form of Registered gross weight, equations expressing price as a function of registered gross