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14 axle loadings (1999 Update of the Arizona Highway Cost taxed on a marginal cost rather than average cost basis, and Allocation Study 1999). those fee structures could be used to address cost responsibility. The economic principles involved have been addressed in the Bridge costs have historically been allocated to highway- 1997 Federal HCAS and recent Oregon studies. user classes based on the size and weight of the vehicles crossing the structures. When assigning these costs, two key In the absence of this preferred set of circumstances, most issues are: states have found proxy allocators used to attribute most cap- ital costs largely based on some measure of travel. In the 1. The definition of increments used in the incremental 1999 Arizona HCAS, an additional distinction was made in analysis of bridge cost responsibility and the methods the simplified model between urban and rural capital costs used to assign vehicles to those increments. with urban costs assumed to have been driven by congestion 2. The methods used to allocate costs among increments, and allocated based on relative shares of VMT. The base including the determination of load and non-load por- Arizona HCAS model in the 1999 Arizona HCAS used axle tions of bridge costs. loadings to assign cost responsibility for the additional pave- ment thickness required to accommodate heavy-truck traffic. Bridge costs are often stratified into three categories: new The 2007 Oregon HCAS assigned responsibility for the base bridges, bridge replacements, and bridge rehabilitation. The increment cost based on a measure of the space that vehicle costs associated with new and replacement bridges have classes take up on roadway systems during congested peri- historically been allocated in many studies based on an in- ods. This allocator is referred to in the Oregon studies as con- cremental analysis of the costs of constructing bridges using gested PCE VMT. The PCE factor is an appropriate allocator different design loadings. This approach was used in the last because congestion is driven by the space that vehicles oc- two federal HCASs and several state HCASs. These loadings cupy on the road system, not simply the number of vehicles are based on hypothetical vehicles for which stresses in the or the total number of miles traveled. Heavy trucks are larger load-bearing members of bridges are calculated and com- and require more braking and acceleration distances to oper- pared with permissible stress levels. As loadings become ate safely on road systems and, therefore, have a greater PCE heavier, the size of bridge members, and consequently bridge factor than lighter, smaller vehicles. costs, must be increased to remain within permissible stress levels. There are elements of any transportation agency budget that have no clear relationship to specific vehicle characteris- When allocating bridge rehabilitation costs, load- and tics. These costs include planning and administrative over- non-load-shares are determined. Bridge rehabilitation ad- head costs. These costs are generally allocated based on either dresses the needs to both improve its structural and func- an assignment of responsibility to a specific highway-user tional condition. The non-load share of bridge rehabilitation class or some general measure of VMT. When considering costs is largely allocated to all vehicles based on relative the allocation of costs to a specific user class, an appropriate shares of VMT. This allocation procedure is based on the example would be the allocation of expenses tied to motor principle that vehicle wear drives rehabilitation costs. The carrier enforcement. These costs would not be incurred in the load share of rehabilitation costs is often allocated to heavy- absence of heavy-truck traffic. Therefore, costs associated truck classes based on some measure that accounts for the with motor carrier enforcement are generally allocated to additional stress placed on bridges by heavy vehicles, such as heavy-truck classes based on the relative shares of VMT for ESAL-miles or heavy-truck VMT (Stowers et al. 1998). each class of heavy trucks. Issues relating to the allocation of pavement and bridge costs are examined in more detail in chapter five. COST ALLOCATION IN A MULTIMODAL ENVIRONMENT Historically, the costs associated with new roadways were allocated based on an incremental analysis of the additional Recent developments in federal policy have served to in- thickness and depth required to sustain heavy trucks. In recent crease the flexibility in the application of federal-aid funds years, however, the costs of constructing new roads in urban and encourage intermodalism. The Intermodal Surface areas have been treated as investment decisions relating to the Transportation Efficiency Act of 1991 (ISTEA) encouraged tradeoffs between congestion and roadway expenditures. Thus, the development of a broad multimodal system through in- new facilities in urban areas are often viewed as designed to re- corporating modes into a National Intermodal Transportation lieve congestion levels on existing facilities. With this view in System. In turn, some have argued for a broader consideration mind, state HCASs are increasingly allocating capital costs in of modes in the cost allocation process. To proponents of urban areas based on the contribution of each highway-user intermodalism, the exclusion of other modes serves to pro- class to congestion. From an empirical standpoint, the ideal mote an incomplete understanding of equity. state would exist if state HCASs examined the costs associated with congestion (including wasted time and fuel, emissions, One study examined the policy implication of applying and noise), states implemented highway-user tax structures that cost allocation across all modes, examined the pros and cons