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Cost Allocation Study Final Report

V. Highway Cost Responsibility

Cost Occasioned Approach

Basis and Philosophy

In HCASs over the past half century, FHWA and its predecessor agencies have used a cost-occasioned approach to allocate highway cost responsibility among different vehicle classes.

During this time specific cost allocation methods have evolved, but the basic cost occasioned approach has remained the basis for cost allocation at both the Federal and State levels. The approach was so widely accepted that Congress, in mandating the 1982 Federal HCAS, stipulated that a cost-occasioned approach be used in the study. Specifically, Section 506 of the STAA directed the Secretary of Transportation to undertake a study of "the costs occasioned in design, construction, rehabilitation, and maintenance of Federal-aid highways by the use of vehicles of different dimensions, weights, and other specifications, and by the frequency of such vehicles in the traffic stream."

The underlying philosophy of the cost-occasioned approach is that each user should pay the highway costs that it creates or "occasions." A key question in cost allocation studies is what costs to consider. Previous Federal and State cost allocation studies have focused on highway agency costs paid from highway user charges. The focus on costs paid from user charges relates to an important objective of most cost allocation studies -- to assess the equity of the highway user charge structure. While there may be many definitions of equity, in cost allocation studies equity has been defined as each vehicle class paying user charges proportionate to its share of highway agency costs.

Some critics of the traditional cost-occasioned approach argue that economic efficiency is more important than equity and that highway user charges should reflect the true cost of each vehicle's use of the highway, not the share of highway agency expenditures allocated to different vehicle classes. Those who advocate focusing cost allocation on efficiency rather than equity generally favor a marginal cost approach to cost allocation whereby vehicles would be charged in proportion to their marginal cost of highway use. Equity and economic efficiency are not mutually exclusive or even conflicting objectives. In many cases user fee changes that would improve equity would also improve economic efficiency and vice versa.

In essence both the cost-occasioned and the marginal cost approaches assign cost responsibility based on principles of cost occasioning. The traditional cost occasioning approach limits the scope of "costs" considered to highway agency obligations or expenditures. The marginal cost approach, on the other hand, does not consider highway agency expenditures, but rather estimates the economic cost of additional increments of highway use by each vehicle class, including both infrastructure costs that ultimately result in highway agency expenditures as well as environmental and other social costs occasioned by operations of each vehicle class that are not reflected in highway agency budgets. The two approaches differ not so much in the determination of which vehicle classes are responsible for the costs, but rather on which costs are allocated.

Economic efficiency may be served by setting user fees in accordance with cost responsibilities estimated using the traditional cost-occasioned approach. This is especially true for pavement costs where there is a direct connection between marginal pavement costs and highway agency investment requirements for pavement preservation. The closer that each vehicle comes to paying its share of highway agency costs for pavement improvements, the closer it comes to paying its marginal pavement costs, and thus the more efficient the allocation of resources. While other agency costs do not vary as directly with use as do pavement costs, it could be expected that economic efficiency would improve if each vehicle came closer to paying its responsibility for highway agency costs.

As noted above, previous cost allocation studies have focused on highway agency costs incurred in the provision and preservation of the highway infrastructure. Each vehicle, because of its highway use and its weight, length, width, and other physical characteristics, contributes to the need for and cost of improvements to provide additional capacity or preserve existing highway facilities. Each user's travel also occasions costs that go beyond the costs of providing and preserving the highway infrastructure, including congestion, environmental, and safety-related costs imposed on others. The 1982 Federal HCAS considered such costs, but only in a supplemental analysis that examined marginal costs of highway use by different vehicles in order to estimate economically efficient levels of prices that those vehicles would have to pay under different conditions.

There are other non-agency costs that previous HCASs have not considered, such as the impacts of constructing highways in sensitive physical and cultural environments. It is difficult to address many of these costs in cost allocation because there are no clear engineering or economic "cause and effect" relationships between the costs and characteristics of the vehicles to which costs must be allocated. Many community costs of highways are less related to the use of the highway by different vehicle classes than to disruption of the physical or cultural environment by the mere presence of the highway.

Traditionally, cost allocation studies at both the Federal and State levels have examined highway agency costs because their primary objective has been to determine the cost responsibility of different vehicles for infrastructure and related costs borne by the highway agency. This cost recovery objective in turn is related to the user fee principle that different vehicle classes should pay for the highway infrastructure in proportion to their share of the costs to provide and preserve that infrastructure. Costs attributable to each vehicle class are estimated using a process that considers how physical and operational characteristics of each vehicle class affect the design of various components of the highway system or the rate at which pavements, bridges, and other elements of the highway infrastructure wear out and must be repaired or replaced.The cost responsibility of different vehicles for pavement, bridge, and certain other types of agency costs varies according to relative amount of travel on different highway functional classes. Since Interstate and other principal arterial highways generally are designed to accommodate higher volumes of heavy trucks, costs per mile of travel by heavy trucks on those highways are lower than on highways that are not designed to handle as many heavy loads. As shown in Chapter II, the distribution of VMT by highway functional class varies considerably among different vehicle classes. Combination trucks used in Interstate commerce travel the majority of their mileage on higher-order systems in rural areas, while single unit trucks used for local trucking travel a significant amount of their annual mileage on lower-order systems that do not have the same high-type design as Interstate highways.

While neither the Federal nor State user fee structures can charge vehicles directly according to the specific highways upon which they operate, average cost responsibilities for different vehicle classes can be estimated based upon their travel and operating weight distributions on different highway functional classes in each State and characteristics of pavements and bridges on each highway class in each State. Details of the estimation of cost responsibilities for pavement, bridge, and other costs are included in appendices to this report.

A consideration in defining the costs to be analyzed in a HCAS is the time frame over which costs are to be estimated. This decision, too, depends on specific objectives of the cost allocation study. Most traditional cost allocation studies have analyzed costs representing the current distribution of agency costs or the anticipated distribution of costs several years into the future. This time frame is appropriate for assessing the equity of the current user fee structure and of potential user fee changes that could improve short run equity. If the objective of the study is to analyze the relative cost responsibility of different vehicle classes over a longer period of time, the time frame over which costs would be evaluated could be extended. While the ideal might be to regularly adjust highway user fees as relative cost responsibilities change, this often is difficult and consideration of both long run and short run cost responsibilities might be appropriate in evaluating potential user fee changes. This is particularly true if significant changes in the composition of the highway program or in such factors as TS&W limits are expected.

While it is appropriate to examine cost responsibilities for agency costs over periods of 1 or more years, some external costs such as congestion costs should be analyzed over hours rather than years if the objective is to estimate true marginal costs for purposes of setting efficient congestion charges. The temporal variation of congestion and some pollution costs makes it difficult to reflect those costs in Federal user fees or in traditional State or local user fees. As will be discussed in other sections of this report, congestion pricing is receiving increasing attention at the local level as a tool to reduce peak period congestion.

Congestion costs vary not only by hour of the day but by geographical area, and many environmental costs also vary geographically. If the cost responsibility of different vehicle classes is to be accurately estimated, it is important to capture geographical variations in those costs. This is especially true if user fees are to be imposed based upon those costs with the expectation that those fees will improve economic efficiency.

As noted above, some HCASs, including the 1982 Federal HCAS, have examined costs other than agency costs in subsidiary analyses to address policy questions other than the equity of the highway user charge structure. In addition to examining marginal costs associated with the operation of different vehicle classes for purposes of estimating how an economically efficient user fee would compare to user fees based on each vehicle's share of highway agency costs, a follow-on to the 1982 Federal HCAS also examined how the responsibility of different vehicle classes for highway costs incurred by all levels of government compared to the responsibility of those vehicle classes for Federal highway costs. This analysis provides a more comprehensive assessment of the extent to which each vehicle class covers its overall highway cost responsibility, but results are not directly applicable to analyses of user fee equity for a particular level of government. Many State cost allocation studies analyze State and Federal costs as well as State-only costs to understand differences in user fee equity when all levels of government are considered.

Applying a cost-occasioned approach for certain types of costs can be difficult and there is no universal agreement on how all costs should be allocated. The greatest certainty is in allocating costs directly related to specific characteristics of different vehicle classes. Pavement and bridge construction costs are among the costs most closely associated with characteristics of different vehicles, although there still is discussion about specific methods for allocating those two categories of costs. For costs that are not as directly related to specific physical characteristics of each vehicle, the uncertainty about how to allocate costs among vehicle classes is more pronounced.

Certain groups of vehicles clearly occasion some specific costs. For example, since truck weigh stations are needed only for trucks, there is general agreement that trucks should be assigned responsibility for weigh station costs, even though all users benefit from weight enforcement programs that help to preserve the infrastructure. The allocation of truck climbing lane costs is not as straightforward. Some might assign the cost responsibility for climbing lanes on long steep grades to larger trucks whose high weight-to-horsepower ratios result in very low speeds that affect safety and delay other users. If there were no trucks, there would be no need for passing lanes. It is the unique characteristics of heavy trucks that necessitate the construction of climbing lanes. Others maintain, however, that climbing lanes are constructed to maintain the capacity and safety of the facility, and that all vehicles should share in the costs in proportion to their PCEs on the steep grade. Using PCEs would assign a significant share of the cost responsibility to heavy trucks, but autos, pickups, and vans would be responsible for most climbing lane costs.

The next section of this chapter discusses the cost occasioning approach used in this study. The chapter describes the composition of the basic cost groups, and then describes the basis for allocating the components of these cost groups to different vehicle classes.

Federal Agency Cost Allocation

As noted earlier, agency costs allocated in this study are obligations from the HTF. Obligations are grouped into 17 categories, 13 of which are identical to improvement types discussed in Chapter III. The other four are subsets of "other" costs in Chapter III that are separated for cost allocation. Table V-1 shows these 17 cost categories along with base period (1993-1995) and Year 2000 obligations for each.

Most highway costs are estimated from detailed FMIS data on obligations by improvement type. The first 11 cost categories in Table V-1 represent specific FMIS improvement types, but FMIS data are refined considerably for cost allocation purposes. The cost categories ridesharing/high occupancy vehicle (HOV) projects, Mass Transit -- Highway Account, and truck related projects are FMIS work types, which are subsets of FMIS improvement types. Those costs are separated for cost allocation purposes because they are uniquely occasioned by different vehicle classes and are allocated differently than other costs. Obligations for Federal lands projects and FHWA administration are not included in FMIS but come from other FHWA accounting records. Obligations from the MTA come from FTA records.

The FMIS subdivides improvement type obligations into several work classes including construction, preliminary engineering, right-of-way, transit and training, planning, and research. The construction category is typically the largest work class within each improvement. Most other work classes are incidental to construction. Costs for other work classes often are not allocated in the same manner as construction costs within each improvement type, so they are broken out and allocated separately. As noted above costs for certain work classes such as transit and truck-related costs are removed completely from the improvement type under which they are reported and are allocated as separate cost categories.

Pavement Cost Allocation Methods

New Pavement, Added Lanes

 Table V-2 shows costs for the two major improvement types that are allocated as new pavement costs. Costs are broken down into nine separate categories, each of which is allocated separately, although some may use the same allocators. The approach used in allocating costs of new pavements and added lanes is similar to the approach developed in the 1982 Federal HCAS. Costs of providing additional lanes of capacity are allocated using a design based two step methodology. The allocation process separates costs into those related to a base facility that provides additional capacity and traffic services and those related to providing the durability to carry projected traffic loadings over the pavement's design life.

The base facility is a hypothetical pavement that would serve a purpose common to all vehicle classes. It is not the minimum facility that a highway agency could build, nor is it the facility that would be required to carry just automobiles. The base facility would provide skid resistance, all-weather capability, and would serve as a "platform" for providing the base and surface thickness required to accommodate projected traffic loadings. The base facility portion of pavement construction costs is related to providing additional capacity to safely accommodate projected future traffic volumes, and the remaining portion of pavement construction costs provides the base and pavement thickness necessary to accommodate projected vehicle loadings.

The construction of new traffic lanes, whether they be new highways on new locations or additions to existing facilities, reflects the need for added highway capacity to relieve congestion and provide higher levels of service for current and future traffic. Some vehicles, because of their size and operating characteristics, have a greater effect than others on traffic flow and highway level of service. For example, trucks consume more physical space on the roadway than automobiles and have a greater effect on traffic flow because they do not accelerate or maneuver as well as automobiles. Traffic engineers have developed a concept called "PCEs" that measures the relative effects of different vehicles on highway level of service. The PCEs for a particular vehicle will vary according to such factors as grades, lane width, and type of highway, and thus the relative contribution of different vehicles classes to congestion and to the need for additional capacity can be measured across a variety of conditions.

In the 1982 Federal HCAS, base facility costs were allocated to all vehicles on the basis of their relative VMT, although consideration was given to allocating those costs in proportion to VMT weighted by the PCEs for each vehicle class. The PCE-weighted VMT was not used as the final allocator because further research into equivalency factors for different vehicles was believed to be needed. For the 1997 Federal HCAS, new research was conducted to estimate PCEs as a function of such key factors as vehicle operating characteristics, highway functional class, time-of-day, and terrain. This research used traffic simulation models that are more accurate and sophisticated than those available 15 years ago. Base facility costs as well as related engineering, right-of-way, and other costs associated with adding new highway lanes are allocated using PCE-weighted VMT.

Table V-3 presents the shares of new flexible and rigid pavement costs by highway functional class that comprise the base facility and are allocated using PCE-weighted VMT. Base facility shares are smaller for higher-order functional classes than for lower-order functional classes. On all highway classes the base facility represents a larger share of total costs for rigid pavements than for flexible pavements.

The load-related portion of new pavement construction costs is allocated based on the relative ESALs of each vehicle class. The ESAL is a measure of the relative contribution to pavement wear associated with different single and tandem axle loads, using an 18,000 pound single axle as the benchmark. Pavement design equations developed by the AASHTO use ESALs as the principal vehicle specific factor in pavement design. Separate allocations are made for rigid and flexible pavement types because the pavement material is highly related to vehicle ESALs.

Pavement cost responsibilities are estimated using three pavement sections/designs for each highway functional class for each State. Pavement design parameters for each State, such as soil strength, terminal PSI value and the other characteristics are considered in this analysis. Design methods reflect the latest State specific and AASHTO design manuals and guidelines. Cost responsibilities for each of the over 300 vehicle class/weight groups are first produced by State and then combined into a national average.

 Table V-4 shows the cost responsibility per mile for new pavement costs for several illustrative vehicle classes along with their share of total new pavement costs. Automobiles account for the largest share of new pavement costs followed by 5-axle tractor-semitrailers. The responsibility of autos for new pavement costs is about 0.05 cents per mile, less than 10 percent of the cost responsibility of 5-axle tractor-semitrailers. Eight-axle and 5-axle twin trailer combinations have the highest average new pavement cost responsibility per mile among the illustrative vehicle classes.

Figure V-1 summarizes shares of new pavement cost responsibility by broad vehicle classes. Passenger vehicles are responsible for over half of all new pavement costs, semitrailer combinations 32 percent, single unit trucks 12 percent, and multi-trailer combinations 3 percent.

 Grading and drainage costs are not separated in FMIS, but a special analysis of grading and drainage costs was conducted for the 1982 Federal HCAS. A survey of several States conducted for this study indicated that grading and drainage factors developed for the 1982 Federal HCAS are still applicable.

Grading and drainage costs associated with new pavement projects are broken into three components, those related to vehicle weight, those related to vehicle width, and those that are not related to any specific vehicle characteristics. In mountainous and rolling terrain, additional grading and drainage expenses are incurred to reduce highway grades so that heavy trucks with high weight-to-horsepower ratios will not slow more than can be avoided. For operational and safety reasons, the maximum speed reduction allowed for vehicles climbing a grade is 15 miles per hour. Thus a critical grade is defined as any combination of length and degree of grade that produces a 15 miles per hour speed reduction. The allocation of additional grading and drainage costs related to vehicle weight is based on established relationships between highway grade features (critical length and degree of grade) and vehicle performance attributes (weight-to-horsepower ratio). The relative cost responsibility of each vehicle class is estimated from the earthwork savings that would result when comparing each vehicle class to the worst performing vehicle. The incremental earthwork savings are computed as a function of the highway cross section and critical grades for given weight-to-horsepower intervals. The enhanced model developed for this study uses updated information on vehicle horsepower derived from vehicle performance models that reflect present day vehicle characteristics and highway design characteristics from the HPMS database.

In the 1982 Federal HCAS, width related costs were allocated using an incremental approach that estimated incremental construction cost savings when designing for vehicles of different widths. Ten different vehicle width categories were defined in the 1982 Federal HCAS going all the way to a hypothetical zero width vehicle, but all except the lightest trucks, automobiles, and motorcycles were in the widest group. The analysis has been simplified somewhat for this study by eliminating the narrowest width groups and allocating a smaller share of total pavement costs on the basis of vehicle width. This is consistent with trends toward designing wider pavements for safety reasons regardless of the relative number of trucks using the roadway. These changes have been applied to both the allocation of additional construction costs related to vehicle width and to additional grading and drainage costs associated with vehicle width.

The portion of grading and drainage costs for new highway lanes that is related to neither vehicle weight nor width is allocated among different vehicle classes on the basis by PCE-VMT. These costs are essential parts of the overall construction costs that are necessitated by needs to provide additional highway capacity, and are allocated on the basis of each vehicle class' contribution to the need for additional capacity.

Table V-5 summarizes the cost responsibility per mile of travel for new pavement costs by broad vehicle groups at various operating weight ranges. Cost responsibility for passenger vehicles averages about 5/100 of a cent per mile. Costs for buses are higher than for autos, pickups, and vans, but bus travel is a small fraction of total passenger vehicle travel and has little influence on the overall average. The average cost responsibility for new pavements for single unit trucks is 0.31 cents per mile compared to 0.66 cents per mile for combination trucks. Within those two truck categories average costs vary from about 0.20 cents per mile for light single units to almost 3 cents per mile for the heaviest combinations.

Table V-6 shows the cost responsibility for new pavement costs for selected vehicles at different operating weights. This table clearly illustrates the relationship between weight and pavement costs for any given vehicle class and also illustrates the important fact that the more axles under a vehicle at any given weight, the lower the pavement costs.

In comparing cost responsibilities for 5-axle tractor-semitrailers and 5-axle twin trailer combinations, the difference in the pavement damage associated with single and tandem axles is evident. The 5-axle tractor-semitrailers has 2 tandem axle pairs while the 5-axle twin trailer combinations has 5 single axles. At every weight the cost responsibility of the 5-axle twin trailer combinations is greater than that of the 5-axle tractor-semitrailers. Similarly, when cost responsibilities of 5-axle tractor-semitrailers and 6-axle tractor-semitrailers are compared, the benefits of the 6-axle tractor-semitrailers tridem axles are apparent. At all but the lowest weights, the 6-axle tractor-semitrailers cost responsibility for is less than 5-axle tractor-semitrailers costs.

Figure V-2 summarizes the overall assignment of new pavement costs to passenger vehicles and single unit and combination trucks. Despite the differences in cost responsibility per mile of travel, passenger vehicles are assigned over half the responsibility for new pavement costs, combination trucks 35 percent, and single units 12 percent.

Pavement Preservation Costs

Pavement 3R costs constitute the largest single category of obligations from the HTF. Table V-7 shows the breakdown of costs for several cost categories that are grouped as pavement preservation costs. Substantial resources were devoted in the 1982 Federal HCAS to developing new techniques for allocating pavement 3R costs based upon the contribution of each vehicle class to various pavement distresses that necessitate pavement improvements. The basic framework for allocating pavement 3R costs in the 1982 Federal HCAS is used in this study, but significant refinements have been made in several areas.

An important contribution of the 1982 Federal HCAS was the use of "mechanistic" pavement distress models that directly relate axle loads and repetitions to the stresses, strains, and other pavement responses leading to pavement deterioration. Several mechanistic models used in the 1982 Federal HCAS are retained, but most have been improved based upon new theoretical work and the availability of pavement performance data from the Long Term Pavement Performance (LTPP) Study. Eleven different pavement distress models are incorporated in the new nationwide pavement cost model (NAPCOM) used for HCA. Together these models represent the state-of-the-art in predicting pavement responses to different axle loads and repetitions.

The 1982 Federal HCAS analyzed pavement distresses on a relatively small number of hypothetical pavement sections. The hypothetical pavements have been abandoned for this study and replaced by actual pavement sections from the HPMS database. The HPMS database is a statistically valid sample of over 100,000 pavement sections representing all non-local pavements nationwide. It is used in evaluating highway investment/performance relationships for the Department's biennial C&P Report as well as other policy analyses.

While the HPMS database contains section properties, traffic volumes, percent trucks and other data related to pavement performance, considerable supplemental data needed for the pavement performance models are added for each pavement section. Using the augmented HPMS database in conjunction with the detailed traffic and operating weight data described in Chapter II provides a much more representative analysis of pavement costs associated with travel by different vehicle classes on different highway systems. Estimates of the relative cost responsibility of different vehicle classes for pavement 3R costs on the different highway functional classes are used to allocate load-related components of 3R obligations to the different vehicles. The models also estimate the shares of total costs that are related to factors such as pavement age and climate rather than axle loads. These nonload-related 3R costs are allocated in proportion to VMT for each vehicle class.

Table V-8 shows the percent of flexible and rigid pavement 3R costs estimated to be attributable to non-load factors on each highway functional class. In general the share of costs attributable to non-load factors is about the same for flexible and rigid pavements although there are minor differences across highway functional classes. Non-load costs are a higher proportion of total costs on lower-order systems than on higher-order systems, ranging from less than 10 percent on rural Interstates to more than 20 percent on urban collectors and local roads.

Preliminary engineering, right-of-way, grading and drainage, and other costs related to 3R improvements are allocated separately. With the exception of grading and drainage, those other costs are allocated in proportion to the VMT of each vehicle class. Grading and drainage costs, which represent a smaller share of total 3R costs than new construction costs, are allocated using the same factors as grading and drainage for new construction.

Reconstruction with added lanes combines elements of both new construction and reconstruction. The FMIS has data on total amounts spent for this improvement type, but amounts for the added lanes cannot be distinguished from amounts for reconstruction of existing lanes from the FMIS data. Each of the work classes under reconstruction with added lanes is allocated using the same allocators as are used for reconstruction projects. If data were available to separate amounts for lane additions from amounts for reconstruction, the portion for new lanes could be allocated in the same way as costs for added lanes, but in the absence of such data all costs are allocated as reconstruction costs. Minor widening is a unique system preservation cost. Minor widening improvements do not add structural capacity and they add only marginally to traffic-carrying capacity. The primary purpose of minor widening projects is to enhance safety through adequate design standards such as: better curve alignments, provision of separation (median) between opposing directional travel, adding or widening shoulders, and similar projects. Since they do not add or restore structural capacity, costs for minor widening cannot be allocated using the same allocators as are used to allocate pavement 3R costs. Nor can they be allocated by PCE-weighted VMT since the improvements typically are not made to increase capacity or reduce congestion. If roadways were widened primarily because of conflicts caused by wide vehicles, there would be some rationale for allocating minor widening costs to the wider vehicles, but that is not believed to be the primary basis for widening decisions. There is no better allocator for minor widening costs than VMT, so VMT is used to allocate all costs related to minor widening.

Table V-9 shows the cost responsibility for pavement 3R costs by broad vehicle class and operating weight range. The cost responsibility per mile of travel is higher than for new pavement improvements, especially for the truck classes. Single unit and combination trucks operating in the heaviest weight ranges have average cost responsibilities greater than 10 cents per mile but that varies widely for specific vehicle classes within those two truck types.

Table V-10 shows variations in the cost responsibility for 3R pavement costs by vehicle classes and operating weight. The same general relationships seen in Table V-5 between weight, the number and types of axles, and cost responsibility are seen in this table. Cost responsibility increases at an increasing rate as the weight of each vehicle class increases. Single axles contribute more to 3R pavement costs than tandem axles and tridem axles contribute less than tandem axles for vehicles with comparable weights.

Figure V-3 shows the overall distribution of pavement 3R cost responsibility among passenger vehicles, single unit trucks, and combination trucks. Whereas Figure V-2 showed passenger vehicles responsible for over half of new pavement costs, those vehicles are responsible for less than one-quarter of pavement 3R costs. The share of cost responsibility for combination trucks, on the other hand, increased from 35 percent for new pavements to 58 percent for pavement 3R improvements.

The Appendix describes methods for allocating pavement rehabilitation costs in more detail. These methods represent major improvements over methods used in the 1982 Federal HCAS, but further refinements will be needed as more data from the LTPP program become available and as our understanding of factors affecting various types of pavement distress improve. In particular, additional data from LTPP sites across the country will enable various distress models to be more thoroughly validated.

Bridge Cost Allocation Methods

Table V-11 shows a breakdown of costs for the four bridge cost allocation categories: new bridge, bridge replacement, major bridge rehabilitation, and other bridge. Costs under each bridge category are broken out into four work classes: preliminary engineering, right-of-way, construction, and other. Almost half of all bridge costs are for bridge replacement and another quarter of the costs are for major bridge rehabilitation.

New Bridges

The cost allocation procedure for new bridges substantially improves upon the 1982 Federal HCAS approach. The basic principles, however, are similar to the 1982 Federal HCAS. Bridge design procedures are used to develop the relationships between vehicle size and weight and the cost associated with providing the bridges necessary to safely accommodate the vehicle fleet. The improved approach addresses some of the major shortcomings of the 1982 Federal HCAS, particularly, the simplifying assumptions underlying the approach. The major differences are summarized in Table V-12. The notable improvements are as follows:

The bridge allocation procedure generally follows the way in which bridges are designed. In simple terms, bridges are designed so that the bridge can withstand the application of the dead load (the weight of the bridge itself) and the live load of the heaviest truck, plus a safety factor. Except for a fatigue criterion, which rarely governs the final design, the number of applications of the vehicle is irrelevant. The premise of this design procedure is that the heaviest vehicle (actually, the vehicle that produces the greatest stress on any key structural member) governs the size/strength of the bridge. Furthermore, any incremental increase in the size of the heaviest vehicle will require an incremental increase in the size/strength of the bridge.

The procedure relates additional costs necessary to make the bridge incrementally stronger to the set of vehicles that occasion these increased costs. Bridges are grouped by functional highway class. The allocation process works by comparing the live load moment of each vehicle class/weight group on the representative bridge (the representative bridge is described by the mean primary span length) of a specific functional class, with the moment produced by the design vehicles. This comparison allows each vehicle class/weight group to be placed in a specific design increment, based upon whether its live load moment is less than or equal to the moment of the design vehicle associated with specific design increments for each functional class. For example, given identical vehicles on bridges of equal spans, the only distinguishing bridge characteristic data in the NBI that can affect the moment produced by the vehicles is support type. Two support types, simple and continuously-supported, are considered. The representative vehicle's axle loads and axle spacings are required to determine live load moments accurately; GVWs acting as point loads do not provide a realistic picture of the moments generated under trucks with different axle arrangements and weights.

Secondly, all vehicles in any specific design increment are allocated the costs associated with that increment based on their relative VMT compared to the other vehicles in the design increment. The VMT is considered the most equitable factor upon which to allocate incremental bridge design costs among vehicles in each increment.

Bridge Replacement

The allocation of bridge replacement costs uses the incremental methodology described above. The percentage of replacement costs assigned to the design increments is estimated using the BNIP, the same model that is used in estimating bridge investment requirements for the Department's C&P Report. The program determines, using several bridge sufficiency ratings in the NBI, how many bridges on each functional highway class must be replaced because they are structurally inadequate, functionally deficient, or functionally adequate but with so many deficiencies that the bridge must be replaced anyway. The percentage of bridges inadequate for each of these reasons is applied to total construction costs for bridge replacements and these amounts are further sub-divided into load related and non-load related costs. The rationale to assign some construction costs to the base increment is that all vehicles occasion costs to remedy non-load related bridge inadequacies. In other words, when the bridge is replaced, walkways, smoother deck surfaces, new and better signing, the value of the useful life remaining in the old bridge, etc. are all part of the new bridge. Load-related costs are allocated to the vehicles that occasion the bridge replacement. In the case of the structurally deficient bridges, the largest cost category, load-related costs are allocated to all vehicles producing live load moments greater than that for the H-15 design vehicle. All vehicles share in the cost responsibility of other costs, with appropriate portions being load related and the remainder non-load related.

Major Bridge Rehabilitation

The process for allocating major rehabilitation costs is similar to replacement costs but more complex because 13 types of rehabilitation are considered. These are rehabilitation of bridge deck, superstructure, or substructure, or some combination of the three. As for bridge replacement, a certain percent of the costs is assigned to various categories and others are calculated on the basis of the number of bridges the BNIP identified as having deficiencies. As for the new bridge costs, all other cost sub-groups are allocated by VMT.

Other Bridge Improvements

Minor bridge rehabilitation and repairs generally are not related to vehicle characteristics. All costs are assigned to the base increment using VMT as the allocator.

Table V-14 summarizes bridge cost allocations to major vehicle categories by improvement type. Two-thirds of all bridge costs are allocated to passenger vehicles, 12 percent to single unit trucks, and 20 percent to combination trucks. These percentages vary by type of improvement. Combination trucks are allocated almost 30 percent of bridge replacement costs and single units 20 percent of replacement costs, whereas those vehicle account for only 5 percent and 3 percent respectively of minor bridge rehabilitation costs.

Table V-15 shows the overall bridge cost responsibility of illustrative vehicles at different weights. Costs are shown on a cents per mile basis since most other costs in this study are summarized in cents per mile, but as noted above, bridge costs, except for fatigue, do not vary directly with VMT. The incremental nature of the cost assignment process is one reason why costs per mile are so high for certain very heavy vehicles in each class. Cost of providing the last increments of bridge strength are assigned only to vehicles that produce the greatest moments, and those vehicles typically account for a relatively small amount of total travel and thus their cost responsibility per mile is high.

System Enhancements

Table V-16 shows the distribution of costs for safety/TSM, environmental, and other improvements classified as system enhancements in this study. Uniquely occasioned costs including ridesharing/HOV projects, transit projects funded from Federal-aid highway funds, and truck-related projects have been removed and are allocated separately as discussed below. Approximately two-thirds of obligations in the safety/TSM improvement type are primarily for traffic operations and other TSM improvements, and one-third are primarily for safety. The distinction between these two general types of improvements is blurred, however, since traffic operations improvements often improve safety and safety improvements may enhance traffic operations. Traffic operations/TSM projects are undertaken primarily to improve highway level of service, reduce congestion, and otherwise improve highway system efficiency. Therefore, construction costs are allocated on the basis of PCE-weighted VMT to reflect the contribution of different vehicle classes to congestion and diminished level of service.

Construction costs for safety improvements also are allocated using PCE-weighted VMT. While the relationship between PCEs, level of service, and safety improvements is not as clear as for TSM improvements, large trucks contribute more to the need for certain safety improvements than do automobiles and light trucks, and some additional safety improvement costs may be incurred to accommodate the operational characteristics of heavy trucks. Other costs within this general category are allocated on the basis of VMT since for the most part they are not related to characteristics of the different vehicle classes. Table V-17 shows the allocation of safety/TSM costs among broad vehicle classes and weight groups in both absolute amounts and cents per mile.

Environmental enhancement costs are allocated on the basis of VMT except for noise-related costs which are allocated among different vehicle classes on the basis of each vehicle classes' contribution to overall noise levels by highway functional class. Methods for estimating each vehicle's contribution to noise levels are described in Appendix E.

Other enhancement costs represent a variety of improvement types that generally are unrelated to characteristics of the different vehicles using the highway and thus are allocated using VMT. These costs include highway beautification; preservation of historic buildings, transportation facilities, and other important features; archeological preservation and salvage; scenic highway programs; wetland mitigation and enhancement; protection of endangered species; hazardous waste remediation; environmental education; tourist information; and vehicle emission inspection and maintenance.

Uniquely Occasioned Costs

Three separate types of uniquely occasioned costs are defined based on data available from FMIS-- ridesharing and HOV costs; transit costs paid from Federal-aid highway funds; and truck-related costs. Ridesharing programs, HOV lanes, and transit improvements focus on reducing congestion, environmental, and other costs occasioned primarily by operations of single-occupant vehicle (SOV) commuters in dense metropolitan areas. For this reason, the costs of these improvements have been allocated to automobiles, pickups, and vans in proportion to their travel on higher-order urban highways. Certainly all vehicles benefit to some degree from congestion relief in corridors where HOV and transit services are improved, whether or not they can directly use the facilities themselves. However, the cost occasioned approach is not based on benefits derived from improvements that are made, but on the contribution of each vehicle class to the need for the improvements. Since the primary purpose of HOV, ridesharing, and congestion-related transit improvements is to improve transportation capacity through alternatives to SOVs, and to reduce their adverse impacts of excessive travel by SOVs, costs for those improvements should be allocated to SOVs.

An alternative to allocating costs for HOV, ridesharing, and transit improvements just to automobiles, pickups, and vans would be to allocate costs to all vehicle classes in proportion to their PCE-weighted VMT. As noted above, that allocator is used for capacity enhancements related both to adding new lanes and to TSM projects. The difference between those types of improvements and HOV, ridesharing, and transit improvements is as follows: while new lanes and TSM meet general needs for new capacity and consider travel demand by all vehicle classes, HOV, ridesharing, and capacity-related transit provide relief for the effects of congestion caused primarily by SOV commuters in dense corridors. While all traffic in these corridors can be said to derive some benefit from the corridor-level efficiencies created by transit improvements, it is the SOV commuters who give rise to the need for additional capacity. Therefore, these users are responsible for the costs occasioned in providing this capacity--in this case, in the form of transit and HOV improvements.

Uniquely occasioned truck-related costs include costs of the commercial vehicle information systems project; motor carrier safety assistance program development and enforcement; commercial drivers license development and enforcement; truck scales; and truck loading, terminal, and transfer facilities. Obligations in each of these areas are focused uniquely on projects to make trucking operations safer, more efficient, and more friendly to the highway infrastructure. Costs are allocated to the various truck classes in proportion to their VMT.

Other Highway Trust Fund Obligations

Obligations discussed above are incurred as part of the Federal-aid highway program with funds being expended in the first instance by State or local highway agencies. There are some obligations of HTF monies that do not go through the Federal-aid highway program including highway improvements on Federal lands, contributions to the NHTSA Section 402 Program, and FHWA administrative expenses. These costs are not included in FMIS and must be estimated from other FHWA accounting information. Federal lands projects are primarily on lower-order rural highways and are allocated among vehicle classes in the same proportions as all other Federal obligations on lower-order rural highways. The FHWA administrative expenses are an overhead expense and are allocated among vehicle classes in the same proportion as all other Federal obligations.

Mass Transit Account

Obligations of funds from the MTA of the HTF are allocated in the same way that obligations for ridesharing, HOV lanes, and transit expenses from the HTF other than the MTA. Like these other cost categories, obligations from the MTA are focused on facilities and services for passengers, especially along high-density corridors handling large volumes of commuter traffic. The transit projects are focused on reducing congestion and other costs associated with the large volumes of SOV traffic. Heavy trucks may benefit from these improvements but, when possible, they try to avoid commuter routes during peak periods and are not the principal contributors to the need for the transit improvements. Thus, costs are allocated to automobiles, pickups, and vans in proportion to their travel on higher-order urban highways.

Summary of Federal Agency Cost Allocation

Table V-18 and Figure V-4 summarize the allocation of Federal program costs among different broad vehicle classes. Passenger vehicles are responsible for all costs from the MTA, 90 percent of system enhancement costs, 50 percent of new capacity costs, 40 percent of system preservation costs, and 70 percent of other costs. The overall Federal cost responsibility of automobiles, pickups, and vans is approximately two-thirds cent per mile of travel while the average overall cost responsibility of buses is about 2.5 cents per mile.

 Single unit trucks are responsible for about 11 percent of total highway costs, ranging from about 16 percent of system preservation costs to no responsibility for costs from the MTA. Their overall cost responsibility is 3.7 cents per mile.

Combination trucks are responsible for 30 percent of total Federal highway cost responsibility. They are responsible for more than 40 percent of system preservation costs and almost 40 percent of new capacity costs. Single trailer combinations, on average, have slightly higher cost responsibilities per mile than multi-trailer combinations. On average, per mile costs of both single and multi-trailer combinations are almost double those of single unit trucks and about 10 times the costs per mile for passenger vehicles.

Buses account for a very small share of overall highway cost responsibility because of their low annual travel. Average per mile costs of bus travel are approximately 2.7 cents per mile. Within these broad vehicle classes, there are large differences in the relative cost responsibility of different vehicle configurations. Figures V-5 to V-7 show cost responsibilities per mile for different vehicle configurations at different operating weights. Within single unit, single trailer, and multi-trailer truck categories, vehicles with more axles generally have lower costs per mile than vehicles with fewer axles. At light weights, differences in costs per mile are relatively small, but at weights where axle loads for particular configurations exceed Federal limits (20,000 pounds on single axles and 34,000 pounds on tandem axles), cost responsibilities per mile increase rapidly.

Registered Weight Federal Cost Responsibilities

Cost responsibilities discussed above reflect costs occasioned by different vehicle classes at various operating weights. Analyzing cost responsibilities over a range of operating weights provides a clear picture of relationships between axle loads, GVW, and highway cost responsibility. Evaluating cost responsibility on an operating weight basis is inappropriate, however, for analyzing user fee equity because vehicles do not always travel at the same weight. Over the course of a year, part of a vehicle's travel typically is empty and, even when loaded, most vehicles do not always operate at the same weight. Cost responsibilities for travel at all weights must be used when comparing costs to user fees paid during the year.

Each vehicle class has a unique average operating weight distribution, and in fact separate operating weight distributions are estimated for different functional highway classes -- rural Interstates, all other rural highways, urban Interstates, and all all other urban highways -- and for different registered weights for each vehicle. Cost responsibilities for vehicles registered at different weights are used in the equity analysis and the primary basis for evaluating the rate structure of the HVUT which is based on registered weights.

Table V-19 compares overall Federal highway cost responsibility for selected vehicles on an operating weight and a registered weight basis. Cost responsibilities in the heavier weight categories for any vehicle class are lower when estimated on a registered weight basis because a large portion of travel for vehicles registered at high weights typically is at weights below the registered weight. This reduces the average cost responsibility per mile compared to costs per mile calculated just for operations at the higher weight.

Table V-20 shows the overall Federal cost responsibility of selected vehicles classes on a registered weight basis. The cost responsibilities  per mile are not as high as cost responsibilities on an operating weight basis because, as noted above, much of the travel of vehicles registered at the upper weight intervals for each vehicle class are at lower weights where costs are not as high.

All Levels of Government Cost Allocation

All levels of government cost responsibility by vehicle class is illustrated in Table V-21. States are allocated about half of the total costs of $125 billion in 2000. Federal and local governments have an equal share of about 25 percent of the total costs. By vehicle class, automobiles have been allocated about half of total costs ($64 billion out of $125 billion) in 2000. As for the freight trucks, combination trucks, registered weighting between 75 to 80,000 pounds have been allocated about 15 percent of total costs ($18 billion out of $125 billion) in 2000.

Allocation of External and Other Social Costs of Highway Use

In Chapter III methods for estimating highway-related costs of air pollution, noise, congestion, crashes, and greenhouse gases were discussed. In this section those costs are allocated among different vehicle classes on the basis of characteristics of each vehicle class that contribute to the costs. References in this section can be found in Appendix E.


Table V-22 shows estimates of high, middle, and low estimates of noise costs developed for this study, in cents per vehile mile. Noise emissions and noise levels at specified distances from the roadway were developed using FHWA noise models. Cost are estimated as the loss in residential property value associated with exposure to various noise levels. Costs in rural areas are much lower than in urban areas for all vehicle classes because many fewer properties are exposed to sufficiently high noise levels to affect their values. It is important to note that these adverse effects of noise on property values often are more than offset by increases in property values associated with improved accessibility afforded by highway improvements and that noise impacts on property values will vary from location to location depending on a number of factors unrelated to highway noise.

Table V-22. 2000 Marginal External Costs for Noise (cents per mile)
  Rural Highways Urban Highways All Highways
  High Middle Low High Middle Low High Middle Low
Automobiles 0.03 0.01 0.00 0.30 0.11 0.03 0.20 0.06 0.02
Pickups and Vans 0.03 0.01 0.00 0.27 0.10 0.03 0.17 0.06 0.02
Buses 0.35 0.13 0.04 4.55 1.72 0.48 2.79 1.06 0.30
Single Unit Trucks 0.27 0.10 0.03 3.14 1.19 0.33 1.85 0.70 0.20
Combination Trucks 0.68 0.26 0.07 9.86 3.73 1.05 4.24 1.61 0.45
All Vehicles 0.08 0.03 0.01 0.64 0.24 0.07 0.42 0.16 0.05

Table V-22 shows the contribution of different vehicle classes to noise costs, expressed in cents per VMT. Combination vehicles have the highest noise costs per mile of travel followed closely by single units trucks and buses. Automobiles, pickups, and vans have low noise-related costs per mile, but because they account for the majority of travel by all vehicle classes, their contribution to total noise costs is substantial.

Figure V-8 shows estimated 2000 noise costs by vehicle class -- combination trucks account for 37 percent of total noise costs; while autos, pickups, and vans, 37 percent; and single unit trucks account for most of the remaining noise costs. It is important to note that these cost estimates are high because they do not account for the many miles of noise barriers that have been constructed to protect properties. It should also be noted that while costs are expressed on a cents per mile basis, the impact on property values is not a cumulative cost as are other environmental costs. The greatest impacts would be on property owners at the time a new highway was constructed or a new lane was added that significantly increased traffic volumes and noise levels. Once the property has been sold, the noise impact will largely have been internalized in either a lower selling price for the property or a longer time before the property could be sold to persons who because of their lifestyle or other factors are not as sensitive to highway noise as most persons.


In analyzing congestion costs, added delays to highway users associated with changes in traffic levels are estimated. The analysis includes both recurring congestion and the added delays due to incidents such as crashes and disabled vehicles. Effects of incidents are estimated using data on the frequency of incidents, their duration, and their impacts on highway capacity for different types of facilities.

The distinction between marginal and average costs is extremely important in considering congestion costs on a per vehicle mile (or per PCE mile) basis. Average congestion costs on a section of highway are calculated as the total congestion costs experienced by all vehicles divided by total vehicle miles. Marginal congestion costs are calculated as the increase in congestion costs resulting from a unit increase in vehicle miles. Because of the nature of the relationship between traffic levels and speed -- at low traffic levels, an additional vehicle mile has little or no effect on the speeds of other vehicles --marginal congestion cost are higher than average congestion costs.

Table V-23 shows high, middle, and low estimates of marginal external congestion costs in cents per vehicle mile. These costs represent added delays to other motorists associated with an additional trip. The costs are external to the trip maker since they are over and above the trip-maker's travel time costs, but they are not external to highway users as a group. Costs are estimated over a range of traffic volumes and vehicle mixes, and include both peak period and non-peak period conditions. The results presented are weighted averages, based on estimated percentages of peak and off-peak travel for different vehicle classes.

Table V-23. 2000 Marginal External Costs for Congestion (cents per mile)
  Rural Highways Urban Highways All Highways
  High Middle Low High Middle Low High Middle Low
Automobiles 3.76 1.28 0.34 18.27 6.21 1.64 13.17 4.48 1.19
Pickups and Vans 3.80 1.29 0.34 17.78 6.04 1.60 11.75 4.00 1.06
Buses 6.96 2.37 0.63 37.59 12.78 3.38 24.79 8.43 2.23
Single Unit Trucks 7.43 2.53 0.67 42.65 14.50 3.84 26.81 9.11 2.41
Combination Trucks 10.87 3.70 0.98 49.34 16.78 4.44 25.81 8.78 2.32
All Vehicles 4.40 1.50 0.40 19.72 6.71 1.78 13.81 4.70 1.24

Overall congestion costs per mile for trucks are approximately twice as high as costs for automobiles. On average, congestion costs per mile are about five times greater in urban areas than in rural areas, but this can vary considerably depending on traffic volumes on particular highways. In urban areas truck costs are approximately 2.7 times those of automobiles while in rural areas their costs per mile are about 2.9 times those of automobiles reflecting the generally higher PCEs in rural areas. Overall, single unit trucks have a slightly higher congestion cost per mile than combinations even though combinations have higher per mile costs in both rural and urban areas. The reason is that a greater share of single unit truck travel is in urban areas where costs per mile are higher.

Figure V-9 shows estimated 2000 congestion costs by vehicle class. Automobiles, pickups, and vans account for about 85 percent of congestion costs. The effect of trucks on traffic flow is partially offset by their relatively low volumes of travel during peak periods when congestion is greatest.

Crash Costs

The marginal cost for highway crashes is the increase in crash costs resulting from a unit increase in highway travel, commonly expressed in cents per added vehicle mile. Marginal costs for highway crashes include costs paid by those undertaking the additional travel as well as costs they cause to other drivers and non-drivers. In studies of highway user taxation, the focus is on costs to other drivers and non-drivers, since drivers deciding whether or not to increase (or decrease) the amount of travel they undertake are presumed to take the added crash costs to themselves into account. Crash costs to other drivers and non-drivers of a unit increase in highway travel are referred to as marginal external crash costs.

Marginal external crash costs were estimated as the sum of two components:

Table V-24. 2000 Marginal External Costs for Crashes (cents per mile)
  Rural Highways Urban Highways All Highways
  High Middle Low High Middle Low High Middle Low
Automobiles 9.68 3.15 1.76 4.03 1.28 0.78 6.02 1.94 1.13
Pickups and Vans 10.21 3.31 1.75 4.05 1.27 0.74 6.70 2.15 1.17
Buses 14.15 4.40 2.36 6.25 1.89 1.08 9.55 2.94 1.62
Single Unit Trucks 5.97 2.00 0.97 2.21 0.71 0.40 3.90 1.29 0.65
Combination Trucks 6.90 2.20 1.02 3.67 1.16 0.56 5.65 1.79 0.84
All Vehicles 9.52 3.09 1.68 3.98 1.26 0.76 6.12 1.97 1.11

Table V-24 shows high, middle, and low estimates of external crash costs for different vehicle classes in cents per vehicle mile. Appendix E contains a detailed explanation of how these crash costs were estimated.

It should be noted that marginal external costs for crashes are much lower than average costs for crashes, since the latter includes costs to both highway users and non-users. For all highways and vehicles, the middle estimates of average crash costs are 18 cents per vehicle mile on rural highways, 9 cents per vehicle mile on urban highways, and 13 cents per vehicle mile on all highways, as compared to the external cost (as shown in Table V-24) of 3.09 cents per vehicle mile on rural highways, 1.26 cents/mile on urban highways, and 1.97 cents/mile on all highways.

Of the five vehicle classes shown, buses have the highest crash costs per vehicle mile. However, buses have much higher occupancy rates than other classes such that they would be lower than the other classes on a per passenger mile basis.

Responsibility for Social Costs

Table V-25 summarizes total 2000 costs of noise, congestion, and crashes allocated to major vehicle groups. These costs include congestion and crash costs borne by highway users and costs that users do not bear but impose on others. Total costs in 1994 dollars are approximately $406 billion of which 69 percent is crash costs borne by highway users and another 14 percent is congestion costs borne by users. Only 12 percent of these social costs are costs that highway users impose on society and do not bear themselves. Air pollution and global warming costs which are borne by non-users are not included in this total. Air pollution costs along with revised total costs will be estimated in an addendum to this report. Regardless of who bears the costs, their magnitude suggests that further steps to mitigate safety, congestion, environmental, and other adverse impacts of highway use should be explored.

In allocating total congestion costs among vehicle classes, the product of marginal congestion costs and VMT was used to distribute costs. Congestion, however, is a highly non-linear phenomenon, and marginal congestion costs for each vehicle class shown in Table V-25 are substantially greater than average cost responsibility per vehicle mile which would be calculated by dividing congestion costs in Table V-25 by VMT for each vehicle class.

Table V-26 shows marginal costs of pavement deterioration, congestion, crashes, and noise associated with operations of several illustrative vehicles on typical rural and urban Interstate highways. Marginal pavement costs are included in this table because in estimating efficient user fee levels at which vehicles cover marginal costs of their travel, pavement deterioration costs must also be considered. Most other infrastructure costs paid by highway agencies do not vary directly with the amount of travel and thus are not truly marginal costs of highway travel. This table illustrates the range over which marginal costs of highway use can vary depending on the type of vehicle and where it travels. Marginal costs for autos operating on rural Interstate highways are about 1.8 cents per mile (excluding air pollution and global warming costs) compared to about 9 cents per mile for operations in urban areas where their contribution to each of the various elements of marginal cost is greater on a per mile basis. Of the illustrative vehicles shown in Table V-26, the 80,000 pound 5-axle tractor-semitrailer operating on urban Interstate highways has the highest marginal cost per mile -- 65 cents per mile excluding air pollution and global warming costs -- over half of which is related to pavement costs. Even though pavements typically may be designed to higher standards in urban areas, the additional cost of resurfacing and rehabilitation in urban areas accounts for the higher pavement costs in urban than in rural areas. It must be emphasized that these costs vary from location to location and are subject to considerable uncertainty as was discussed above.

Table V-27 illustrates the degree to which just one element of marginal cost -- congestion costs -- can vary on the same highway class. This table shows congestion costs on low volume (24,000 vehicles per day) and high volume (88,000 vehicles per day) 4-lane Interstate highways during peak and off-peak periods. Even on low-volume highways costs per mile during peak periods are over three times greater than during off-peak periods. On high volume highways marginal congestion costs are over six times greater during peak than off-peak times.


C.B. Breed, C. Older and W.S. Downs, A Study of Highway Costs and Motor Vehicle Payments in the United States, Association of American Railroads, Washington, D.C., 1993.

FHWA, The Final Report of the Federal Highway Cost Allocation Study, FHWA, Washington, D.C., 1982.

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