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46 GLOSSARY Alternative fee--a fee that is charged to some vehicles in and when deferred maintenance moves the costs associated place of the usual fee (e.g., a lower registration fee for with one period's use into another period. publicly owned vehicles). Cross-subsidization--a condition where some vehicles Arterial--a road or highway used primarily for through overpay and others underpay relative to their respective traffic. responsibilities. Attributable costs--costs that are a function of vehicle size, Dead load--load on a bridge when it is empty. weight, or other operating characteristics and therefore Debt financing--funding current activities by issuing debt can be attributed to vehicle classes based on those to be repaid in the future. characteristics. Debt service--funds used for the repayment of previously Average daily traffic--average number of vehicles passing incurred debt (both principal and interest). a given point or using a given highway per day. Deck--the roadway or surface of a bridge. Average daily truck traffic--average number of trucks Depreciation--the amount of decrease in the value of a passing a given point or using a given highway per day. physical asset owing to aging in a time period. Axle-miles of travel (AMT)--product of VMT multiplied Efficiency--the degree to which potential benefits are real- by the number of axles. Because trucks, on average, have ized for a given expenditure. roughly twice as many axles as cars (i.e., four versus two), Efficient pricing--a system of setting prices for the use of their share of the total AMT on any given highway system highway facilities so that each vehicle pays the costs it will be approximately double their share of VMT on that imposes at the time and place it is traveling. It promotes system. the most efficient use of existing facilities and gener- Axle weight or axle load--gross load carried by an axle. ates the right amount of revenue to build the most Beltway--a controlled-access arterial encircling an urban efficient system and perform the optimal amount of area. maintenance. Collector--a road that connects local roads with arterial Equity--generally interpreted as the state of being just, im- roads. partial, or fair. Horizontal equity refers to the fair treat- Common costs--expenditures that are independent of vehi- ment of individuals with similar circumstances. Vertical cle size, weight, or other operating characteristics and so equity refers to the fair treatment of individuals in differ- cannot be attributed to any specific class of vehicles. ent circumstances. These expenditures must therefore be treated as a com- Equity ratio--the ratio of the share of revenues paid by a mon responsibility of all vehicle classes and are most highway-user group to the share of costs imposed by that typically assigned to all classes on the basis of a relative group. A user group that meets 110% of its cost responsi- measure of use such as VMT. bility would be assigned an equity ratio of 1.1. Equity Cost allocation--the analytical process of determining the ratios above 1.0 are assigned to user groups who are cost responsibility of highway-system users. paying more than their cost-responsible share, where pay- Cost-occasioned approach--one that determines responsi- ments from user groups assigned equity ratios of less than bility for highway expenditures/costs based on the costs 1.0 fall short of the costs imposed by the group. occasioned or caused by each vehicle class. Such an Equivalent single-axle load (ESAL)--are calculated based approach is not based solely on relative use, nor does it on the pavement stress imposed by a single axle with attempt to quantify the benefits received by different an18,000-lb axle load. ESAL-miles are equivalent single- classes of road users. axle loads times miles traveled. Research has concluded Cost responsibility--based on the principle that those that the relationship between axle weight and ESALs is an who use the public roads should pay for them and, more approximate third- or fourth-power exponential relation- specifically, that payments from road users should be ship; ESALs therefore rise rapidly with increases in axle in proportion to the road costs for which they are re- weight. sponsible. It is the proportionate share of highway costs Excise tax--a tax levied on the production or sale of a spe- legitimately assignable to a given vehicle class user cific item such as gasoline, diesel fuel, or vehicles. group. Expenditure--the amount of money spent in a time period. Cost-based approach--one in which the dollars allocated to External cost--one imposed on individuals who do not use the vehicle classes are measures of the costs imposed the facility. during the study period, rather than expenditures made Federal highway funds--those collected from federal high- during the study period. The difference between the cost- way-user fees and distributed to states by the FHWA for based and expenditure-based approaches is most evident spending on transportation projects by state and local when considering large investments in long-lived structures governments.

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47 Functional classification--a system of classification of roads well as deterioration from age and environmental factors, according to their general use, character, or relative impor- taking into account the soil type, road-base depth, pave- tance. Definitions are provided by the FHWA for Rural ment material, pavement thickness, and climate zone. Interstate, Rural Other Principal Arterial, Rural Minor Non-divisible load--large pieces of equipment or materials Arterial, Rural Major Collector, Rural Minor Collector, that cannot be feasibly divided into smaller individual Rural Local, Urban Interstate, Urban Other Expressway, shipments. All states issue special permits for non- Urban Other Principal Arterial, Urban Minor Arterial, divisible loads that would otherwise violate state and Urban Collector, and Urban Local. federal gross vehicle weight, axle weight, and bridge Functionally obsolete--a functionally obsolete bridge is formula limits. one that no longer meets minimum standards, but may Operating weight--the actual weight of a vehicle at a par- continue to operate with load restrictions. ticular time. Gross vehicle weight--the loaded weight for a vehicle. Overhead costs--costs that vary in proportion to the overall Highway cost allocation study (HCAS)--a study that esti- level of construction and maintenance activities, but are mates and compares the costs imposed and the revenues not directly associated with specific projects. paid by different classes of vehicles over some time Passenger car equivalent--a measure of road space effec- period. tively occupied by a vehicle of a given type under given Highway Performance Monitoring System--a system terrain, vehicle mix, road type, and congestion conditions. whereby the FHWA collects and reports data about a The reference unit is the standard passenger car operating sample of road segments in every state in a common under the conditions on the road category in question. format. Registered weight--the weight that determines the registra- Highway user--a person responsible for the operation of a tion fee paid by a single-unit truck or a tractor. For a trac- motor vehicle in use on highways, roads, and streets. In tor, it is typically the highest of that vehicle's declared the case of passenger vehicles, the users are the people in weights. the vehicles. In the case of goods-transporting trucks, the Revenue attribution--the process of associating revenue user is the entity transporting the goods. amounts with the classes of vehicles that produce the Incremental cost--the additional costs associated with revenues. building a facility to handle an additional, heavier (or Right-of-way--the strip of land, property, or interest therein, larger) class of vehicle. over which a highway or roadway is built. Incremental method--one that assigns responsibility for Seismic retrofit--the work done on an existing structure highway costs by comparing the costs of constructing and intended to increase its resistance to earthquakes. maintaining facilities only for the lightest class of vehi- Social (or indirect) costs--costs that highway users impose cles and for each increment of larger and heavier vehicles. on other users or on non-users. Costs typically included in Under this method, vehicles share the incremental cost of this category are those associated with noise, air, and a facility designed to accommodate that class as well as water pollution; traffic congestion; and injury and prop- the cost of each lower increment. erty damage resulting from traffic accidents. Light (or basic) vehicles--the lightest vehicle class, usually Span--a section of a bridge. including passenger cars, vans, and pickups. State highway system--comprises all roads under the juris- Live load--the additional load on a structure by traffic diction of state agencies. (beyond the dead load imposed by holding itself up). Statewide Transportation Improvement Program--a Load-related costs--those costs that vary with the load program where each state, following federal law guide- imposed by traffic on a facility. lines, produces and regularly updates a list of intended Marginal cost--the increase in total cost that results from future transportation improvements. producing one additional unit of output. With respect to Structurally deficient--one that fails to meet the desired highway use, the marginal cost is the increase in total level of structural integrity. Weight limits often are placed highway costs that results from one additional vehicle on structurally deficient bridges. trip. Economic efficiency is achieved when the price Tax avoidance--the legal avoidance of a tax or fee. charged to the user is equal to the marginal cost. Tax evasion--the illegal failure to pay a tax or fee. National Highway System--a set of highways throughout Truck--a general term denoting a motor vehicle designed the United States that have been designated as national for transportation of goods. The term includes single-unit highways by the federal government. The FHWA sets trucks and truck combinations. design and maintenance standards and provides funding User charge--a fee, tax, or charge that is imposed on facil- for national highways; however, the highways are owned ity users as a condition of usage. by the states. User revenues--highway revenues raised through the impo- National pavement cost model--a model of pavement sition of user charges or fees. costs that incorporates the wear-and-tear costs imposed by Value pricing--a system where prices are set in proportion vehicle traffic of different weights and configurations as to the benefits or value received by road users.

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48 Vehicle class--any grouping of vehicles having similar Vehicle miles of travel--equal the sum of vehicles divided characteristics for cost allocation, taxation, or other pur- by the number of miles each vehicle travels within a time poses. The number of vehicle classes used in a cost period. responsibility (allocation) study will depend on the needs, Vehicle registration fees--fees charged for being allowed purpose, and resources of the study. Potential distinguish- to operate a vehicle on public roads. ing characteristics include weight, size, number of axles, Weight-mile tax--a graduated fee based on the weight of a type of fuel, time of operation, and place of operation. vehicle and the miles the vehicle travels.