II. Trends and Forecasts of Highway Use


Most highway improvement costs are directly related to the use of the highway system by various classes of highway users. Too much use of particular facilities causes congestion and necessitates capacity improvements, demand management strategies, transit improvements, or similar investments to improve the level of service and efficiency of transportation facilities within the corridor. Similarly, system preservation improvements are required to rehabilitate pavements, bridges, and other highway features when use of those facilities has deteriorated their physical condition. Many environmental, congestion, safety, and other external highway costs also are related to highway use.

This chapter presents information on travel characteristics of the major types of vehicles using the highway system, including distributions of travel at various weights and on various highway functional classes. The range of registered weights of vehicles in each vehicle class is shown along with operating weight distributions for vehicles at each registered weight. This latter information is particularly important for evaluating options to improve highway user fee equity because each vehicle's overall cost responsibility and user fee payments are best compared over the full range of weights at which vehicles operate during the year.

Vehicle Classes

The classification of vehicles for HCA should reflect differences that affect either their highway cost responsibility or the highway user fees that vehicles pay. At the most general level, vehicles are grouped into five categories: automobiles, pickups and vans, buses, single unit trucks and combination trucks. Additionally, vehicles are grouped into 5,000 pound weight categories ranging from 5,000 pounds to 150,000 pounds to capture differences in cost responsibility that vary with weight. Finally, the single unit and combination trucks are divided into 17 classes reflecting differences in the number of cargo carrying units and the number and types of axles. The 20 vehicle classes used for this study are:

The classification of vehicles for this study is somewhat different from the classification used in the 1982 Federal HCAS. The earlier study had broken down automobiles into large and small autos, and distinguished three types of buses, school buses, transit buses, and intercity buses. It did not have as many truck classes. Because the current study is being done in conjunction with Department's Comprehensive TS&W Study (1997 U.S. DOT TS&W), some detail in the automobile and bus categories was sacrificed for greater detail in the truck categories.

Base and Future Year Vehicle Stock

The number of registered vehicles within each vehicle class is often referred to as the vehicle stock. Vehicle stock is important for estimates of HURs contributed by different vehicle classes because several user fees including the HVUT, the excise tax on new trucks and trailers, and the tire tax are related at least in part to the vehicle stock.

Estimates of the number of registered vehicles in each of the 20 vehicle classes used in this study were developed from several sources, including R. L. Polk, data reported by the States and summarized in FHWA's Highway Statistics, and the Truck Inventory and Use Survey (TIUS). First control totals for broad groupings of the vehicle classes (e.g., automobiles, light trucks, single unit trucks, and combination trucks) were estimated using Polk and Highway Statistics data. Then these broad vehicle classes were subdivided into the 20 vehicle classes based primarily on TIUS data. In some cases data from other sources including truck weight data collected by the States and recent State cost allocation studies were used. Table I-1 and Figure I-1 provide information about the population of specific vehicle classes.

Estimates of 2000 stock by vehicle class and registered weight were developed assuming that increases in the stock of vehicles would be proportionate to increases in travel by each vehicle class. Thus the average miles per vehicle and the mix of vehicle types (among the vehicle classes) in 2000 are the same as in 1994. The actual vehicle population and mix of vehicles is influenced by business cycles, current economic conditions, and shippers' demand for specific truck types. A simplified future year scenario facilitates analysis of alternative scenarios by avoiding compound effects caused by manipulating more than one input variable. This is of particular concern for the 1997 U.S. DOT TS&W Study.

Table II-1 shows estimates of the number of vehicles in each of the 20 vehicle classes for 1994 and 2000. Annual growth rates from 1994 to 2000 for the various vehicle classes are estimated to be 2.2 percent for autos, pickups, and vans; 2.6 percent for single unit and combination trucks, and 2.4 percent for buses.

Autos, pickups, and vans account for over 96 percent of all vehicles. The largest truck class is 2-axle single units which accounts for 2 percent of all vehicles. The next largest truck class is the 5-axle tractor-semitrailer which accounts for one-half percent of all vehicles.

The bus fleet is made up of three general types of vehicles: school buses, transit buses, and intercity buses. Of the total bus population of 654 ,000 vehicles in 1994, 71 percent were school buses, 24 percent were transit buses, and 5 percent were intercity buses. Each type of bus has different operations that result in wide variations in average annual travel. School buses average about 11,000 miles of travel each year, transit buses 22,000 miles, and intercity buses 66,000 miles per year.

Vehicle Population By Weight Groups

Vehicle weight is an important factors affecting highway cost responsibility. While the number, type, and spacing of axles interrelate with vehicle weight, weight is nevertheless one of the primary determinants of cost responsibility. Tables II-2 and II-3 show the number of vehicles in each vehicle class by registered weight in increments of 10,000 pounds for 1994 and 2000. For most vehicle classes, registered weight distributions are estimated based upon maximum declared weights in the TIUS database. For some vehicle classes, additional sources of information, including FHWA's Highway Statistics, truck weight study, and prior State HCASs, were also used. Details of how registered weight distributions by vehicle class were estimated are discussed in Appendix C. Registered weight distributions are assumed to be the same in 2000 as in 1994.

Figures II-1, II-2, and II-3 show the operating weight distributions of single unit trucks, combination trucks with semitrailers, and multi-trailer combinations. Among the single unit trucks, there are three distinct distributions that correspond to maximum allowable weights for those configurations based on Federal and State axle load limits. Among the semitrailer combinations, the 5- and 6-axle combinations have sharp peaks at 80,000 pounds, the maximum allowable weight on Interstate highways. More 6-axle tractor-semitrailers operate above 80,000 pounds than do 5-axle tractor-semitrailers. The 3- and 4-axle single trailer combinations do not have pronounced peaks in their registered weight distributions, reflecting the variety of uses of those vehicles. Among the multi-trailer combinations, there is a pronounced peak at 80,000 pounds for the 5- and 6-axle combinations, and lesser peaks at 80,000 pounds for 7- and 8- or more axle multi-trailer combinations. There is another peak for the 7- and 8- or more axle multi-trailer combinations about 110,000 pounds, the maximum allowable weight in several Western States that have grandfather rights to allow higher weights on the Interstate System. allowable weight in several Western States that have grandfather rights to allow higher weights on the Interstate System.

Base Year and Future Year Highway Travel

The VMT by each vehicle class is critical to virtually all aspects of HCA. It enters into estimates of HURs paid by each vehicle class, cost responsibilities of different vehicles, and the allocators that are used to assign different costs to different vehicles. Travel is estimated for each of the 20 vehicle classes by 5,000 pound operating weight interval and by each of 12 functional highway classes.

Table II-4 shows the 12 highway functional classes that have been designated for planning purposes by the AASHTO in cooperation with FHWA. Analysis of travel by highway functional class is important because higher-order systems (Interstate, Other Freeways and Expressways, and Other Principal Arterials) are designed differently from lower-order systems and the various vehicles analyzed in this study have substantially different travel patterns by highway class.

Methods for estimating VMT involved several steps. Total VMT reported in FHWA's 1994 Highway Statistics was the control total for broad vehicle classes in the base case analysis. Using the truck VMT control total from Highway Statistics and data from the 1992 TIUS on the distribution of VMT across the various truck classes and weight groups, control totals for the various truck vehicle classes were derived.

For the distribution of VMT across vehicle classes, weight groups, and highway functional classes, an additional analysis was conducted on 12,000,000 truck weighings from truck weight study data collected by the States and several recent HCASs. A major reason for incorporating other data sources than the Highway Statistics and Highway Performance Monitoring System (HPMS) data was that some vehicle classes were substantially different than those usually used by States in collecting traffic count and truck weight data.

Figure II-4 shows the distribution of total VMT by all vehicles across the 12 highway functional classes. About 60 percent of total travel is in urban areas; in both rural and urban areas there is more travel on the higher order systems than on lower order systems.

Figure II-5 shows the distribution of travel on higher order and lower order systems for selected vehicle classes. Sixty-five percent of automobile VMT is in urban areas with the majority being on higher order systems. The distribution of 3-axle single unit truck VMT is more evenly split -- 53 percent in urban areas and 47 percent in rural areas. Sixty-three percent of 5-axle tractor semitrailer VMT is in rural areas, with the majority of that being on the higher order systems. Only 20 percent of tractor-semitrailer traffic is on lower order systems in either rural or urban areas.

Tables II-5 and II-6 show estimated VMT by vehicle class and highway functional class. As noted earlier, the assumption is that the distribution of VMT by highway functional class will not change between the base period and 2000.

Tables II-7, II-8, II-9, and II-10 show the distribution of VMT by vehicle class and registered/operating weights. The data are derived primarily from truck weight study data submitted to FHWA by the States. As with the distribution of VMT by functional class, it is assumed that operating weight distributions for the various vehicle classes will not change on average between the base period (1993-1995) and 2000. Figure II-6 shows the operating weight distribution for selected vehicle classes. The operating weight distribution of 3-axle single units has a single peak of about 17 percent at 25,000 pounds after which the percentage of travel falls steadily. Only about 5 percent of travel by those vehicles is at weights greater than 50,000 pounds. The 5-axle tractor-semitrailer has a bi-modal operating weight distribution with one mode at about 35,000 pounds and another mode at 75,000 pounds. About 5 percent of 5-axle tractor semitrailer travel is at weights greater than 85,000 pounds. The 5-axle twin trailer combination also has a slightly bi-modal operating weight distribution with one mode at about 35,000 pounds and another at about 75,000 pounds.

Similar to the method used for the number of vehicles, the base future VMT in the Year 2000 employed relatively simple growth rates (to facilitate alternative HCAS revenue scenarios and TS&W policy options). Proportional vehicle class and weight group shares were held constant, with all values being grown by the appropriate growth rate for automobiles, light trucks, other trucks, or buses. The future year vehicle VMT was based on three general assumptions for the different vehicle groupings (based on recent trends). Automobile and light truck VMT were assumed to increase by 2.2 percent per year from 1994 to 2000. The VMT for all truck classes was assumed to increase 2.6 percent per year. Bus VMT was assumed to increase by 2.4 percent per year. These projected VMT values are shown in Table II-11.

Relationship Between Operating and Registered Weights

As noted earlier, vehicles pay certain taxes on the basis of their registered weight, and the overall equity of highway user fees is best evaluated by comparing user fee payments and highway cost responsibility over the full range of operations by vehicles at each registered weight during the year. However, relationships between vehicle weight and pavement and bridge costs are best understood by examining cost responsibilities at different operating weights. In fact, all cost responsibilities for each vehicle class are estimated in the first instance on the basis of VMT at each operating weight. To compare user fee payments and cost responsibilities on a registered weight basis for each vehicle class, the annual distribution of operating weights for vehicles at each registered weight must be estimated. These relationships were initially developed based on an analysis of the R. L. Polk registered weight data and the operating weight data in the 1992 TIUS. Because States differ on their definition of registered weight, only States that define registered weight as the GVW were analyzed (see Appendix C).

Figures II-7 and II-12 show relationships between operating weights and registered weights for different vehicle classes. Figures II-7 to II-9 compare operating weight distributions with registered weight distributions for 3-axle single units, 5-axle tractor-semitrailers, and 8-or-more axle twin trailer combinations. These figures show that registered weight distributions tend to have one or more peaks, usually centered around maximum GVWs in different States. Figures II-10 to II-12 illustrate operating weight distributions for 3-axle single unit trucks at three registered weights: 35,000 pounds, 45,000 pounds, and 55,000 pounds; a 5-axle tractor-semitrailer at three registered weights: 65,000 pounds, 80,000 pounds, and 90,000 pounds; and an 8-or-more axle twin trailer combination at three registered weights: 110,000 pounds, 120,000 pounds, and 130,000 pounds. These figures show that the distribution of operating weights, and thus the overall highway cost responsibility, for the same vehicle at different registered weights can be quite different. 

Table II-11. Trends and Projections of VMT by Vehicle Class
(millions)
  1990 1994 2000
AUTO/LT4 1,997,283 2,183,153 2,487,659
Single Unit Trucks 64,114 71,239 83,150
Combination Trucks 89,257 99,176 115,639
Buses 5,822 6,416 7,397
TOTAL 2,156,476 2,359,984 2,693,845

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