Highway Statistics 1998
This section contains data on the physical, operational, use, extent, and performance characteristics of public roads in the United States.
IMPACTS OF DATA REPORTING PROCEDURES
These data are derived from the Highway Performance Monitoring System (HPMS). The HPMS is a combination of sample data on the condition, use, performance and physical characteristics of facilities functionally classified as arterials and collectors (except rural minor collectors) and system-type data for all public roads within each State. When a State does not provide complete current year data, FHWA may supplement the State data with estimates based on State provided aggregate totals or on prior year(s) data. Footnotes explain missing data, data which have been estimated by FHWA, or data which are from previous year(s). Readers should read all the footnotes as well as this text prior to using data in this section.ORGANIZATION OF TABLES
Nearly all tables in this section contain rural and urban stratifications; in some tables, urban may be further stratified into small urban (5,000 to 49,999 population) and urbanized (>50,000 population).
An urbanized area is an area with 50,000 or more persons that encompasses at a minimum the land area delineated by the Bureau of the Census. The Bureau of the Census establishes urbanized area boundaries based on the density of the population (persons per square mile). The adjusted Census urbanized area boundary reflected in this publication is usually enlarged to include such additional areas as airports, satellite cities/towns, strip developments adjacent to high-use roadways, and other areas and facilities that are important to or serve the urbanized area. In some cases, the adjusted urbanized area includes land that will become urban in some predetermined amount of time (such as 3-5 years). These boundaries are decided upon by State and local officials with the approval of the FHWA.
The tables in this section have been organized into seven general areas:
1. Public Road Length--Tables HM-10, HM-12, HM-16, HM-18, and HM-20.
2. Federal-Aid Highways--Tables HM-14, HM-15, HM-30, HM-31, HM-33, HM-35, HM-36, HM-37, HM-39, HM-42, HM-47, and HM-48.
3. Functional System--Tables HM-50, HM-51, HM-53, HM-55, HM-57, HM-59, and HM-60.
4. Urbanized Areas--Tables HM-71 and HM-72.
5. State Highway Agency-Owned Public Roads--Tables HM-80 and HM-81.
6. Highway Use and Performance--Tables VM-1, VM-2, VM-3, VM-4, TC-3, HM-61, HM-63, HM-64, FI-10, and FI-20.
7. Metric--All Tables; electronic versions only.
All length tables in this publication, except table HM-30, include only the centerline length of roads classified as public roads in accordance with the Code of Federal Regulations (23 CFR 460). These include existing roads that are open to public travel and maintained by a public authority. Where a highway route includes more than one roadway, the centerline length is the average of the independent directional roadways measured along their respective centerlines. Other existing roads and trails that are privately owned or not publicly maintained or restricted from use by the general public are excluded. Table HM-10 contains the total public road length in the United States, classified by ownership. Table HM-12 is a national summary of length by type of surface and ownership/functional system. Table HM-15 contains length of Federal-aid highways and is in the same format as its companion travel table, VM-3, and lane length table, HM-48. Table HM-16 is a nationwide summary which contains length of Federal-aid highways by ownership. Another nationwide summary table, HM-18, contains length of Federal-aid highways by functional system. The latter two tables also include small urban and urbanized area summaries as well as total urban. Table HM-20 contains public road length by functional system and is in the same format as table VM-2. Table HM-60 is also in the same format and contains the lane length. Table HM-30 contains length of NHS roads that are open to traffic as well as those planned but not yet built.
Except for minor amounts of Federal public road length under the U.S. Forest Service, Bureau of Reclamation, Bureau of Indian Affairs, National Park Service, Fish and Wildlife Service, and military, the rest of the public roads in the United States are owned by the State, county, town or township, Indian tribe, municipal or other local government, or an instrumentality thereof with public authority. Public roads in Federal forests and reservations may be part of the State and local road systems and are included with the lengths reported for those systems. Length of public roads by ownership are identified in tables HM-10, HM-12, HM-14, HM-16, and HM-50.
The surface classification used in the tables identifies only the visible surface types on the traveled portion of the roadway. Many highways, either by original design or because of reconstruction, consist of more than one major type of construction material. No data relative to vertical composition are presented, except for the composite pavement type where the reporting agency has indicated that a concrete roadway has been overlaid with one or more inches (25 or more millimeters) of compacted bituminous material.
Federal-aid highways are segments of State and local road systems eligible for Federal-aid construction and rehabilitation funds because of their service value and importance. Except for connectors on the National Highway System and bridges, roads that are functionally classified as rural minor collector or rural and urban local are not eligible for Federal aid. The designation of a public road as a Federal-aid highway does not alter its ownership as a State or county road or city street.
The Intermodal Surface Transportation Efficiency Act (ISTEA) of 1991 eliminated the historical Federal-aid Systems and created the National Highway System (NHS) and other Federal-aid highway categories. The NHS was selected in consultation with appropriate local officials; it consists of the highway routes and connections to transportation facilities depicted on the maps submitted by the Secretary of Transportation to the Congress on May 24, 1996. The NHS was approved by the National Highway System Designation Act of 1995. Highways designated as part of the Interstate System are included in the NHS. Nonroadway NHS routes, such as ferry boats, are not included in Highway Statistics.
Historical tables linked to the superseded Federal-aid systems have been changed to reflect the NHS and other Federal-aid highways, as applicable. These include tables HM-14, HM-15, HM-16, HM-18, the HM-30 series, HM-42, HM-47, HM-48, VM-3, and FI - 20. About 2,000 miles of NHS intermodal connectors are included in these tables as NHS.
The Eisenhower National System of Interstate and Defense Highways was originally established by the Federal-Aid Highway Act of 1944. The Federal-Aid Highway Act of 1956 and the companion Highway Revenue Act of 1956 further defined the purpose and extent of the system and, as subsequently amended, dedicated a group of Federal excise taxes on motor fuel and automotive products to support Federal-aid highway activities. The Interstate System extends over 46,000 miles (74,000 kilometers) and connects, as directly as practicable, the Nation's principal metropolitan areas, cities, and industrial centers; serves the National defense; and connects at suitable border points with routes of continental importance.
Length related characteristics
Federal-aid highways are classified according to system, surface type, lane width, traffic lanes, access control, and traffic volume in tables HM-31 through HM-39. Tables HM-30, HM-42, and HM-47 classify length of NHS according to open and not open to traffic, volume-service flow ratio, and measured pavement roughness, respectively.
The functional systems used in tables HM-50 through HM-80 result from the grouping of public roads according to the character of service they are intended to provide. Because most travel involves movement through a network of roads, individual public roads do not serve travel independently in any major way. Functional classification defines the role that a particular public road plays in serving the flow of trips through a highway network. The functional systems are: (1) arterial highways, which generally handle the long trips; (2) collector facilities, which collect and disperse traffic between the arterials and the lower level; and (3) local roads, streets, and other public ways, which serve the land access function to the residential areas, businesses, individual farms and ranches, and other local areas. The functional system lengths are identified by ownership, surface type, lane width, traffic lanes, access control, and traffic volume in tables HM-50 through HM-59.
Table HM-60 contains estimated lane length by functional system. FHWA assumes two lanes for the rural minor collector and rural/urban local functional systems lane length estimate. A companion table, HM-48, is similar except the lane length is for the Federal-aid highways.
Table HM-71 contains length and daily travel data by functional system for the adjusted Census urbanized areas having a population of 50,000 or more persons. The daily travel contained in this table times 365 days (366 days for leap years) equals annual travel. Table HM-72 contains urbanized area characteristics and rates based on State-reported mileage, travel, population, and land area data. An estimate of freeway lane length in each urbanized area is also provided; it includes Interstate and other freeway and expressway length only and is based on HPMS universe data. Where urbanized areas cross State boundaries, the tables contain information for the entire area without regard to State boundaries. Both tables are in sort by urbanized area population.
State highway agency-owned public roads
State highway agency-owned public road length by functional system is contained in table HM-80 while lane length, daily vehicle travel, and annual average daily traffic per lane estimates are provided in companion table HM-81. As with tables HM-71 and HM-72, the daily travel may be made to equal annual travel by multiplying by 365 (366 for leap years). These tables are based on HPMS universe data.
HIGHWAY USE AND PERFORMANCE
Table VM-1 is a summary of estimated travel by vehicle type and system; travel values are derived from the highway functional systems data contained in table VM-2, while the vehicle types are derived from the categories used in table VM-4. The other rural arterial roads category includes the other principal and minor arterial functional systems; the other rural roads category includes the collector and local functional systems. All urban systems, except the urban Interstate System, are included in the other urban public roads category.
The highway use of motor fuel and the motor-vehicle registrations from tables MF-21 and MV-1 are used to estimate average distance traveled per vehicle, average fuel consumption per vehicle, and average distance traveled per gallon of fuel consumed. The estimate of distance traveled per person is based on the most current information from the Nationwide Personal Transportation Survey, the Truck Inventory and Use Survey, and the National Transportation Statistics report.
Table VM-2 is a summary of the States' estimated highway travel based on traffic counts by functional system; it is a companion to table HM-20. Table VM-3 contains estimated highway travel on Federal-aid highways; it is a companion to length table HM-15.
Table VM-4 shows the distribution of travel activity by vehicle type for arterial functional systems. Data in some cases may exclude motorcycles, combine some vehicle types, and/or be limited in count duration and seasonal coverage. Users should refer to the Traffic Monitoring Guide for vehicle descriptions and sampling guidelines.
Table TC-3 presents a distribution of historic Interstate System traffic volumes and loadings; it shows the interrelationship among vehicle types, volumes, and axle loadings. The data, which are from the Truck Weight Study, are collected by the States from weigh-in-motion systems. The accompanying graph, Comparison of Growth in Volumes and Loadings on the Rural Interstate System, shows the growth in loadings relative to volumes. The data points on the graph are 3-year moving averages.
Volume-service flow ratio
Table HM-61 shows arterial and collector functional system length by State for various ranges of volume-service flow ratio using HPMS expanded sample traffic volume and peak period service flow data. Some use this metric as a measure of peak period traffic congestion. A companion table, HM-42, provides similar information for the NHS.
Present serviceability rating (PSR)
Table HM-63 shows a distribution of mileage by PSR value for the rural major collector, urban minor arterial, and collector functional systems. The PSR is a subjective measure of pavement condition based on an assessment of ride and visual pavement distress by a rating panel. Users of these data are advised to see the discussion of PSR in the section "General Limitations In Use of Data."
International Roughness Index (IRI)
Table HM-64 shows a distribution of mileage by IRI value for the principal arterial and rural minor arterial functional systems. A companion table, HM-47, provides similar information for the NHS. IRI, a measure of pavement roughness, is an objective equipment-based rating reported in the HPMS in inches per mile (meters per kilometer). Users of these data are advised to see the discussion of IRI in the section "General Limitations In Use of Data."Injury highway crashes
Tables FI-10 and FI-20 show highway fatalities by functional system and Federal-aid highways. The fatality data are from the NHTSA Fatality Analysis Reporting System.
Metric tables are marked with an "M" at the end of the table number. A soft conversion from English to metric has been made using 1 mile = 1.609344 kilometers for the individual values; totals may not agree with the equivalent English-based tables.
GENERAL LIMITATIONS IN USE OF DATA
Information included in the HPMS data base is the result of a cooperative effort between the FHWA and State and local governments. All HPMS data are provided to the FHWA through State Departments of Transportation and are usually obtained from existing State or local government data bases, including those of Metropolitan Planning Organizations (MPOs). Typically, the existing data bases and record keeping systems of these State and local governmental units are designed and are maintained to meet their specific, individual business needs; most items in their data bases are not specifically designed to meet Federal reporting purposes. Some data required for the HPMS are not collected by all the States in their own data bases, and some State data base adjustments may have been made in order to meet the requirements of the HPMS.
As a result, data quality and consistency of HPMS information are dependent upon the programs, actions and maintenance of sound data bases by numerous data collectors, suppliers and analysts at the State, metropolitan, and other local area levels. In general, specific data items that are used by the collecting agency are likely to be of better quality than data items which are collected or estimated solely for the use of the FHWA. Data quality and consistency are also dependent upon the nature of the individual data items and how difficult they are to define, collect, or estimate. Particularly in recent years, limitations on staff and financial resources at the State and local levels have seriously impacted many of the data collection programs that feed the HPMS data base.
HPMS data are collected in accordance with the Highway Performance Monitoring System Field Manual for the Continuing Analytical and Statistical Data Base. This document contains standard collection, coding, and reporting instructions for the various data items to be reported with the objective of creating a uniform and consistent data base. The degree to which these instructions are followed by the reporting agencies has a direct impact on the quality and consistency of the data and, therefore, on the utility of these data as an indicator of the condition, performance, and use of the Nation's highway systems. State reported HPMS data are reviewed by FHWA for completeness, consistency, and adherence to reporting guidelines. Data are generally used as reported; any adjustments are accomplished in close working relationship with data providers.
The HPMS data base is constructed primarily to facilitate national level analyses of the condition, performance, and use of the Nation's highway system. Users of the HPMS data, as reported in Highway Statistics and in other media, should not necessarily expect to find consistency among all States for all data items, due to State-to-State differences in the way the data are defined, collected, or estimated. Even when data are consistently collected and reported, users need to recognize that HPMS information may not be comparable across all States due to inherent State differences such as size, population density, degree of urbanization, extent of system, administrative responsibility, climate, etc. When making State level comparisons, therefore, it is inappropriate to use these statistics without recognizing and accounting for the differences that may impact comparability.
The HPMS contains data from two pavement rating systems. One is a subjective rating system based upon the Pavement Serviceability Rating (PSR) or equivalent data adapted from a State's pavement serviceability index (PSI), sufficiency ratings, or ratings from a pavement rating table contained in the HPMS Field Manual. The second is an objective rating measure, the International Roughness Index (IRI). Both are used as indicators of pavement surface condition. Pavement rating data are not reported for local or rural minor collector functional systems.
Present serviceability rating
PSR and other subjective measures use a numerical value ranging from zero to five, reflecting poor pavement condition at the lower end of the scale and very good pavement condition at the higher values. These indices provide a judgement of pavement condition based upon an assessment of ride and visual pavement distresses by a panel of road users. PSR and PSI were adapted from the American Association of State Highway (Transportation) Officials Road Tests conducted in the late 1950's and early 1960's. Since they are subjective, ride-based rating schemes, and because of the various methodologies used by the States to collect these data, the ratings may not be consistent or comparable among the States. If a State's PSR, PSI, or other subjective rating process has been carefully constructed and executed, it should provide a useful basis for rating roadways within a State. However, because of the subjectivity of the rating process and because of State-to-State differences in pavement types, traffic, weather, soil conditions, and other influencing factors, comparisons of pavements based on these indices may not be valid.
In general, PSR, or equivalent subjective pavement rating data, are reported to the HPMS on an annual basis where IRI data are not reported. PSR data are input to the HPMS data base as reported by the States; sampled PSR data are expanded using the sample expansion factors in HPMS to represent the functional systems for which they are reported. Although PSR is primarily a measure of current ride quality, PSR data are used by FHWA in national level models to predict pavement deterioration, deficiencies, and investment needs.
Measured pavement roughness
Measured pavement roughness is an objective equipment-based rating reported in the HPMS as IRI in inches per mile (meters per kilometer). These ratings are collected by various mechanical devices, some of which may require calibration through correlation to "known profiles" established via precise measurements. The IRI is a numerical value that is an accumulation of the inches (meters) of vertical movement of a vehicle over a roadway surface, adjusted to reflect a rate per mile (kilometer). Low values indicate a smooth riding quality, while higher values are indicative of a rough road. Because IRI is a more objective, mechanically measured index, IRI should be more consistent between and among States when similar pavement types and surface textures have been measured using devices that have been properly calibrated.
Variability in IRI measurements can arise from differences in the equipment used to measure IRI, and differences in the measurement protocols used. In an effort to improve data consistency, FHWA recommends that States follow AASHTO Provisional Standard PP37-99 when reporting IRI data to the HPMS. Additional guidelines intended to improve consistency are included in the HPMS Field Manual and users are encouraged to consult that document. Improvements to measurement equipment and instrument standardization for the most part rely upon Federal and State research activities such as those carried out through the Road Profiler User Group. While IRI data may be more suitable for comparative purposes than PSR data, the user needs to consider and account for the variability introduced by these and other factors, such as pavement type, when making comparisons.
The FHWA believes that the IRI data, which for the most part are available from State pavement management systems, are of reasonably good quality. As additional protocols for IRI equipment and measurement techniques, such as filtering, measurement intervals, sensor use, lane location, speed, etc., become available and are developed and adopted by the States, FHWA believes that IRI data consistency should continue to improve.
In general, IRI data are reported to the HPMS on an annual basis for the arterial functional systems and all NHS routes. IRI data are used as reported by the States without manipulation or adjustment. Although IRI is a measure of ride quality, the IRI data are also used by FHWA in national level models to predict pavement deterioration, deficiencies, and investment needs.
IRI is a measure of surface condition; to have a comprehensive measure of pavement condition, data on other pavement distresses such as rutting, cracking, and faulting are needed. Provisional standards for these measures are being developed by AASHTO, and States will be encouraged to include them in their pavement management systems in the future.
Public Road Length
In 1997, FHWA instituted a new method for creating mileage based tables. Previously, adjustments to tables developed from sampled HPMS data were made using areawide mileage data provided by the States. These adjustments are no longer made; instead, expanded sample data are used as reported. In addition, in the past rounding and expansion related differences were spread across table cells so that all table-to-table mileage totals on related tables matched precisely. While these steps made all functional system table totals match, an unintended result is that the tables are not reproducible from the data set by any other users. As a result, FHWA decided to discontinue the factoring and spreading process, and users may note minor differences in table-to-table mileage totals. For record purposes, FHWA considers the mileage totals from Table HM-20, Public Road Length, Miles by Functional System to be the controlling totals should a single value be required.
Lane length is a computed value; it is a product of the centerline length and the number of through lanes. FHWA uses lane length as a primary measure of road supply. The number of through lanes are the prevailing number of lanes in both directions carrying through traffic in the off-peak period. It excludes lanes used for parking, turning, collector-distributor operations, weaving, service ramps, bus pullouts, climbing lanes and vehicle run away ramps, etc.
Number of lanes is required for all principal arterials, other NHS, and all standard samples. As a result, lane length can be computed for the Interstate, other principal arterials, and NHS directly. For minor arterials, rural major collectors, and urban collectors, the lane length is calculated using the standard sample length and number of through lanes and expanded from the sample to the functional system total. For the rural minor collector and the rural/urban local functional systems, the number of lanes is assumed to be two; lane length is the product of the functional system centerline length times two.
Highway vehicle travel
The FHWA uses daily vehicle-miles of travel (DVMT) [daily vehicle-kilometers of travel(DVKT)] as the primary measure of travel activity on the Nation's highway systems. The daily travel times 365 days (366 days for leap years) equals annual travel.
Travel is a calculated product of the annual average daily traffic (AADT) and the centerline length of the section for which the AADT is reported. In the HPMS, travel is accumulated for each universe section to develop appropriate totals for the higher functional systems. AADT is required for each section of Interstate, NHS, and other principal arterial; as a result, travel is computed for these functional systems on a 100-percent basis. For minor arterial, rural major collector and urban collector systems, travel is calculated from samples using the AADT, centerline length reported for each sample section and the HPMS sample expansion factor for each section. The DVMT is not adjusted for the functional systems where sample data are used; data are used as reported. Travel for the NHS is computed directly from the universe AADT data.
For the most part, travel for the rural minor collector and rural/urban local functional systems is calculated by the States using their own procedures and are provided in HPMS on a summary basis. Some States use supplemental traffic counts outside of the HPMS procedures; others employ estimating techniques, such as fuel use, to determine travel on these systems. In general, these methods are used in both rural and urban areas, including the donut areas of nonattainment areas to meet Clean Air Act requirements.
Travel estimates reported via the HPMS should be of reasonable quality particularly for the higher order functional systems. AADT and travel data are edited by the HPMS software for unusual values and for unusual changes to previously reported values. FHWA routinely works with State data providers to modify reported AADT values that do not appear to be reasonable before final use. Although AADT is required to be updated annually in HPMS, counts are only required to be updated on a 3-year cycle. For any reporting year, AADT for uncounted sections is to be derived by factoring the latest year's count for those sections. States that follow the HPMS sampling instructions in developing traffic counting programs (Appendix K in the HPMS Field Manual) and the standard practices advocated in the Traffic Monitoring Guide have adequate counting and classification tools to prepare quality AADT and travel estimates for HPMS. The consistency of the sampling and counting procedures should also provide comparable State-to-State traffic data.
Most States generally follow the recommended sampling, counting, and estimating procedures contained in the Traffic Monitoring Guide, although some State traffic count programs exceed the recommended 3-year cycle. The calculation and application of various adjustment factors to 24- or 48-hour coverage counts to enable them to represent AADT is as much art as science. Classification counts, which are needed to adjust pneumatic tube counts collected for three or more axle vehicles, are difficult to collect and to apply on a statewide basis. Equipment used to obtain count information is only accurate within certain limits and can suffer from malfunctions and breakdowns, factors which can affect the reliability of traffic counts. The user must recognize the shortcomings of the data collection and traffic estimation processes when using HPMS travel data. The degree to which recommended procedures are followed can impact the accuracy and consistency of the travel estimates in HPMS. Differences in State and local practices need to be taken into account when attempting to make State level comparisons.
The FHWA uses traffic volume-service flow ratio (V/SF) and hours of delay/1,000 VMT(1610 VKT) as the primary measures of congestion on the Nation's highway systems; FHWA also makes use of volume per lane (AADT/lane) for some purposes.
The V/SF is a computed numerical value based upon traffic volume information and roadway capacity calculated for each sampled section of roadway. The calculation of the peak hour capacity is done in accordance with an estimating process contained in the Highway Capacity Manual (HCM), which was developed by the Transportation Research Board. Delay is the estimated difference between actual travel speed and free flow travel speed; it is a modeled estimate, not a measured value. Both V/SF and delay use reported HPMS data.
HPMS data used for these estimates includes AADT, the design hour volume as represented by the 30th highest hour volume, the directional factor for all urban and rural multilane facilities, and the peak hour capacity of the roadway section. Additional HPMS data items relating to roadway characteristics such as geometry, speed, signalization, facility type, and facility location, among others, are also used.
Since V/SF and delay are products of complex estimating procedures, they are more susceptible to State-to-State and year-to-year variability than a measured congestion parameter such as measured travel time or AADT/ lane might be. The user of these data has a responsibility to assure that the application of V/SF, delay, or AADT/lane from HPMS is suitable to the purpose for which the congestion analysis is being conducted. Note that these values are indicators of recurring congestion only; they are not useful in evaluating non-recurring congestion.
Beginning with 1993, the major decrease in the percent of congested travel (chart "Urban Interstate System Congestion Trends") is an artifact of the change in capacity calculation procedures based on the Highway Capacity Manual. A revised HCM was issued in 1994 and subsequent calculations of capacity have been based on the revised procedures. The change in procedures was based on research that showed that drivers were willing to follow each other more closely and at higher speeds than previously. Although this change in driving habits occurred over a period of years, the change in procedure occurred abruptly. These trends have been back cast to 1993 using the 1994 HCM procedures for users convenience in comparing more recent congestion trends.
For the most part, the HPMS provides a generally uniform, consistent, statistically valid, and credible national level data base built from State-provided data. The HPMS is the most comprehensive and accurate data base available on the extent and performance of the Nation's highways. Users need to recognize that the HPMS data and the tables generated from the HPMS files in Highway Statistics must be used with full understanding of the data reporting and estimating processes and a recognition of their shortcomings. When making State-to-State comparisons, the user must be keenly aware of the differences that exist between the States being compared; it is inappropriate to use these statistics without recognizing, and accounting for, the major differences that may impact comparability.
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