|Highway Statistics 2001||OHPI > Highway Statistics > 2001 > Roadway Extent|
This section contains data on the physical, operational, use, extent, and performance characteristics of public roads in the United States. Public roads include those that are open to public travel and maintained by a public authority (see Code of Federal Regulations (23 CFR 460)). Other roads and trails that are privately owned or not publicly maintained or restricted from use by the general public are excluded.
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 level data for all public roads within each State. When a State does not provide complete current year data, FHWA works with the State to develop suitable estimates of missing data; as a result of non-reporting, however, some tables may contain blank cells. Data anomalies are explained in footnotes to each table; readers should consult all footnotes as well as this text prior to using data in this section.
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 population (persons per square mile). The adjusted 1990 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). Adjusted boundaries are set by State and local officials with the approval of the Federal Highway Administration.
The tables in this section have been organized into nine general areas:
All length tables in this section, except table HM-30, reflect the centerline length of public roads. 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 State, county, town, township, Indian tribe, municipal or other local governments, or instrumentalities thereof. 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 is identified in tables HM-10, HM-12, HM-14, HM-16, and HM-50. Table HM-10 contains total public road length in the United States; table HM-15 contains the Federal-aid highway length; table HM-40 contains NHS length, and table HM-50 contains length by functional system. Table HM-30 contains length of NHS roads that are open to traffic as well as those planned but not yet built.
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 (NHS), roads that are functionally classified as rural minor collector or rural or urban local are excluded. 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 Dwight D. 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 several 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.
The Intermodal Surface Transportation Efficiency Act (ISTEA) of 1991 eliminated the historic Federal-aid systems and created the National Highway System and other Federal-aid highway categories. The NHS was selected in consultation with appropriate State and 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. Non-roadway NHS routes, such as ferry boats, are not included in Highway Statistics.
Historic 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 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 roads; and (3) local roads, streets, and other public ways, which serve a land access function to homes, businesses, individual farms and ranches, and other uses. The length of road by functional system is 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 through traffic lanes for the lane length estimate for rural minor collector and rural/urban local functional systems. Number of through traffic lanes as reported by the States is used to estimate lane length for all other functional systems. Tables HM-43 and HM-48 contain lane length for the NHS and Federal-aid highways.
The surface classification used in the Tables HM-31 and HM-51 identifies the pavement type used 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, although material and layer thickness criteria are used to define the reporting categories.
Table HM-63 shows a distribution of length by Present Serviceability Rating (PSR) and International Roughness Index (IRI) values for the rural major collector, and the urban minor arterial and collector functional systems. A mix of measurement methods is used for these systems.
Table HM-64 shows a distribution of length by IRI value for the principal arterial and rural minor arterial functional systems. A companion table, HM-47, provides similar information for the NHS.
PSR is a subjective measure of surface pavement condition based on an assessment of ride and visual pavement distress by a rating panel.
IRI is an objective, instrument-based measure of pavement roughness, reported in the HPMS in inches per mile (meters per kilometer).
Users of these data are advised to see the discussion of PSR and IRI in the section "General Limitations in Use of Data."
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. Table HM-42 provides similar information for the NHS.
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 selected urbanized area characteristics and rates based on State-reported length, 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 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.
Table VM-1 is a summary of estimated travel by vehicle type and system; travel values are derived from the highway functional system data contained in table VM-2, while the vehicle types are derived from the categories shown in the Traffic Monitoring Guide. 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 consumed 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. Estimated travel on the NHS is shown on table HM-44.
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.
Tables FI-10 and FI-20 show highway fatalities by Federal-aid Highways and functional system. 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. All metric tables are available in electronic format only.
Information included in the HPMS database 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 databases, including those of Metropolitan Planning Organizations (MPOs). Typically, the existing databases and record keeping systems of State and local governmental units are designed and maintained to meet their specific, individual business needs; most items in their databases are not specifically designed to meet Federal reporting purposes. Some data required for the HPMS are not collected by all the States for their own databases; some State adjustments to State collected data 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 databases 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 that 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.
HPMS data are collected in accordance with the Highway Performance Monitoring System Field Manual. 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 database. 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 database 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.
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 judgment of pavement condition based upon an assessment of ride and visual pavement distress 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 database 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 where IRI data are not available.
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 similar 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; 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 by AASHTO and the Road Profiler Users 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.
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.
In 1997, FHWA instituted a new method for creating length-based tables. Previously, adjustments to tables developed from sampled HPMS data were made using area-wide length 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 length 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 adjustment and spreading process, and users may note minor differences in table-to-table length totals. For record purposes, FHWA considers the length 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. The FHWA uses lane length as a primary measure of road supply. The number of through lanes is 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.
Starting in 1999, number of lanes was required for all functional systems except the rural minor collector and the rural/urban local systems, and for all NHS routes; lane length is computed on a 100-percent basis. For the rural minor collector and the rural/urban local functional systems, the number of through lanes is assumed to be two; lane length is the product of the functional system centerline length times two.
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. Travel for the NHS on all functional systems is computed 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 is 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 designated air quality non attainment areas to meet traffic monitoring requirements of the Clean Air Act.
AADT and travel estimates are edited by the HPMS software and reviewed by FHWA 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 required to be taken only on a 3-year cycle. Usually, AADT for uncounted sections is estimated by factoring the latest year's count. States that follow the HPMS sampling instructions in developing traffic counting programs (Appendix F in the HPMS Field Manual) and the practices advocated in the Traffic Monitoring Guide have adequate counting and classification tools to prepare quality AADT and travel estimates for HPMS. These 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 that 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 a traffic volume-service flow ratio (V/SF) as a measure of congestion in Highway Statistics. 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 capacity is done in general accordance with estimating procedures contained in the latest Highway Capacity Manual (HCM), which was developed by the Transportation Research Board.
Since V/SF is a product of complex estimating procedures, it is 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. As a measure of congestion, the V/SF metric does not adequately reflect the effects of peak spreading. Also, changes to the HCM calculation procedures used in the estimate of service flow mask the actual congestion experienced by the highway user. (HCM procedures were changed in 1994, 1998, and 2000.) The user of these data has a responsibility to assure that the application of V/SF from the HPMS is suitable to the purpose for which the congestion analysis is being conducted.
The HPMS provides a generally uniform, consistent, statistically valid, and credible national level database built from State-provided data. The HPMS is the most comprehensive and accurate database available on the extent and performance of the Nation's highways. 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 recognition of their strengths and weaknesses. When making State-to-State comparisons, the user must be 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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