This section contains data on the physical, operational, usage, extent, and performance characteristics of public roads in the United States as of December 31, 1997.
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 road facilities within each State. When a State does not provide current year data (or complete current year data), various means are used to present information that is reasonably current. This may include estimates based on State provided aggregate totals or other estimates based on prior year(s) data. Footnotes have been included where appropriate to explain missing data or data which have been estimated from previously available data. Readers are advised to read all the footnotes as well as this text prior to using data in this section.
ORGANIZATION OF TABLES
Almost all tables in this section contain rural and urban breakouts where urban may be further broken down 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 (1,000 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 LengthTables HM-10, HM-12, HM-16, HM-18, and HM-20.
2. Federal-Aid HighwaysTables 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 SystemTables HM-50, HM-51, HM-53, HM-55, HM-57, HM-59, HM-60, and HM-65.
4. Urbanized AreasTables HM-71 and HM-72.
5. State Highway Agency-Administered Public Roads Tables HM-80 and HM-81.
6. Highway Usage and PerformanceTables VM-1, VM-2, VM-3, VM-4, TC-3, HM-61, HM-63, HM-64, and FI series.
7. MetricTables HM-10M, HM-12M, HM-14M, HM-15M, HM-16M, HM-18M, HM-20M, HM-30M, HM-36M, HM-50M, HM-60M, VM-1M, VM-2M, VM-3M, and FI-1M.
All length tables in this publication, except table HM-30, include only centerline length classified as public roads in accordance with 23 Code of Federal Regulation 460. These include existing roads that are open to public travel and maintained by a public authority. Where a highway route comprises 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 generally not publicly maintained or private 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 jurisdiction. Table HM-12 contains length by type of surface and jurisdiction/functional system (national level only). Table HM-15 contains length of Federal-aid highways and is in the same format as its companion travel tableVM-3 and lane length tableHM-48. Table HM-16 is a nationwide aggregate table which contains length of Federal-aid highways by jurisdiction. Another nationwide aggregate 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 all 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 roads that are open to the public as well as those intended to be built within the next few years.
The increase in public road length by construction is relatively small each year. Most construction is for the improvement of existing public roads, such as paving, resurfacing those previously surfaced, widening facilities, adding lanes, reducing grades, minimizing curves, eliminating grade crossings, and other improvements that provide safer, more efficient public roads having greater traffic and load-bearing capacities. Most new length falls in the local functional system category that serves residential or business uses. Decreases in public road length do occur. In recent years, this has been due to the removal of some timber, forest access, and natural resource roads from general public use.
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 instrumentality thereof with public authority. Public roads in Federal forests and reservations may be part of the State and local jurisdictional systems and are included with the lengths reported for those systems. Length of public roads by jurisdiction are identified in tables HM-10, HM-12, HM-14, HM-16, and HM-50.
The designation of a public road as a Federal-aid highway does not alter its ownership or jurisdiction as a State or county road or city street. A Federal-aid highway simply means that, because of its service value and importance, it has been made eligible for Federal-aid construction and rehabilitation funds.
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.
The Federal-aid highways are basically segments of State and local systems length eligible for Federal aid. Except under special circumstances (i.e., connectors on the National Highway System and bridges), this excludes roads that are functionally classified as rural minor collector or rural and urban local.
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 National Highway System Designation Act of 1995 approved the NHS established under ISTEA. (For purposes of this publication, the NHS excludes any nonroadway routes, i.e., ferry boat routes.) The NHS was prepared in consultation with appropriate local officials and was submitted to and approved by the United States Congress. The NHS 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. Highways designated as part of the Interstate System are included in the NHS.
All 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 the FI series. 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. By law, the Interstate System is limited to 43,000 miles (69,202 kilometers) plus other incorporated routes that are logical additions or connections and meet Interstate System design standards. The Interstate System in Alaska and Puerto Rico is to be designed in accordance with geometric and construction standards as are adequate for current and probable future traffic demands and the needs of the locality of the highway.
The Interstate System 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
Within this section, 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. The categories for a given data item vary within each table to provide representation of the characteristics of the particular Federal-aid highway category. 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 jurisdiction, 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.
Count of structures
Table HM-65 contains a count of the bridges and other structures greater than or equal to 20 feet (32 meters) in length from the Structure Inventory of the Nation's Bridges by functional system.
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 characteristic type data and rates that are based on the State-reported mileage, travel, population, and net land area aggregate data for the same urbanized areas. An estimate of freeway lane length (including the Interstate System) in each urbanized area is also provided; it is based primarily on the 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 current urbanized area population.
State highway agency-administered public roads
State highway agency-administered (jurisdiction) public road data by functional system, based on the HPMS universe data, is contained in table HM-80. Lane length, daily vehicle travel, and annual average daily traffic per lane estimates are provided in the companion table HM-81. As with the HM-71 and HM-72 tables, the daily travel may be made to equal annual travel by multiplying by 365 (366 for leap years). These estimates are derived from the HPMS universe information.
HIGHWAY USAGE AND PERFORMANCE
Table VM-1 contains the estimated travel for passenger cars, motorcycles, buses, and trucks for the rural Interstate System, other rural arterial roads, other rural public roads, urban Interstate System, and other urban public roads. The rural and urban travel values are based on a summary of the highway functional systems contained in table VM-2. The vehicle types are based on a summary of vehicle distributions as shown in the table VM-4. These distributions are normalized to account for temporal variation. The other rural arterial roads category includes all other principal and minor arterials, excluding the Interstate System. The other rural roads category includes the collector and local functional systems. All urban systems, except the urban Interstate System, are included within the other urban public roads category.
Table VM-1 for each vehicle type contains the number of vehicles registered, total fuel consumption, and average fuel consumption. Also included by vehicle type are the calculated annual average distance traveled and average distance traveled per gallon. The highway use of motor fuel and the motor-vehicle registrations contained in tables MF-21 and MV-1 are used to obtain the related items of average distance traveled per vehicle, average fuel consumption per vehicle, and average distance traveled per gallon of fuel consumed. In addition, table VM-1 contains FHWA's estimate of distance traveled per person 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 contains a summary of the States' estimated highway travel based on traffic counts taken along selected highway sections which are grouped into functional systems, according to the character of service they provide. It is a companion to table HM-20.
Table VM-3 contains the estimated highway travel carried by Federal-aid highways. It is a companion to the length table HM-15 that reflects the approved NHS as indicated at the front of this section's text.
Table VM-4 presents a 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. Analysts are cautioned to refer to the individual table footnotes in using these data. Also, refer to the Traffic Monitoring Guide for vehicle descriptions and sampling guidelines.
Table TC-3, along with the graph, contains a percentage distribution of Interstate System traffic volumes and loadings historically. The table 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 graph, Comparison of Growth in Volumes and Loadings on the Rural Interstate System, portrays the rapid growth in loadings relative to volumes. The data points on the graph are 3-year moving averages.
Volume-service flow ratio
Table HM-61 contains the length for each rural and urban arterial and collector (except rural minor collector) functional system by State for various ranges of volume-service flow ratio. These data are expanded from the traffic volume and peak service flow (capacity, as defined by the 1994 Highway Capacity Manual) data reported on or calculated from other data in the HPMS sample sections, and are a useful measure of traffic congestion existing on the States' functionally classified facilities. A companion table, HM-42, is similar except the lengths are for the NHS.
Present serviceability rating (PSR)
Table HM-63 contains mileage that uses the PSR, as reported in HPMS, to group the data into various ranges for the rural major collector, urban minor arterial, and collector systems. The PSR is 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. Users of these data are advised to see the discussion of PSR in the section "General Limitations In Use of Data."
Measured pavement roughness
Table HM-64 contains ranges of lengths of measured pavement roughness for all principal arterials and rural minor arterials. A companion table, HM-47, is similar except that the roughness ranges of lengths are for the NHS. Measured pavement roughness is an objective equipment-based rating reported in the HPMS as the International Roughness Index (IRI) 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
The FI series contains the total fatal and nonfatal injury crashes along with the total number of fatalities, most seriously injured persons, total nonfatal injuries, and corresponding rates for all highways in each State. Fatality data are based on the 30-day definition; i.e., only crash victims who die within 30 days of their crashes are counted as fatalities. The tabulations contain numbers and rates [per 100 million vehicle-miles (vehicle-kilometers)] of crashes and casualties by functional system and Federal-aid highway categories.
Readers are provided several of the length and travel tables in metric. These tables are marked with an "M" at the end of the table number. A soft conversion from English to metric has been made of 1 mile = 1.609344 kilometers for the individual values; however, the totals are the sum of the individual converted values and 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 or transportation plans and programs, 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 usage of the Nation's highway systems. State reported HPMS data are reviewed by FHWA for completeness, consistency, and adherence to reporting guidelines. Where necessary, data are adjusted to improve completeness, consistency, and uniformity among the States. The review and adjustment process is accomplished in close working relationship with the data suppliers.
The HPMS data base is constructed primarily to facilitate national level analyses of the condition, performance, and usage 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 FHWA currently uses 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 measure, the International Roughness Index (IRI). Both may be reported in HPMS and are used as indicators of pavement condition, although only one is required and IRI is preferred. Pavement rating data are not reported for local or rural minor collector functional systems.
Present serviceability rating
The indices of 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 pavement condition 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 quality 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 is not reported. The data are subject to variability given the subjective nature of the evaluation process and, FHWA believes, may be more likely to be estimates. PSR data are not manipulated by FHWA; that is, they are reported by FHWA as they are input into the HPMS data base 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, needs and investments. IRI is converted to equivalent PSR values when PSR is not provided for HPMS sample sections.
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 reported to HPMS can arise from a number of sources. Some sources of variability include differences in the type of pavement surface being rated (i.e., concrete vs. bituminous), differences in the equipment used to measure IRI, and differences in the measurement protocols used. While the FHWA does not specify a particular type of equipment, it does specify a particular measurement protocolnamely that the measurement be provided for the right wheel track of the right most lane. Other guidelines are also provided; however, the FHWA is aware that not all States follow these protocols for all cases of IRI measurement, which diminishes to some extent the consistency of the reported data. Improvements to measurement equipment and instrument standardization for the most part rely upon Federal and State research activities such as those carried out by 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 when making any comparisons. IRI data are required for Interstate, other principal arterial, and rural minor arterial functional systems, although IRI is recommended for all functional systems other than the local or rural minor collector systems. The IRI data are collected on a universal basis for the NHS and principal arterial systems and on a sample basis for the rural minor arterial system. Sampled data are expanded using the sample expansion factors in HPMS to represent the complete functional systems for which they are reported.
The FHWA believes that the IRI data, which for the most part are available from State pavement management systems, are of reasonably good quality. IRI data are disseminated by FHWA without further 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, needs and investments. 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.
Finally, it should be noted that, to have a comprehensive assessment of pavement condition, additional measures of pavement distress 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.
Public Road Length
Beginning with this issue of Highway Statistics, the FHWA has instituted a new method for creating mileage based tables derived from HPMS data. Previously, adjustments to tables developed from sampled data were made using area wide mileage data provided by the States; these adjustments are now being made using the universe totals from the HPMS data set. In addition, rounding and expansion related differences were spread across table cells so that all table- to-table mileage totals on related tables matched precisely. While this cosmetic step makes 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 made a decision to discontinue the 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. FHWA is open to comments on this change from the user community; please direct any comments on this change to email@example.com.
The FHWA currently develops lane length as a primary measure of roadway area available for vehicular travel on the Nation's highway systems. 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, short truck climbing channels, and vehicle run away ramps, etc.
The lane length is a computed value that is a product of the centerline length and the number of through lanes. In the HPMS, the number of lanes is required to be reported for all principal arterials, other NHS, and all standard samples. In addition, some States report lane information for all minor arterials and collectors. As a result, lane length can be computed for the Interstate, other principal arterials, and NHS on a 100-percent basis.
For minor arterials, rural major collectors, and urban collectors, the lane length is calculated based on the standard sample sections using the reported number of through lanes, length of section, and expansion factor. Using the expanded sample, a check is made that the accumulated total expanded centerline length for the State's functional system matches the universe functional system length (Table HM-20). If not, a correction factor of the universe length to the deficient expanded total centerline length is applied to the applicable State's functional system lane length.
For the rural minor collectors and the rural/urban locals, the number of lanes is assumed to be two. Thus for these latter systems, the lane length is the product of the functional system centerline length times two.
Highway vehicle travel
The FHWA currently 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.
In concept, travel is a calculated value that is a 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 can be accumulated for each
universe section to develop appropriate totals for the higher functional
systems. AADT is required to be reported for each section of Interstate,
NHS, and other principal arterial; as a result, travel can be 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 will be adjusted
for the functional systems where sample data are used if the universe and
expanded sample length do not equal. 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 unknown methods. For these systems, travel values are developed by the States using their own procedures and are provided in HPMS (via areawide data). 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 for purposes of meeting EPA travel monitoring 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 incorporating them into a final master file. 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.
In practice, FHWA is aware that not all States rigorously follow the recommended sampling, counting, and estimating procedures contained in the Traffic Monitoring Guide. Reporting of AADT based on actual traffic counts, on all Interstate and principal arterials on a 3-year cycle, is a required but not necessarily followed protocol for HPMS reporting. 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 as well as for other HPMS items, 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. These differences in State and local practices need to be taken into account when attempting to make valid State level comparisons.
FHWA uses reported AADT to conduct congestion, needs, performance, and impact analyses and for modeling. Areawide travel reported through the HPMS as well as calculated universe and expanded sample travel for selected areas can also be used by the EPA to benchmark nonattainment urbanized area travel and to monitor growth of travel over time in nonattainment areas. This provides EPA another means to track whether or not air quality nonattainment areas are meeting highway travel reduction goals of State clean air implementation plans.
The FHWA currently uses traffic volume-service flow ratio (V/SF) as the primary measure of congestion on the Nation's highway systems; however, FHWA also makes use of volume per lane (AADT/lane) and hours of delay/1,000 VMT as congestion measures for some purposes.
The V/SF is a computed numerical value based upon traffic volume information and roadway capacity as calculated from HPMS data. It is one among many measures that has been and is being actively considered to measure congestion. As a measure of congestion, V/SF has many legitimate uses; however, other measures of congestion, such as AADT/lane, may be more or less suitable depending upon the purpose for which the measure is being used. The objective of this discussion is not to develop a treatise on the most appropriate measure of congestion; however, the user of these data has a responsibility to assure that the application of V/SF 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.
In general concept, the V/SF is calculated for each sampled section of roadway included in the HPMS. The calculation uses the 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. All of these values are reported in the HPMS, with the exception of capacity which is calculated by the HPMS software using procedures outlined in the 1994 Highway Capacity Manual (HCM). While traffic data are reported to the HPMS in a 100-percent basis for principal arterials and NHS (and on a sampled basis for the other regular sampled systems), the design hour volume and directional factors are available only on a sample basis. In addition, the calculation of peak hour capacity introduces additional complexity into the process of calculating V/SF. The calculation of the peak hour capacity is done in accordance with an estimating process contained in the HCM, which was developed by the Transportation Research Board. The estimating procedures use many additional HPMS data items relating to roadway characteristics such as geometry, speed, signalization, facility type, and facility location, among others. For the most part, the additional information needed to estimate peak hour capacity from the HPMS data is also sample based.
Since V/SF is the product of a complex estimating process, it is more susceptible to State-to-State variability than a measured congestion parameter such as travel time or AADT/ lane might be. Some areas of variability that should concern the user of V/SF as a congestion measure include the applicability of the generalized HCM procedures to the particular case under analysis and the completeness and accuracy of the various data elements used to compute the V/SF if State level comparisons are attempted. At a national level, the desire for consistency for planning purposes mitigates for the use of uniform procedures, as represented in the HCM. Since AADT/lane does not need to use calculated capacity values, it is less encumbered by these estimating shortcomings and may be a better choice for some purposes.
Note that, 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. FHWA is exploring the use of AADT/capacity as an additional measure of congestion.
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 this publication 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.