Section V
ROADWAY EXTENT, CHARACTERISTICS, AND PERFORMANCE
This section contains data on the physical,
operational, usage, extent, and performance characteristics of
public roads existing in the United States as of December 31, 1995.
IMPACTS OF DATA REPORTING PROCEDURES
The current data are based on the Highway Performance
Monitoring System (HPMS). The HPMS comprises 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); certain condition and use data
for all rural arterials and urban principal arterials; and system-type
data for all public road and street facilities within each State.
When a State fails to provide current year data (or complete current
year data), various means are used to prepare information that
is reasonably current. Where this is true, the States may have
provided current aggregate area-wide totals and the included tables
have used these data to the extent possible. Footnotes have been
included where appropriate to explain missing data or data which
have been estimated from previously available data.
ORGANIZATION OF MILEAGE AND TRAVEL 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 development 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
35 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 eight general areas:
EXISTING MILEAGE
All mileage tables in this publication, except
tables HM-30 and HM-45, include only mileage classified as public
road mileage in accordance with 23 United States Code 402. Table
HM-10 contains the total public road mileage of roads and streets
in the United States, classified by jurisdiction. Table HM-12
contains mileage by type of surface and jurisdiction/functional
system. Table HM15 contains mileage of Federal-aid highways
and is in the same format as its companion travel table-VM3.
Table HM-16 is a nationwide aggregate table which contains
mileage of Federal-aid highways by jurisdiction. Another nationwide
aggregate table, HM18, contains mileage of Federal-aid highways
by functional system. This table also includes small urban and
urbanized area summaries as well as total urban. Table HM20
contains all public road mileage by State and by functional system.
This table is in the same format as table VM-2 and contains the
mileage that corresponds to the travel contained in that table.
Tables HM-30 and HM-45 contain mileage that is intended to be
built within the next few years in addition to mileage that is
already open to the public travel.
The increase in total road and street mileage
by construction is relatively small each year. Most construction
is for the improvement of existing highways and streets, such
as resurfacing those previously surfaced, widening facilities,
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 mileage falls in the local functional system category that
serves residential or business uses. Decreases in public road
mileage 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.
Roads in Federal forests and reservations may
be part of the State and local jurisdictional systems and are
included with the mileages reported for those systems. Mileage
directly under Federal control is identified in tables HM10,
HM12, HM14, HM16, and HM50.
The designation of a road or street 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.
Surface types
The surface classification used in the tables
identifies only the visible surface types on existing streets
and roadways. 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 of compacted bituminous material.
FEDERAL-AID HIGHWAY MILEAGE
Except for minor amounts of Federal highway
mileage under the U.S. Forest Service, Bureau of Land Management,
Bureau of Indian Affairs, National Park Service, Fish and Wildlife
Service, and military, practically all of the roads and streets
in the United States are under the jurisdiction of State and local
governments. The Federal-aid highways are, basically, segments
of State and local systems mileage eligible for Federal aid. Except
under special circumstances, 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. 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 HM14,
HM15, HM16, HM18, the HM30 series, HM-42,
HM-47, HM-48, VM3, and the FI series.
Interstate System
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 the support of Federal-aid highway activities. By law, the
Interstate System is limited to 42,500 miles under Section 103
of Title 23 and other routes incorporated under Section 139(a)
of Title 23 that are logical additions or connections and meet
Interstate System design standards. The Interstate System under
Section 139(c) is limited only to Alaska and Puerto Rico. Highways
may also be designated as part of the Interstate System under
provisions of Section 1105(e) of ISTEA as amended under Section
332 of the National Highway System Designation Act of 1995.
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.
An estimate of future additions to the Interstate
System are shown in table HM-45. These additions reported include
segments applicable under Sections 103 or 139(b) or Section 332
of the National Highway System Designation Act of 1995.
Mileage characteristics
The Federal-aid highway mileages are classified
according to system, surface type, lane width, traffic lanes,
access control, and traffic volume in tables HM-31 through HM39.
The categories for a given data item vary within each table to
provide representation of the characteristics of the particular
Federalaid highway category. Tables HM-30, HM42,
and HM47 clasify mileage of NHS according to open and not
open to traffic, volume-service flow ratio, and measured pavement
roughness.
MILEAGE BY FUNCTIONAL SYSTEM
The functional systems used in tables HM50
through HM80 result from the assignment of streets and highways
into groups according to the character of service they are intended
to provide. Because most travel involves movement through a network
of roads, individual roads and streets do not serve travel independently
in any major way. Functional classification defines the role that
a particular road or street 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 and streets, which
serve the land access function to the residential areas, businesses,
individual farms and ranches, and other local areas. The functional
system mileages are identified by jurisdiction, surface type,
lane width, traffic lanes, access control, and traffic volume
in tables HM-50 through HM59.
Lane mileage
Table HM-60 contains estimated rural and urban
lane mileage by functional system. FHWA assumes two lanes for
the rural minor collector and urban and rural local functional
system lane mileage estimate. A companion table, HM-48, is similar
except the lane mileage is for the NHS.
Volume-service flow ratio
Table HM-61 contains the mileage 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 mileages 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 and the 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 measured pavement roughness
for the higher systems that include all principal arterials and
rural minor arterials. A companion table, HM-47, is similar except
that the IRI mileage ranges 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. Users of these data are advised to see the discussion of
IRI in the section "General Limitations In Use of Data."
Count of bridges
Table HM-65, Highway Bridges Greater Than or Equal to 20 Feet, contains a count of the bridges from the Structure Inventory of the Nation's Bridges by functional system for rural and urban areas. A companion table, HM41, is similar except that the bridge counts are for Federal-aid systems; this table has been discontinued temporarily.
Minor collector and local functional
systems
Table HM-67 contains mileage estimates of minor
collector and local functional systems by groupings of average
daily traffic volume and surface type for rural and urban areas.
Estimates are supplied by the States in the areawide (aggregate)
portion of the HPMS data submittals.
Urbanized area mileage, travel, and other
characteristics
Table HM-71 contains mileage 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)
would equal annual travel. Table HM72 contains rate and
characteristic type data 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 mileage
(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 mileage
State highway agency-administered (jurisdiction)
mileage by functional system, based on the HPMS universe data,
is contained in table HM80. Lane mileage, daily vehicle-miles
traveled, and annual average daily traffic per lane estimates
are provided in the companion table HM81. As with the HM71
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 on the HPMS universe information.
HIGHWAY USAGE CHARACTERISTICS
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 roads, urban Interstate
System, and other urban streets. The rural and urban cate-gories
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 series. 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 streets 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 Study,
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
mileage table HM15 that reflects the approved NHS as
indicated at the front of this section's text.
The VM-4 series first appeared in the 1993
edition of Highway Statistics. It describes distribution
of travel activity by vehicle type by 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.
The FI series contains the total fatal and
nonfatal injury accidents along with the total numbers 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 accident victims who die
within 30 days of their accidents are counted as fatalities. The
tabulations contain numbers and rates (per 100 million vehicle
miles) of accidents and casualties on the functional systems and
Federal-aid highway categories.
Table TC-3, along with the graph, Comparison
of Growth in Volumes and Loadings on the Rural Interstate System,
displays the interrelationship among vehicle types, volumes, and
axle loadings. The data, which are from the Truck Weight Study,
are collected by the States for varying periods of time and are
not adjusted to typify annual system averages. Note that the data
for passenger cars, buses, and light single-unit trucks are combined.
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 3year moving averages. It should be noted that
the increase in loadings is a function of increased axle weights
as well as an increase in the number of trucks.
METRIC TABLES
Readers are provided several of the mileage
and travel tables in metric. These tables are marked with an "M"
at the end of the title 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.
GENERAL LIMITATIONS IN USE OF DATA
Readers are advised to refer to the previous
paragraphs as well as all footnotes contained on each table prior
to using the data in this section.
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 databases 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 States supplying the data.
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 major data subject areas are noted below.
Pavement ratings
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 are reported in HPMS and are used as indicators of pavement condition.
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, State-to-State 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 for functional systems
where IRI is not completely reported or is not required to be
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. The data are not processed or manipulated
by FHWA; that is, they are reported by FHWA as they are input
into the HPMS data base by the States. Where IRI data are not
reported, the PSR data are collected on a universal basis for
the principal arterial systems and on a sample basis for the remaining
sampled functional systems. PSR data are not collected for local
or rural minor collector functional systems. Sampled data are
expanded using the sample expansion factors in HPMS to represent
the complete 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.
Measured pavement roughness
Measured pavement roughness is an objective equipment-based rating
reported in the HPMS as IRI in inches (meters) per mile (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 protocol-namely 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 protocol 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.
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 conditions, additional measures
of pavement distress such as rutting, cracking, and faulting are
needed. Consistent protocols for these measures are now being
developed and States will be encouraged to include them in their
pavement management systems.
Highway vehicle travel
FHWA currently uses daily vehicle-miles of
travel (DVMT) as a the primary measure of travel activity on the
Nation's highway systems.
In concept, DVMT is a calculated value that
is a product of the annual average daily traffic (AADT) and the
length of the section for which the AADT is reported. In the HPMS,
DVMT is accumulated for each reported section to develop appropriate
totals. AADT is required to be reported for each section of Interstate,
NHS, and other principal arterials; as a result, DVMT is computed
for these functional systems on a 100-percent basis. For other
functional systems down through the rural major collector and
urban collector systems, DVMT is calculated from sample sections
using the AADT and length reported for each sample section. DVMT
for each of these latter functional systems is developed by expanding
the sample using HPMS sample expansion factors. For the most part,
DVMT for the rural minor collector and rural and urban local functional
systems is calculated by the States using unknown methods. For
these systems, DVMT values are provided in HPMS by the States
using their own procedures. 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 rural as well in urban areas,
including the donut areas of nonattainment areas for purposes
of meeting EPA travel monitoring requirements.
DVMT estimates reported to the HPMS should be of reasonable quality
particularly for the higher order functional systems. AADT and
DVMT 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 that follow the standard practices advocated in the
Traffic Monitoring Guide, have adequate counting and classification
tools to prepare quality AADT and DVMT 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 AADT or 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
of AADT and travel data.
DVMT for eligible segments is calculated and used by FHWA as an
apportionment factor for Interstate Maintenance funds. In addition,
FHWA uses reported AADT to conduct congestion, needs, performance,
and impact analyses and for modeling. Areawide DVMT reported through
the HPMS as well as calculated universe and expanded sample DVMT
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 VMT
reduction goals of State clean air implementation plans. DVMT
is annualized and reported as VMT in Highway Statistics
and most other dissemination media.
Congestion
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) as
a congestion measure for some purposes.
The V/SF is a computed numerical value based upon traffic volume
information and roadway capacity estimates as reported by the
States to the HPMS. 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 calculated V/SF is then expanded
using the sample expansion factors in the HPMS to represent each
entire functional system. The calculation uses the AADT, the design
hour volume as represented by the 30th highest hour volume, the
directional factor, 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 most current version (1994) of
the Highway Capacity Manual (HCM). While traffic data are
reported to the HPMS on 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. In addition, at the national level, anomalies in input
data item values have a tendency to even out and to represent
aggregate average values when summed across all States as a whole.
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 the major decrease in the1995 percent
of congested travel that appears in this publication (Page V-67)
is an artifact of the change in capacity calculation procedures
based on the Highway Capacity Manual (HCM). 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.
Of course, this change in driving habits occurred over a period
of years, but the change in procedure occurred abruptly. This
causes the break in the trend that invalidates comparisons between
1995 measures of capacity and previous years.
Summary
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 serves its intended Federal
purposes well, and it is the most comprehensive and accurate database
available on the extent and performance of the Nation's highways.
It meets FHWA's responsibilities for collecting and disseminating
the most reasonable transportation data available. Users of HPMS
data need to recognize that these data must be used with full
understanding of the data reporting and estimating processes and
a recognition of their shortcomings. The data are not perfect.
When making State-to-State comparisons, the user must be keenly
aware of the differences that exist between the States being compared.
Users 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, type of terrain and extent of highway grades and
curves, climate, etc. When making State level comparisons, therefore,
it is inappropriate to use these statistics without recognizing,
and accounting for, the major differences that may impact comparability.