APPENDIX F

Traffic Monitoring Procedures for the HPMS

## Introduction {#introduction .HPMS-Heading-3}

High quality traffic data are critical to the many uses of the HPMS data
set:

-   Traffic derived travel data from the HPMS are used in the
    Federal-aid Highway fund apportionment formulae;

-   Traffic data are critical to the analyses that support the Condition
    and Performance Reports to Congress which are subsequently used for
    national highway budgeting purposes;

-   HPMS derived travel data are required to meet Clean Air Act
    requirements; and

-   Travel data are central to estimating several of the Department's
    performance indicators such as vehicle crash and fatality rates and
    delay.

Traffic monitoring is also a key activity in the development and
maintenance of the HPMS data set. Traffic data drives the HPMS sample
stratification and selection process; the validity of the entire HPMS
sample expansion procedure depends on the maintenance of adequate
traffic monitoring programs.

## General {#general .HPMS-Heading-3}

State maintenance of an adequate traffic counting program is a primary
HPMS concern. A State's count program should cover all Interstate,
principal arterial, other NHS, and HPMS sample sections on a 3-year
maximum cycle; at least one-third should be counted each year. It should
cover all roads, not just State-owned roads, and should include counts
on those systems made on the State's behalf by MPOs, cities, or
counties. The program should provide for a sufficient number of
automatic traffic recorder (ATR) stations and classification count
stations to permit factoring of 48-hour tube-type counts to estimates of
annual average daily traffic (AADT). The State should also have an
underlying short count program that assures that traffic counts on all
roads on all systems are obtained over a longer term (6-year maximum)
cycle for basic traffic monitoring purposes. For further details, see
the discussion of Data Obsolescence Counts in Chapter 2 of the *AASHTO
Guidelines for Traffic Data Programs*. The information provided by the
State's overall count program should be of sufficient detail to permit
the State to develop and maintain comprehensive traffic flow maps in
rural and urban/urbanized areas.

Traffic information in a comprehensive count program may be available
from several monitoring sources including State, MPO, city, or county.
Automatic traffic recorders provide continuous monitoring of existing
traffic conditions around the State. Vehicles on freeways/expressways
and other important multi-lane facilities can be monitored by route
under a multi-year statewide or MPO count plan that includes input from
real-time intelligent transportation system (ITS) deployments. Other
highway functional systems, both State and off-State, can be monitored
by geographic area, such as by county or highway district.

A schedule should be developed for the State's comprehensive count
program. Areas of the State selected for counting in a program year
should be selected on a random basis. Areas of high growth should be
counted more often than those with low growth; although all areas should
be comprehensively counted at least every 6 years. To make the most of
available resources, an area traffic count plan should consider using
cluster count techniques. Counts scheduled and obtained under other
programs should be incorporated into the count plan to avoid duplication
of monitoring sites. Coordination and cooperation with local governments
to implement a shared data, comprehensive count program is highly
desirable; however, the State ultimately maintains responsibility for
ensuring that these data meet minimum collection and quality
requirements. To meet these responsibilities, the State should have a
comprehensive quality assurance program that includes data collection,
the conversion of traffic counts into AADT values, and equipment testing
provisions.

Information collected from all monitoring programs should be maintained
in a computerized database, preferably in a geographic information
system. If several years of data are maintained, traffic growth by
location will be more readily available for developing trends,
determining changes to volume group breakpoints, scheduling of future
monitoring, and other uses. Note that although some planning and design
activities, such as urban planning models, may make use of average
weekday traffic (AWT) estimates, the HPMS makes use of AADT estimates.

## Volume Group Assignments {#volume-group-assignments .HPMS-Heading-3}

The State's comprehensive traffic count program can be used to develop
traffic volume group assignments for all road sections if the program
has been formulated to adequately monitor both high and low volume
roads, including those off the State system. To facilitate this process,
count station locations should be selected to represent expected AADT
volume group breakpoints for the volume ranges of both the standard and
nonattainment area samples. This may require locating count stations at
one per 5 to 10 miles in rural areas and more closely in urban areas;
for homogeneous traffic sections, more than one section may be
represented by a single traffic count station. Selection of count
station locations should be based on previous count experience, recent
land developments, and the existence of uncounted sections along the
routes.

Generally, traffic counts in addition to those taken for the HPMS are
needed to establish the assignment of road sections to their respective
volume groups in both rural and urban areas. A well-designed
comprehensive count program that includes off-State system roads should
provide the needed additional counts. Traffic mapping techniques also
can be applied to maximize the use of HPMS universe and sample counts,
other coverage counts, and counts taken for project planning or
operational purposes in assigning road sections to volume groups.

## Traffic Monitoring Guide {#traffic-monitoring-guide .HPMS-Heading-3}

A detailed discussion of recommended procedures for developing reliable
estimates of travel characteristics, including AADT, is contained in the
*Traffic Monitoring Guide* (TMG). However, a general discussion of some
elements of a typical traffic volume count program and their
applicability to the HPMS follow.

Continuous ATRs

Automatic Traffic Recorders (ATRs) are used to provide continuous
traffic count coverage at selected locations. ATR data are also used to
develop seasonal or monthly, day-of-week, and growth factors which are
then used to adjust short coverage counts to AADT. Analytical procedures
to determine the appropriate level of effort and to develop the needed
traffic estimates are described in the TMG.

Continuous count data are essential for converting coverage counts to
AADT. The State's documentation of its continuous count program should
discretely identify the number of continuous counters on the rural and
urban portions of the PAS/NHS system. Whenever possible, the State
should have at least one continuous counter on each major PAS/NHS
highway route. At a minimum, each continuous counter should have at
least two full days of data for each day of the week for each month.

Short Counts

Although short counts can cover lesser periods, the TMG recommends using
48-hour counts (two full 24-hour days) for all HPMS universe, standard
sample, and donut area sample sections whether on or off the State-owned
highway system. Short counts should be randomly scheduled geographically
throughout the State and temporally throughout the calendar year to
ensure adequate representation and to minimize bias. Where axle
correction factors are needed to adjust raw counts, they should be
derived from facility specific vehicle classification data obtained on
the same route or on a similar route with similar traffic in the same
area. Factors that purport to account for suspected machine error in
high traffic volume situations should not be applied to HPMS traffic
counts or traffic count programs used for HPMS purposes, such as volume
group assignment. In high volume situations, such as controlled access
facilities, it is more appropriate to use ramp counts in conjunction
with strategic mainline counting, or other appropriate technology, than
to use short counts and adjustment factors.

Vehicle Classification Data Collection

Data reported in the HPMS should reflect the use of statistically valid
data collection procedures employing automatic vehicle classification
equipment. Summary vehicle classification data reporting requirements
are outlined in Chapter III; percent trucks data are reported in Items
81-84 for each HPMS standard sample section (see Chapter IV). Axle
corrections based on vehicle classification data should be applied to
all counts where the counting device uses axle sensors. State
documentation of the vehicle classification activity should illustrate
that:

a.  Classification data are representative of specific functional
    systems.

b.  Each season of the year is represented in the development of axle
    correction factors.

c.  Classification sessions are long enough to account for the changes
    in vehicle mix from day to day. The TMG recommends that vehicle
    classification sessions be at least 48-hours. Data for less than 24
    continuous hours is not appropriate.

d.  The total volume of vehicles observed is at least equal to that for
    an average day.

e.  Classification counts are well distributed among rural and urban
    locations.

f.  Classification counts are collected, at a minimum, over a 3-year
    cycle, one-third of the counts per year.

g.  There are sufficient classification categories to represent vehicles
    with two to seven axles.

## Application of Traffic Count Procedures to HPMS {#application-of-traffic-count-procedures-to-hpms .HPMS-Heading-3}

Traffic count data reported for all Interstate, other principal
arterial, and other NHS sections provide most of the travel data used
for apportionment and other purposes (see Chapter 1, Table I-2). The
HPMS standard sample design provides an appropriate statistical base for
the development of traffic estimates for each sampled section and of
systemwide travel for the sampled systems. By incorporating vehicle
classification and truck weight information, the TMG structure provides
the capability of estimating classified VMT and Equivalent Single Axle
Loads (ESALs) from the HPMS data.

One-third of the Interstate, other principal arterial, and NHS road
sections should be counted each year. In addition, one-third of the HPMS
standard sample sections on each functional system should be counted
each year. The sections to be counted should be randomly selected.
Samples should be selected from each sample stratum (volume group),
although minor adjustments may be necessary for strata with numbers of
sections not divisible by three or having less than three samples. A
single count may be used for several sections between adjacent
interchanges on controlled access facilities.

The development of section AADT estimates from count data must include
the use of short count and other appropriate adjustment factors. AADTs
reported to the HPMS for standard sample and non-sample PAS/NHS sections
not counted during the current year must be updated to current AADT
estimates by use of appropriate growth factors.

Estimates of Daily Vehicle-Miles of Travel (DVMT) can be developed by
direct computation for the Interstate, other principal arterials, and
other NHS sections and by expansion of the HPMS standard sample on a
functional system basis for other systems. This is done by multiplying
the standard sample section AADT by the section length and by the
standard sample expansion factor and summing the result to the HPMS
stratification level desired (functional system, total rural, etc.); the
HPMS software will perform these calculations by functional system.
Since HPMS standard sample expansion procedures are based on the ratio
of universe to sample mileage, mileage totals at any stratification
level should be exact. A comprehensive count program, good count
practices, a well-distributed HPMS standard sample, and appropriate AADT
estimation techniques will result in highly reliable DVMT estimates.

The same procedures can be used in preparing and reporting count based
AADT data for donut area sample sections; a unique expansion factor for
each applicable NAAQS nonattainment area is used to prepare DVMT
estimates.