Chapter VII

Sample Selection and Maintenance

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

The purpose of this Chapter is to address a number of subjects related
to the selection and maintenance of the HPMS sample and sample panels.
The data reported for sampled roadway sections are the source of the
condition, use, and operational information provided by the HPMS.
Expanded sample data are used for apportionment of funds, for monitoring
trends and impacts in performance data over time, and for analyses in
support of national budgeting for highway improvements through the
*Condition and Performance* reports to Congress. Selection and
maintenance of an adequate, up-to-date HPMS sample should be a high
priority, continuing activity.

Although developed for the HPMS standard sample, most of the following
discussion applies equally for donut area supplementary sample panels;
Appendix G contains additional discussion of the donut area sample
panel.

## Sample Panel Considerations {#sample-panel-considerations .HPMS-Heading-3}

The HPMS sample includes the arterial and collector functional systems,
excluding the rural minor collector system. While it is assumed that
there is a "technically best" way to collect sample data, a sample
design also must be simple and cost efficient to meet manpower and cost
considerations; this involves tradeoffs. The required number of samples
for HPMS are derived by formula from the normal dispersion
characteristics of AADT values within a framework of preselected AADT
groups (strata). The sample size requirements relate to the critical
data element, AADT, whose values can be conveniently stratified.
Information obtained from the existing sample or universe data in each
State is used to optimize and maintain the sample panel.

Procedures for determining necessary sample size based on the analysis
of existing data are described in this chapter. In order to obtain
cost-effective, valid comparisons of system performance over time and to
reduce technical effort, the sample was designed as a fixed sample
panel. With a fixed sample panel, the same sections that are inventoried
are then updated in future years on a cyclical basis. This means of
obtaining data is efficient because:

> 1\. The need for the periodic drawing of a complete new sample is
> eliminated.
>
> 2\. The need to update or reinventory all data elements for every
> cycle is eliminated.
>
> 3\. Only those data elements that change over time need be updated on
> a cyclical basis.

The length of the cycle is determined by the known statistical
characteristics of individual elements, the intended use and accuracy
needed, and the time and cost required to collect and report the data.

However, the use of fixed panel sections is not without disadvantages.
These include:

-   the possible loss of the sample\'s representativeness as the highway
    networks and traffic patterns change, and

-   the inability to assess the correctness of the estimates by
    comparing them with those of a different sample.

Procedures have been developed to ensure the representativeness of the
sample and FHWA has determined that the practicability of the fixed
panel approach outweighs the disadvantages. HPMS sampling procedures are
both simple and efficient and, if applied properly, the selected sample
will achieve the predetermined levels of desired precision and yield an
adequate sample for performance monitoring.

## Sample Stratification and Precision Levels {#sample-stratification-and-precision-levels .HPMS-Heading-3}

Data needs vary for rural, small urban, and urbanized areas; this
variation is reflected in the sample design. The design is capable of
producing valid estimates of highway condition, operational, and
performance characteristics on a State-by-State basis. Rural and small
urban functional systems are sampled on a statewide basis; all
Federal-aid urbanized areas must be sampled individually.

The sampling plan consists of the random selection of a panel of road
sections within predetermined AADT volume groups (strata) for each
functional system in the rural, small urban, and urbanized areas of the
State. The stratification of sections (sampling units) into relatively
homogeneous AADT volume groups produces estimates of greater accuracy
with respect to VMT for a smaller number of samples at the functional
system (summation) levels. Although stratification for sample selection
is based on the critical data element AADT, tests have shown that AADT
stratification is compatible with the sampling of nonvolume related data
elements.

The required sample size is a function of the variability of AADT within
a volume group, the functional system volume group precision level, and
the number of sections available for sampling in the volume group (the
universe). The term "precision level" is defined as the degree of
confidence that the sampling error of a produced estimate will fall
within a desired fixed range. For a precision level of 80‑percent
confidence with 10-percent allowable error (80-10), there is the
probability that 80 times out of 100 the error of a data element
estimate will be no greater than [+]{.underline}10 percent of its true
value. The precision levels specified in Appendix C represent minimum
FHWA requirements for rural, small urban, and urbanized area functional
system volume groups. The precision levels determined for the sample
design apply specifically to the individual volume strata. The sample
size estimating procedures shown in Appendix D are used to determine the
required number of samples to meet the target precision levels at the
volume group level. The sample adequacy software described in Appendix K
produces a table that contains a standard sample size estimation based
on Appendix D criteria.

Sample size requirements by functional system will vary by State
according to the total number of road sections, the number of
predetermined volume groups, the validity of the State\'s AADT data, and
the design precision level. The HPMS sample size requirements are more
stringent for the arterial systems, where a higher level of precision is
needed because of higher Federal interest. In rural and small urban
areas, the sample sizes are based on a 90-5 precision level for the
volume groups of the principal arterial system, 90-10 for the minor
arterial system, and 80-10 for the collector (excluding minor collector)
systems. The sample for individually sampled urbanized areas is broken
into two major categories of precision levels:

> 1\. For individual urbanized areas with a population of 200,000 or
> more and those that are in an NAAQS nonattainment or maintenance area,
> the design precision levels are 90-10 for the arterial systems and
> 80-10 for the collector system.

2\. For other individual urbanized areas with a population of less than
200,000, the design precision levels for individual volume strata are
80-10.

The statewide summation of individual urbanized functional system data
element estimates will result in an overall precision level of at least
80-10.

The higher precision levels at the State level are necessary to obtain
comparable urban and rural precision levels and to obtain precision
levels that can adequately accommodate desired levels of accuracy for
estimates of proportionate values as well as average and aggregate
values. That is, although the HPMS sample is designed to measure AADT,
the same samples are used to estimate the proportionate values of data
such as pavement condition. Since the level of accuracy for estimated
proportions is closely related to sample size, precision levels have
been set sufficiently high to produce reasonable proportionate estimates
at the functional system level.

## Preparation for Sample Selection {#preparation-for-sample-selection .HPMS-Heading-3}

Before a sample can be drawn, the universe from which it is selected
must be defined. This is very important since expansion factors, and the
reliability of the expanded sample data, relate directly to the universe
definition. Initial steps include:

-   First, delimit the boundaries between rural, small urban, and
    urbanized areas using FHWA-approved, adjusted urban boundaries.

-   Next, identify the functional system of all arterial and collector
    routes within each of these areas.

-   Then, break the arterial and collector routes into logical roadway
    sections.

-   Finally, assign all road sections in these functional systems to the
    predetermined AADT groups shown in Appendix C.

An AADT volume group assignment is required for all roadway sections on
functional systems subject to sampling (all but rural minor collector
and rural and urban local). Assigning sections to proper volume groups
and maintaining proper volume group assignments is an important step.
Because of economic growth and development, AADT growth may require
periodic adjustments to volume group assignments over time. The HPMS
software will assign a volume group for each section where an AADT is
provided. Maintaining accurate volume groups requires States and other
data providers to maintain comprehensive, high quality, traffic count
programs (see Appendix F).

If volume groups other than the predetermined volume groups shown in
Appendix C are selected, the AADT limits of these volume groups must be
reported to FHWA, and the State will need to assign each section to the
appropriate volume group.

Each HPMS section should be relatively homogeneous as to geometrics,
traffic volume, cross section, and condition, and should be long enough
to constitute a logical section for various analyses such as determining
highway investment requirements. In general:

-   Rural section lengths should range from 0.5 to 16.1 kilometers (0.3
    to 10.0 miles)

-   Urban access controlled facility section lengths should usually not
    exceed 8.0 kilometers (5.0 miles)

-   All other urban section lengths should range from 0.2 to
    4.8 kilometers (0.1 to 3.0 miles).

These suggested lengths are intended to keep the sample normalized on a
national basis. Shorter lengths may be warranted where there are
nonhomogeneous roadway elements; longer sections reduce the number of
universe sections and result in a somewhat smaller number of initial
samples. However, longer sections may have to be split in later years in
order to maintain sample homogeneity; this will increase the number of
universe sections and may result in an increase in the required number
of samples.

Finally, it is important to precisely document the exact location of
each sampled section to assure that yearly and cyclical updates, field
reviews, traffic counts, etc., are performed on the appropriate roadway
sections.

## Calculation of Expansion Factors {#calculation-of-expansion-factors .HPMS-Heading-3}

The purpose of the HPMS sample panel is to provide an expandable base
for rural, small urban and urbanized area data in each State, stratified
by functional system and traffic volume group. An expansion factor is
calculated for each volume stratum within each functional system. This
is accomplished by dividing the total kilometers (miles) in the stratum
by the kilometers (miles) included in that stratum\'s sample. As noted
above, the total universe length in each stratum is a known value based
on the AADT volume group identifier. Expansion factors are calculated by
the HPMS software for each sample section. The expansion factor allows
sample data to be expanded to represent entire functional systems for
rural, small urban, and urbanized areas.

## Sampling Rural and Small Urban Areas {#sampling-rural-and-small-urban-areas .HPMS-Heading-3}

Both rural and small urban area data are sampled on a statewide basis,
stratified only by functional system and volume group. The volume group
for each road section in the sampling universe must be identified using
the tables in Appendix C before sample selection can begin. The number
of sections to be included in the sample is determined using the
calculation procedure in Appendix D. A minimum of three unique sample
sections is required for each volume group; if less than three universe
sections exist in a volume group, they must all be sampled and the
expansion factor will be 1.000. Sections should be selected from the
universe of each functional system and volume group using a random
number table or random number generation computer software, until the
required number of samples is reached.

## Sampling Individual Urbanized Areas {#sampling-individual-urbanized-areas .HPMS-Heading-3}

Urbanized area data are sampled on an individual area basis stratified
by functional system and volume group. Each State must individually
sample urbanized areas regardless of population size. The volume group
for each road section in the sampling universe must be identified using
the tables in Appendix C before sample selection can begin.

The number of sections to be included in the sample is determined using
the calculation procedure in Appendix D. As with rural and small urban
areas, sections should be selected from the universe of each functional
system and volume group using a random number table or random number
generation computer software, until the required number of samples is
reached; a minimum of three sections per stratum is required.

Each State must sample its part of a multi-State urbanized area
individually. The sample in each State should not be less than its pro
rata share for the entire urbanized area by functional system and volume
group nor in any case less than one section per applicable volume group.
If length does not exist in a particular volume group in one (or more)
portion(s) of a multi-State urbanized area, all of the sampling should
take place in the State(s) where the length does exist. In such areas,
expansion factors must be calculated separately for each State\'s
portion. To ensure a consistent sampling approach, States having
multi-State urbanized areas are urged to coordinate with the appropriate
neighboring State(s) so that all portions of an urbanized area are
sampled in the same manner.

An individual sampling approach must be applied to all parts of
multi-State urbanized areas if expanded estimates are desired for the
complete urbanized area.

## Sample Maintenance {#sample-maintenance .HPMS-Heading-3}

An HPMS sample adequacy review should be performed annually as part of a
State's sample maintenance activities. The review should be completed
shortly after the annual submittal of the HPMS data set; this permits
the data provider to assess the adequacy of the sample in time to make
changes for the next reporting cycle. Timing of the State's HPMS data
processes is an important issue since sampling is dependent to a certain
degree upon up-to-date traffic and functional classification data. A
number of elements should be considered when making a review of HPMS
sample adequacy. These should include not only the assessment of number
of samples by volume group, but also checks for potential sample biases.

When conducting a sampling review, the State also should check for
biases that may have been inadvertently introduced into the sample.
Although the HPMS sample was designed as a fixed panel sample,
additions, deletions, and other changes may have been made to the sample
to account for system and other changes that occurred over time. And,
although changes to the HPMS sample are to be made on a random basis,
this may or may not have been a closely observed practice.

As a result, sample bias may have been introduced in areas such as
samples on State versus non-State owned roads, subarea biases by highway
district or county, or nonrandom selection of adjacent roadway sections
as new samples. Some of these biases may be disclosed by comparing the
number of miles sampled; for instance, the percent of State owned miles
sampled compared with the percent of non-State owned miles sampled.
Others may require a more detailed examination of the sample and its
distribution; are samples clustered in groups on the same facility, for
instance. A periodic review of the sample provides an opportunity to
identify if any of these problems exist; as further changes are made to
the HPMS sample, any biased sample selection procedures should be
eliminated to improve sample randomness.

Need for Sample Panel Adjustments

There are any number of occurrences that may result in a need to
reconsider the suitability of the existing sample panel. Some of the
more common reasons for considering sample panel adjustments include:

-   The decennial census of population is likely to require changes in
    HPMS sample panels. The sampling basis may need to change because
    the numbers of small urban areas and individual urbanized areas may
    change, and/or the FHWA approved, adjusted Census urban boundaries
    of existing urban areas may be altered.

-   The addition of new areas and the expansion of current urban
    boundaries are likely to require the functional reclassification of
    roadways within the new boundaries. This will in turn likely require
    transferring universe and sample sections from one area's panel to
    another and randomly drawing additional samples to satisfy urban
    area requirements. Also, the loss of samples caused by movement from
    rural to small urban or from rural or small urban to urbanized areas
    may cause a deficiency in the rural or small urban area panels.

-   The EPA's designation or redesignation of NAAQS nonattainment or
    maintenance areas. This may result in new or revised donut areas
    depending on the geographic boundaries of the new nonattainment or
    maintenance areas, requiring adjustment to the donut area sample
    panel. Please see Appendix G regarding the donut area.

-   Changes in the existing functional system length and HPMS sample
    panels are likely to result from functional reclassification,
    non-Census related changes in urban boundaries, or new road
    construction.

-   Migration of sections among and between volume groups may also
    result in a need to change HPMS sample panels. Each volume group
    contained in a functional system is a separate sampling universe;
    normally, over the short term (less than 3 years), there should be
    only minor changes in sample section and universe length assignments
    to specific volume groups as a result of traffic increases or
    decreases. Universe volume group information for each roadway
    section must be kept up to date so that correct volume group
    reassignments can be made (see Appendix C).

A thorough sample adequacy review, conducted on a 3-year cycle basis,
provides an opportunity to update the HPMS sample panels when necessary
to meet the changed conditions reflected above.

Making Sample Panel Adjustments

Sample panel adjustments should be made as necessary upon completion of
a sample adequacy review; use of a 3-year cycle will minimize the burden
of completing this task. The following general procedures should be
considered when adjusting sample panels:

-   Functionally reclassify roadway sections that have moved from rural
    areas into new or expanded urban/urbanized areas or out of
    contracting urban areas into rural areas; use appropriate
    classification criteria and good engineering judgment to determine
    the extent of change warranted.

-   Stratify the reclassified roadway sections within these same areas
    into traffic volume groups consistent with the groups established
    for the latest HPMS sample.

-   Transfer rural, urban, or urbanized sample sections that have moved
    from one area type to another into the appropriate functional
    systems and volume groups in the new panel.

-   Calculate the required number of standard samples required for the
    revised rural, small urban, and urbanized area panels in accordance
    with Appendix D procedures and select additional samples where
    necessary.

In using these general procedures, the user should keep in mind that:

-   When small urban or urbanized areas contract in size, changes to
    small urban or rural sample panels will occur; universe and sample
    sections affected by such changes should be assigned to the correct
    functional system and volume group in the new panel.

-   Make adjustments to standard and donut area sample panels
    independently. It is better to update the standard sample panels
    prior to updating the donut area sample panels, since existing
    standard samples in the donut areas become donut area samples; in
    general, the same procedures apply (see Appendix G).

-   If a new urbanized nonattainment area is designated by EPA, a new
    donut area sample must be drawn for that nonattainment area in
    accordance with the procedures in Appendix G.

Selecting Additional Samples

The selection of additional sample sections for a given volume group is
straightforward for most sample panel updates. Basically, the number of
existing sample sections is compared to the required number as
determined from the Appendix D procedures, and additional sample
sections are randomly drawn from the nonsampled universe sections in the
same volume group to cover any shortfalls. Again, maintaining accurate
volume groups requires States and other data providers to maintain
comprehensive, high quality, traffic count programs (see Appendix F).
This procedure is to be used for the standard sample panels in rural,
small urban and individually sampled urbanized areas, and for the donut
areas of nonattainment areas; it also is to be used when newly
designated urbanized areas are sampled as individual areas.

Sample Permanence

Once a roadway section has been selected for a sample panel, it must be
maintained as a sample regardless of changes in volume group assignment,
functional system, or geographic area. Sample sections transferred to
other geographic areas become part of the sample base for those areas.
Samples may be dropped in cases where a roadway is truly abandoned and
not relocated, where sample sections are reclassified to the rural or
urban local or rural minor collector functional systems, or where sample
sections are dropped from use as a result of a sample reduction plan.
When samples are deleted, the State must submit a list of the sample
numbers, the reasons for the deletions, and where the deletions will
occur.

Deleting Samples

Since the standard sample panel has been in existence for some time, the
addition of samples and the movement of universe and sample sections
from one volume group to another are likely to have caused over sampling
in some volume groups. Significant oversampling is not encouraged
because of cost and efficiency impacts; sample reductions should be
considered a normal part of sample maintenance. Before proceeding with a
sample reduction exercise, the State should prepare a sample reduction
plan and provide it to the FHWA for evaluation. A sample reduction plan
should take account of the following:

> 1\. All sampling criteria must be met; sample size requirements in
> Appendix D must be maintained for each standard sample functional
> system.
>
> 2\. Sample AADT and universe section volume group data must be up to
> date and accurate.
>
> 3\. Individual volume group reductions of less than three sample
> sections in any volume group should not be considered.
>
> 4\. Random deletion of the samples within each over sampled volume
> group is required.
>
> 5\. The three samples per volume group minimum must be maintained.
>
> 6\. Trends of sample/universe section migration among volume groups
> should be examined; volume groups that continually gain samples may
> warrant keeping a few excess samples.

7\. An expansion factor maximum of 100.000 should be observed.

> 8\. A State using the HPMS analytical package or the HPMS data base
> for other purposes may want to keep an oversampled sample panel intact
> or consider using higher precision levels.
>
> 9\. The sample reduction process may require more effort than the
> apparent resulting benefit of maintaining fewer samples; however, a
> periodic review and adjustment of the sample is needed to maintain the
> overall viability of the HPMS sample program.

10\. A sample reduction should be considered as part of the annual
sample review.

Updating Expansion Factors

When updating sample panels, any change in sample length and/or the
length of the sampling frame (the universe) requires updating the
expansion factors related to affected volume groups. Expansion factors
are recalculated before every HPMS submittal to ensure that all changes
to volume groups (both universe and sample), whatever the cause, have
been properly accommodated. Expansion factor recalculation is one of the
final data preparation steps when the HPMS submittal software is used.

A Tabular Summary

The following table provides an overview of conditions which generally
require making changes to HPMS sample panels, both standard and donut
area. It is divided into those changes triggered either directly or
indirectly by Bureau of Census actions, or by changes unrelated to
Census actions. The table outlines the "causes" for potential sample
panel change and the "Recommendations" to deal with the change.

  -------------------------------------------------------------------------
  **CAUSE**                   **RECOMMENDATION**
  --------------------------- ---------------------------------------------
  **CENSUS RELATED**          

  New Small Urban Areas       Adjust all rural sample section records
  (Rural to Small Urban)      within the new area to urban requirements.
                              Verify statewide rural and small urban area
                              sample and universe bases and select
                              additional samples as necessary.

  New Urbanized Areas (Small  Adjust all rural and small urban area sample
  Urban and/or Rural to       section records within the new area to
  Urbanized)                  urbanized area requirements. Procedures for
                              drawing new standard samples for individual
                              panels are discussed above. Procedures for
                              drawing donut area samples are discussed in
                              Appendix G. Verify all sample and universe
                              bases and select additional samples as
                              necessary.

  Expansion of the Adjusted   Adjust all affected rural sample section
  Boundaries of Small Urban   records to urban requirements. Verify all
  or Urbanized Areas (Rural   affected sample and universe bases and select
  to Small Urban and Rural    additional samples as necessary.
  and/or Small Urban to       
  Urbanized)                  

  Functional System           Reassign reclassified sections (universe and
  Reclassification\--Any Area sample) to appropriate areas and volume
                              groups. Sample new sections as necessary to
                              maintain required volume group precision
                              levels.

  Losses in Urban Population  No action until Census area designation
                              changes.

  Major Revision of           Redraw sample panel and include old samples,
  Boundaries Based on New     if possible.
  Census                      

  Changes or Additions to     Updates to the donut area samples are made
  Nonattainment/Maintenance   based on the procedures in Appendix G.
  Area(s).                    

  **NON-CENSUS RELATED**      

  New Length by Functional    Verify sample and universe base; sample new
  System                      sections, if necessary.

  Functional System           In addition to the movement of sections
  Reclassification in Any     because of reclassification, there may be a
  Area                        need for possible volume group changes for
                              universe and/or sample sections, precision
                              level changes, and additional samples.

  Volume Group Reassignment   Reassign sample sections but no further
  of Sections                 action is needed if changes are minor; if
                              changes are major, verify volume group sample
                              and universe bases for all affected volume
                              groups and add samples, if necessary.

  Expansion Factor            Adjust expansion factor values for sample
                              section records in the affected group.
  -------------------------------------------------------------------------

Splitting Samples

Sample sections should be split only when significant HPMS related
changes in an existing sample occur. The most critical reasons for
splitting samples involve changes in traffic (AADT) on the section,
county code, functional system, urban/rural/urbanized status, and number
of lanes. Many times these changes are the product of adjacent land
development, which may result in the improvement of a portion of an
existing sample, an increase in traffic on the improved portion, and/or
the inclusion of the improved portion in an urbanized area boundary. In
general, a change in any of the following key HPMS data items on an
existing sample section should result in splitting an HPMS sample:

  --------------------- -------------------------------------------------
  **Item Number**       **Data Item**

  4                     County Code

  13                    Rural/Urban Designation

  15                    Urbanized Area Code

  16                    NAAQS Nonattainment Area Code

  17                    Functional System

  19                    National Highway System

  25                    Governmental Ownership

  26                    Special Systems

  27                    Type of Facility

  33                    AADT

  34                    Number of Through Lanes
  --------------------- -------------------------------------------------

It may be necessary to split HPMS samples when there are changes in
other HPMS data items; however, the State needs to make a reasoned
judgment of the particular case beyond these minimum specifications. In
most cases, for changes in other HPMS data items on existing sample
sections, it is more than adequate to code the predominant or typical
condition on the existing sample section and retain the entire sample.
If a shoulder type, for instance, is changed on a portion of an existing
sample section, it is acceptable to code the resulting predominant type
of shoulder on the entire section in lieu of splitting the sample. If
part of a sample is improved and the remaining portion is to be improved
in the next construction season, it is likewise acceptable to code the
predominant condition and retain the entire sample. Samples should not
be split for changes in non-HPMS related State inventory items, such as
guardrail changes or highway district boundary, etc.

When splitting an existing sample section, the State should select the
portion to remain as the sample based either on length - keep the
longest portion - or by random pick; the selected portion becomes the
sample and retains the existing sample identifier. Although FHWA
suggests that the longest section be retained as the sample, either is
acceptable. The remaining portion should be converted to a universe
section or merged with an adjoining universe section. It is important
that all count information is adjusted to reflect actual conditions on
the retained sample section. Two examples follow.

![](media/image1.wmf)

## Eliminating Short Sample Sections {#eliminating-short-sample-sections .HPMS-Heading-3}

As part of sample maintenance activities, existing samples should be
reviewed to see if they can be deleted or recombined with adjacent
sample sections. In the past, excessive splitting of HPMS sample
sections has resulted in the accumulation of many short adjacent sample
sections. Adjacent short samples not meeting minimum length requirements
should be recombined into longer sample sections if they have similar
roadway characteristics and the key data items listed under the
"Splitting Samples" discussion are the same. Excessive short samples
resulting from previous sample splits can also be considered for
deletion if HPMS-critical roadway characteristics are the same for a
number of adjacent samples. In this case, the State may wish to retain
the longest sample and recombine the remaining samples with an adjacent
universe section or merge them into a new universe section. The sample
section to be retained can also be selected randomly. Either way, a
reduction in an excessive number of short samples may save the State
financial and personnel resources and will improve sample
representation.