TRAFFIC MONITORING GUIDE

**May 1, 2001**

**U.S. Department of Transportation**

**Federal Highway Administration**

**Office of Highway Policy Information**

Traffic Monitoring Guide\
EXECUTIVE Summary

This document summarizes the recommendations in the Traffic Monitoring
Guide (TMG). The complete guide should be referenced as needed to
understand the technical analysis behind these recommendations.

Actual implementation will vary from agency to agency. Each State or
local highway agency has its own traffic counting needs, priorities,
budgets, geographic, and organizational constraints. These differences
cause agencies to select different equipment for data collection, use
different collection plans for obtaining traffic data, and emphasize
different data reporting outputs. However, all highway agencies collect
the same basic types of data, and each can benefit from using a similar
basic data collection framework.

Traffic monitoring has a long tradition and each agency has an
established legacy program. The TMG offers suggestions to help improve
and advance current programs with a view towards the future of traffic
monitoring. A basic program structure for traffic monitoring is
presented. The guide provides specific examples of how statewide data
collection programs should be structured, describes the analytical logic
behind that structure, and provides the information highway agencies
need to optimize the framework for their particular organizational,
financial, and political structures.

Data Collection Framework

The basic recommended program design consists of:

portable short duration counts, and

permanent continuous counts.

The short duration counts ensure geographic diversity and coverage. The
continuous counts help the agency understand the time-of-day,
day-of-week, and seasonal travel patterns and allow development of the
mechanism needed to convert short duration counts into accurate
estimates of annual conditions. Adjustments to short duration count data
are normally required to remove temporal bias from data used for annual
average daily traffic (AADT) computation.

The TMG recommends that the short count data collection consist of a
periodic comprehensive coverage program over the entire system on a
6-year cycle. The coverage plan includes counting the Highway
Performance Monitoring System (HPMS) sample and universe sections on a
shorter 3-year cycle to meet the national HPMS requirement.

The coverage program is supplemented with a "special needs" element
where additional counts are performed as needed to meet other more
specific data needs. The "special needs" program represents many
different operations and may include the following:

pavement design counts performed to provide data for pavement design,
maintenance, repair, rehabilitation, and reconstruction

traffic operations counts performed to provide inputs to traffic control
studies (e.g., the creation of new signal timing plans)

traffic counts for other special purpose studies.

The specific requirements (what is collected, when and where it must be
collected) for these and other "special needs" studies change from
agency to agency. The ways in which agencies balance these
all-encompassing needs against their limited traffic counting budgets
lead to the very different data collection programs that exist around
the country.

The TMG recommends a coverage program structure for both volume and
vehicle classification programs. Substantial amounts of classification
data are needed to better understand truck travel on highways. Highway
agencies should collect classification data (which also supply total
volume information) in place of simple volume counts whenever possible.
The TMG recommends that State highway agencies initially aim to take 25
to 30 percent of their short duration coverage counts with
classification counting equipment. Agencies that can exceed this figure
are encouraged to do so. The ability to meet or exceed this goal depends
on agency perspective and is a function of the equipment available and
the nature of the road system. Classification data are difficult to
collect in many urban settings because of safety or equipment
limitations. Therefore, a city may decide to collect considerably less
than 30 percent of its counts as classification counts.

Access to data collected from continuous counters is necessary for all
highway agencies. Considerable benefit can be obtained by sharing these
data collection resources. Agencies should work together to reduce
duplication in the number and location of permanent, continuous data
collection devices. Agencies should share the data they collect (e.g., a
State DOT should use seasonal and day-of-week information collected at
permanent sites operated by a county or city as part of developing
adjustment factors for a specific urban area). A single count location
can supply information for many purposes (e.g., permanent, continuous
weigh-in-motion scales supply weight, classification, and volume data).
Opportunities to share data exist not only among agencies but within
agencies. Ensuring that planning, operations, maintenance, and
construction groups share the data they collect can substantially
increase the availability of traffic monitoring data while reducing the
overall cost of data collection.

A key source for urban traffic data will be the traffic surveillance
systems used for traffic management and control. These systems,
currently being installed, expanded, and improved as part of the
Intelligent Transportation System (ITS) program offer highway agencies
the ability to collect continuous traffic monitoring data at high volume
locations. Access to these data requires proactive efforts by the
traffic monitoring groups, as archiving and analysis of surveillance
data are traditionally less important to the operations groups that
build, operate, and maintain these ITS systems. Without proactive
efforts by the traffic monitoring groups, the benefits of ITS data can
be lost because operations groups spend their scarce resources on
operational improvements rather than on the archiving and analysis
software needed to convert surveillance data into useful traffic
statistics.

The TMG recommends that each agency improve the quality of reported
traffic data by establishing quality control processes for traffic data
collection and processing. Subjective editing procedures for identifying
and imputing missing or invalid data are discouraged, since the effects
of such data adjustments are unknown and frequently bias the results.
Each highway agency should have formal rules and procedures for their
quality control efforts.

Volume Counts

The measurement of traffic volumes is one of the most basic functions of
highway planning and management. Traffic volume counts are the most
common measure of roadway use, and they are needed as input to most
traffic engineering analyses. While several traffic volume statistics
are used in traffic analyses, two are of primary interest for the design
of statewide traffic monitoring programs: annual average daily traffic
(AADT) and average daily vehicle distance traveled (DVDT). Because DVDT
is computed by multiplying the roadway segment AADT by the length of
that segment, the primary goal of most traffic monitoring programs is to
develop accurate AADT estimates, which can then be expanded to estimates
of travel. To achieve this goal, the recommended traffic monitoring
program consists of two basic components, a continuous count program and
a short duration count program.

Continuous Count Program

All highway agencies should have access to data collected from
continuous counters. These data are needed to understand temporal
(day-of-week, month) changes in traffic volume. However, not all
agencies need to operate these devices. Agencies should work together to
ensure that enough data are collected and shared to allow calculation of
accurate seasonal adjustment factors needed to convert short duration
traffic counts into estimates of AADT. The TMG provides considerable
guidance on how to structure continuous count programs, how to determine
the appropriate number of counters for adjustment factor development,
and how to apply those factors.

Short Duration Counts

The short count program is designed to provide roadway segment-specific
traffic count information on a cyclical basis. The TMG recommends the
collection of 48-hour periods with counters that record hourly data. To
compute AADT, the volume data from the short counts must be adjusted to
annual conditions. The adjustments include:

axle correction (for counts taken with single axle sensors)

day-of-week (for counts taken for less than one week)

month (to account for volume changes from month to month).

Since AADT is desired for the current year, growth factors need to be
computed and applied to counts not taken during the current year. Also,
counts of less that 24-hour duration, usually taken as a last resort and
not recommended in the TMG, must be adjusted to represent a full 24-hour
period.

Short Count Program Design

Highway agencies perform short duration counts for a variety of
purposes, including meeting federal reporting needs (HPMS), supplying
information for individual projects (pavement design, planning studies,
etc.), and providing broad knowledge of roadway use. The short duration
counting program can be most efficient if these various data collection
efforts are coordinated so that one count session meets multiple needs.
To produce that efficiency, the TMG recommends the following steps to
program design:

Divide the road system into homogeneous traffic volume segments,
determine the count locations needed to cover the system over a maximum
cycle of six years.

Determine the count locations required to meet the HPMS needs.

Determine the count locations and data collection needs of specific
projects that will require data in the next year or two. This entails
working with the offices that will request these data to determine their
data needs.

Overlay the counts[^1] on maps of the highway system including the
location of functioning continuous counters.

Determine how counts can be combined to make best use of available
counting resources.

Schedule the counts to efficiently use the available data collection
crews and equipment.

This program design is intended to reduce count duplication and increase
the efficiency of the data collection staff.

HPMS Counts

Of particular importance to all highway agencies is the collection of
the HPMS sample and universe section traffic data. Volume data from the
HPMS are used to apportion Federal-Aid funds. Significant portions of
these funds are allocated by each State highway agency to lower
jurisdictions, highway districts, or local agencies. Consequently, each
highway agency has a direct financial interest in the validity of data
submitted to the FHWA under the HPMS.

In addition, the outcome of many studies based on the HPMS data affect
highway agencies. The HPMS data are used in a number of key analytical
tools, including the HPMS Analytical Package, the Surface Transportation
Efficiency Analysis System (STEAM), the Highway Economic Requirements
System (HERS), and the ITS Deployment Analysis System (IDAS), as well as
a host of State-specific planning and performance modeling systems.

The HPMS traffic data collection requirement was initially designed as a
statistical sample of highway sections to meet federal data needs. The
HPMS data collection system evolved into a combination of a universal
count program for the National Highway System and other principal
arterials (that is, every HPMS roadway section must be counted) and a
statistical sample for the remaining highway systems.

Each State highway agency is responsible for reporting traffic data to
the HPMS as specified in the HPMS Manual. To support the HPMS reporting
requirement, each NHS, principal arterial, and sample section must be
counted at least once every three years. Note that the HPMS covers roads
on and off the State highway systems.

In addition, each State should maintain cyclic count coverage data on
all arterial and collector roadways covered by the HPMS sample so that
those sections can be accurately assigned to HPMS volume strata. This is
necessary to expand the HPMS sample counts into accurate estimates of
statewide VDT.

State highway agencies may not need to physically count all HPMS sample
or universe locations. In some cases, States rely on local governments
to collect and report these data. In other cases, procedures such as
"ramp balancing" can be used to estimate traffic volumes on roads where
portable counts cannot be safely performed. Regardless of how these data
are collected, the State highway agency is responsible for the quality,
completeness, and accuracy of all submitted HPMS traffic data.

Vehicle Classification Counts

This edition of the TMG significantly departs from the vehicle
classification recommendations in previous editions. The new
recommendation follows the same basic design as the volume count program
previously described. It consists of a coverage count element supported
by a continuous count program.

One reason for this change is that the statistical sample previously
collected met only a single objective efficiently, the estimation of the
average percentage of travel by truck type by functional class of
roadway. It did not meet the data collection needs of many other users
and did not supply sufficiently accurate data on the percentage of
trucks operating on HPMS sample sections.

This change in data collection methodology has other implications. The
new approach acknowledges that many uses other than statewide travel
estimates must be met. To meet these needs it is necessary to be able to
estimate annual average truck travel. Therefore, the TMG recommends a
strong continuous classification count program. The continuous
classification count program is specifically aimed at providing users
with a better understanding of time-of-day, day-of-week, and seasonal
variation in truck volumes.

To apply the knowledge of truck volume variation, the TMG recommends new
procedures to create and apply factors from the continuous
classification count program to the short duration classification counts
being collected. The TMG recommends that seasonal and day-of-week
adjustment factors be developed for three or four broad vehicle
classifications. These factors are similar to those currently applied to
traffic volume counts, but recognize that truck travel patterns are very
different from those of passenger vehicles. The factors are needed to
create accurate estimates of annual truck traffic.

Short Duration Classification Counts

The objective of the short duration classification count program is to
ensure that highway agencies have valid truck volume information for all
highways under their jurisdiction. This means that agencies need to
count truck volumes on all arterial and major collector roads. A
specific emphasis is placed on the collection of classification data on
the HPMS sample segments, since these data are used in many nationally
significant analyses. At the same time, structuring the coverage program
on the HPMS sample provides a geographically diverse set of roadway
locations to address most other needs. Additional needs would be covered
under the \"special needs\" criteria.

The basic data collection recommendations follow:

Highway agencies should collect classification counts rather than volume
counts whenever equipment and staffing limitations allow. As a rule of
thumb, 25 to 30 percent of the volume counts should be classified.

Each agency should perform at least one vehicle classification count on
each route each year.

For roads that change character and/or sustain significant truck volume
changes over their length, one count should be taken on each segment of
that roadway each year.[^2]

Where practical, these counts should be performed at existing HPMS
standard sample sections.

The classification counts should cover a 48-hour monitoring period and,
if possible, should use the standard FHWA 13 vehicle categories. In some
locations, equipment limitations prevent the collection of the 13 FHWA
categories. This usually occurs in high volume, multi-lane situations
where vehicle speeds are highly variable, such as congested urban areas,
or where traffic signal systems cause vehicles to accelerate or
decelerate while being counted. At these locations, highway agencies are
encouraged to use any truck classification equipment that can accurately
classify trucks, even if that means using a different (usually
simplified) classification scheme. If unable to classify under the
13-class scheme due to equipment limitations or safety issues, the TMG
recommends the use of four (or three) classes based on total vehicle
length.

To meet these guidelines, many State highway agencies will need to
increase the number of classification counts they conduct. As old
traffic counting equipment is replaced, the new equipment should be
capable of classifying as well as counting. For statewide monitoring
purposes, highway agencies should attempt to collect classification data
whenever possible, given equipment limitations and the need for
efficient staff and equipment utilization. The goal for every highway
agency should be to collect enough data to provide a valid estimate of
truck volume on each route. These estimates should be based on actual
traffic counts conducted on the roadways in question.

Permanent, Continuous Classification Counts

The continuous classification count program has one major goal, the
creation of factors needed to estimate annual average daily truck volume
from short duration classification counts. To accomplish this goal it is
necessary to measure day-of-week and seasonal variation in truck traffic
and to apply the knowledge to short duration counts. Truck volumes vary
significantly by time of day and day of week as illustrated by Figures 1
and 2.

Sufficient continuous counters are needed to measure each of the
different truck volume patterns found in a State or region. This means
that continuous counters should be placed on different functional
classes of roads and in different geographic locations. It is especially
important to be able to measure the differences in truck volume patterns
between roads that carry primarily local truck traffic and those that
serve through-traffic.

A good rule of thumb is that the continuous classification count program
should be roughly the same size as the traditional continuous volume
count program (the ATR program). In fact, the design of the continuous
count program is very similar to the design of the ATR program. While
the recommended continuous count program requires a significant number
of count locations, it is important to note that continuous classifiers
also serve as ATRs. Thus it is possible to use the classification
counters in place of ATRs at the same time they are used to supply
continuous classification data. Such a step significantly reduces the
number of continuous counters an agency needs and reduces unnecessary
duplication.

![](media/image1.wmf){width="5.999305555555556in"
height="4.374305555555556in"}**Figure 1: Basic Time of Day Patterns**

![](media/image2.wmf){width="5.995138888888889in"
height="4.0993055555555555in"}

**Figure 2: Typical Day of Week Travel Pattern**

Factoring of Short Duration Classification Counts

For many years highway agencies have developed and applied adjustment
factors to short duration volume counts in order to estimate annual
average volumes. Annual average estimates of truck volumes are key
inputs to pavement design analyses, trend analyses, revenue studies,
accident analyses, and a variety of other studies of high importance and
visibility to highway agencies. Therefore, highway agencies must develop
adjustment procedures that allow accurate estimation of annual average
truck volumes from short duration counts. The definition of trucks in
this analysis applies to the longer wheelbase mostly cargo vehicles not
to pickup trucks or vans.

Research has shown that truck volumes, like car volumes, vary by time of
day, day of week, and season, but truck volumes follow patterns that are
significantly different than those of passenger vehicles. Therefore,
applying the adjustment factors already computed for volume induces bias
in the computation of annual truck volume estimates. What each highway
agency needs are adjustment factors specifically designed to convert
short duration truck volume counts into estimates of annual average
daily truck traffic (AADTT).

These factors and factoring procedures need to be developed by each
State highway agency. The development of truck factor procedures is a
new endeavor and as such it will take time to mature. The TMG suggests a
factoring approach that uses the nature of the road's freight traffic
and, if needed, geography to categorize roads into factor groups. The
recommended roadway characterization includes identifying whether the
truck traffic on that road is predominantly "locally" oriented or that
road carries large "through" truck movements. Roads with mostly local
truck traffic tend to have travel patterns that are heavily oriented
toward business day travel (that is, few trucks at night and on
weekends.) Roads that carry heavy through-movements have higher weekend
and nighttime truck volumes.

Geographic stratification for the truck factor groups is suggested for
States in which economic activity changes significantly from one part to
another. For example, if the southern half is heavily agricultural,
while the northern half is heavily oriented toward manufacturing, these
two geographic areas are likely to have different seasonal trucking
patterns.

Table 1 presents a suggested grouping scheme for the creation of truck
factor groups. A key recommendation on the development of truck factors
is to create factors for only three or four broad categories of
vehicles. The suggested classes for factoring are:

passenger vehicles

single-unit trucks

single combination trucks (trucks and tractors with a single trailer)

multi-trailer trucks.

States that have few multi-trailer trucks should reduce further to three
categories by consolidating the single combination and multi-trailer
truck categories.

Table 1: Example Truck Factor Groups[^3]

  ----------------------------------- -----------------------------------
  Rural                               Urban

  Interstates and arterial major      Interstate and arterial major truck
  through-truck routes                routes

  Other roads (e.g., regional         Interstates and other freeways
  agricultural roads) with little     serving primarily local truck
  through-traffic                     traffic

  Other non-restricted truck routes   Other non-restricted truck routes

  Other rural roads (e.g., mining     Other roads (non-truck routes)
  area)                               

  Special roads (e.g., recreational,  
  ports)                              
  ----------------------------------- -----------------------------------

The use of urban or rural breaks may be necessary due to the differences
in patterns and volumes at these locations. In many States, such a break
may not be considered appropriate.

The aggregated classification scheme for factoring short duration
classification counts is recommended for several reasons. In many
States, the volumes in many of the FHWA 13 vehicle categories are very
low and highly variable. When volumes within a vehicle class are low,
the factors computed for those vehicle categories become very unstable
and inaccurate. The factors and estimates can change drastically based
on a few vehicles. Aggregating vehicle classes allows the factoring
process to keep the majority of truck volumes (by class) high enough to
provide stability to the factors produced.[^4] The aggregation also
reduces the computational process to create and apply the factors, since
computing factors for 13 classes would become a very cumbersome process.

Other Recommendations

Calibration and Quality Control

A key component of the vehicle classification program is the
establishment of quality control procedures including the calibration
and testing of equipment used to collect truck volume data. Each State
must periodically calibrate, test, and validate the performance of its
classification equipment to ensure that the equipment is operating as
intended. This includes testing new classifiers received from the
manufacturer. The quality control program should include a short field
test whenever a classifier is placed in traffic to ensure that the
counter is working correctly.

Use of Multiple Classification Schemes

The classification schemes that can be collected are a function of the
data collection equipment used and of road conditions. Many States use
different classification equipment in different operating conditions and
are confronted with the task of dealing with different classification
schemes at different points in their roadway network. Each State highway
agency must understand the different classification schemes they use and
develop conversion rules. For example, if the State uses the FHWA 13
classes but also utilizes length categories from inductance loops on
urban freeways, it must develop appropriate length-based classification
rules to make both schemes compatible.

To understand how different classification schemes relate to each other,
the highway agency needs to periodically perform specific studies to
determine the make-up of the different classification schemes. In the
example above, WIM data that contain both axle spacing and overall
vehicle length information can be used to determine how vehicles
categorized with the FHWA 13-category scheme are placed within the
vehicle length categories.

Truck Weight Data

The new TMG recommends changing the focus of the truck weight data
collection program from collecting data at a random number of locations
to adjusting the number of locations to fit the level of variability in
truck weights. This is done in recognition of the major cost and
difficulties involved in collecting accurate truck weight data. The
objective of the new program recommendations is to ensure that each
State collects accurate truck weight data to meet agency needs. This is
accomplished by:

defining truck weight roadway groups (so that each road within a group
experiences truck weights per vehicle type that are similar to those of
other roads within that group)

collecting weight data from at least six sites within each group

collecting data on the day-of-week and seasonal changes in vehicle
weights that occur within each group

paying specific attention to the calibration of the WIM equipment used
for that data collection.

While structuring a truck weight program similarly to the volume and
classification data programs would be preferable, (i.e., a few
continuous count locations supported by a large number of
geographically-dispersed short duration counts), the cost of weight data
collection and the limitations in available equipment make such a design
unrealistic. Instead, the program recommends collecting data at a
relatively small number of locations designed to be representative of
much larger groups of roads. The truck program design is similar to the
continuous count elements for volume and vehicle classification. One
major difference for the truck weight data collection program is that
most of the weight monitoring sites need not operate continuously. The
program is designed to ensure that current operational WIM sites become
the base of the program. This base can be modified as needed to form the
groups.

Truck Weight Roadway Groups

The TMG recommends that each State define its roadway system into "truck
weight roadway groups," so that each road within a group experiences
truck loading patterns (in terms of vehicle weights per vehicle, not
total tonnage using the roadway) similar to those of other roads within
that group. Further, it recommends using the characteristics of the
freight moved on the roads to help create the roadway groups. This can
be accomplished by understanding the type of commodities carried, the
vehicles used, and the freight movement function performed by each road.
(For example, does the road serve primarily as a through-truck route?
Does it serve as a farm to market road? Does it provide access to
specific types of heavy industry or mining areas? Does it serve
conventional urban/suburban development patterns?)

Small, reasonably homogenous States (e.g., Rhode Island, Vermont) may
only need one or two truck weight groups. For example, they might have
roads with a large percentage of through-trucks versus roads that are
primarily used for local freight movements. Large, diverse States (e.g.,
California, Texas) may have several different truck weight groups.

States are encouraged to adopt "truck weight groups" that:

can be easily applied within the State

can provide a logical means for discriminating between roads that are
likely to have very high load factors and roads that have lower load
factors.

The truck weight groups need not be the same groups that are used to
create vehicle classification factoring. However, the information
developed from the vehicle classification groups will be a great help in
the development of the truck weight groups. The truck weight groups
should follow the vehicle classification groups as much as possible.
However, since the number of WIM sites will be much lower than the
number of permanent classifiers, the number of truck weight groups will
be lower.

The truck route grouping process should, as much as possible,
incorporate knowledge about specific types of heavy trucks, so that
roads that carry those heavy trucks are grouped together, and roads that
are not likely to carry those trucks are treated separately. For
example, roads leading to and from major ports might be treated
separately from other roads in that same geographic area because of the
high load factor that is common to port facilities. These "specialty
roads" should be treated just as "recreational" routes are treated when
continuous volume count information is collected (that is, as an
important but "special" case).

Recommended Number, Length, and Location of Counts

Vehicle weights within each truck weight group should be measured by a
number of WIM sites[^5] located within the group. For most truck weight
groups, a minimum of six sites should be monitored.

At least one of the WIM sites within each group should operate
continuously throughout the year to measure temporal changes in the
loads carried by trucks operating on those roads. Where possible, more
locations within each group should be monitored continuously to provide
a more reliable measure of seasonal change. The proper number of
additional continuous sites is primarily a function of the State's
ability to supply the resources needed to monitor those sites and the
need to monitor differences in seasonal weight characteristics.[^6]

The remaining WIM sites should be monitored for no less than 24
consecutive hours to account for time-of-day differences in vehicle
weights. Data collection sessions of longer than 24 hours are encouraged
whenever practical. In particular, when in-ground weight sensors are
being used and the data collection electronics can be safely left to
operate without on-site staff, a minimum of one-week counts are
recommended at all measurement locations that are not being operated
continuously.

Given the recommended data collection design, the size of any State's
weight data collection program will be a function of the variability of
the truck weights (the number of weight groups created) and the accuracy
and precision desired to monitor and report on those weights (the more
count locations measured within a weight group, the better the highway
agency will understand the weights present on that group of roads.)

For a small State that has only two basic truck weight road groups, the
basic recommendation would be for a minimum of about 12 weighing
locations and two to four continuously operating weigh-in-motion sites.
The number of locations can be further reduced by data-sharing
agreements with neighboring States to collect "joint" vehicle weight
data.

A large State with diverse trucking characteristics could have 10 or
more distinct truck weight groups, and therefore 60 or more WIM sites,
with a corresponding increase in the number of continuously operating
WIM locations. Most States will have far fewer weight groups, at least
as a starting point. The number of weighing locations in a State should
fall somewhere between the extremes of 12 and 90 locations.

The truck weight monitoring locations cannot be selected in a random or
even semi-random manner due to equipment and site selection
considerations. Instead, the TMG recommends that each State start with
its existing WIM sites and add sites as needed. A plan should be
developed to establish the criteria for site selection.

When data collection at existing sites becomes inappropriate, because of
pavement failure surrounding the WIM sensors or failure of the WIM
equipment itself, or because weight data is no longer needed at that
site, the plan should guide the decision to remove or maintain that
site. At sites where data are still desired, the equipment should be
reinstalled after any necessary pavement repair/rehabilitation takes
place. Where a site is no longer necessary, the WIM equipment should be
moved to a different site where either vehicle weights are not known or
additional data are needed.

Highway agencies should collect WIM data at a variety of locations. This
includes moving at least some data collection activity to new roads or
new locations whenever the opportunity presents itself. In this fashion,
insight can be continuously gained on the truck weight patterns found
throughout the State. However, this desire for better geographic
distribution of data collection sites must be balanced against equipment
and resource limitations and the need to ensure that each site selected
for WIM data collection has the physical attributes (flat, strong
pavement in good condition with constant vehicle speeds) that allow for
accurate WIM system operation.

Data Aggregation and Reporting

The collection of data without effective processing of the data and
reporting of information to users reduces the value of the program.
States need to improve the processing and analysis of their collected
WIM data either by making use of appropriate software or developing it.
The TMG contains specific recommendations for the development of load
summary tables from the collected WIM data. The Vehicle Travel
Information System (VTRIS) package developed by the FHWA allows easy
analysis of current WIM data.

For each of the truck weight groups, State highway agencies should
develop estimates of:

average gross vehicle weights (GVW) by vehicle class

axle load distributions by type of axle (single, tandem, tridem, etc.).

These summaries should then be made readily accessible to users so that
this information gains widespread use. Widespread use will translate
into more agency-wide support for the data collection activity and
better transportation decisions. Of particular interest to many highway
agencies will be the need to produce these types of loading estimates
for the new AASHTO Pavement Design Guide currently under development.

Need for Calibration

Heavy emphasis is placed on the calibration of WIM data collection
equipment. Quality information is more important than the quantity of
data collected. It is far better to collect small amounts of
well-calibrated data than to collect large amounts of data from poorly
calibrated scales.

All equipment at WIM sites should be carefully calibrated before the
actual collection of data. In addition to periodic re-calibration of
continuously operating WIM equipment, highway agencies need to perform
ongoing quality control functions for the data collected and processed.
When questionable data are observed, the performance of that equipment
must be investigated, and, if necessary, repairs made and new
calibration efforts undertaken.

Acronyms

**Acronym Meaning**

3S2 3-axle tractor with a 2-axle semi-trailer

ADT Average Daily Traffic

AADT Annual Average Daily Traffic

AADTT Annual Average Daily Truck Traffic

AASHTO American Association of State Highway and Transportation
Officials

ADUS Archived Data User Service

ARTS Advanced Rural Transportation Systems

ASTM American Society for Testing and Materials

ATIS Advanced Traveler Information System

ATMS Advanced Traffic Management System

ATR Automatic Continuous Traffic Recorder

AVC Automatic Vehicle Classification

BMS Bridge Management System

CAAA Clean Air Act Amendments (1990)

CMS Congestion Management System

CVC Continuous Vehicle Classifier

CVO Commercial Vehicle Operations

DVDT Daily Vehicle Distance Traveled

EAL Equivalent Axle Loading

EPA Environmental Protection Agency

ESAL Equivalent Single Axle Loading

FHWA Federal Highway Administration

GIS Geographic Information System

GPS Global Positioning System

HPMS Highway Performance Monitoring System

IRI International Roughness Index

ISTEA Intermodal Surface Transportation Efficiency Act (1991)

ITS Intelligent Transportation Systems

LTPP Long Term Pavement Performance

MADT Monthly Average Daily Traffic

MPO Metropolitan Planning Organization

NHS National Highway System

OFE Other Freeways and Expressways

OPA Other Principal Arterial

PMS Pavement Management System

PSR Present Serviceability Rating

PTR Permanent Traffic Recorder (another name for ATR)

SHRP Strategic Highway Research Program

TEA21 Transportation Equity Act for the 21^st^ Century

TMG Traffic Monitoring Guide

TVT Travel Volume Trends

TWS Truck Weight Study

VDT Vehicle Distance Traveled

VMT Vehicle Miles Traveled

VTRIS Vehicle Travel Information System

WIM Weigh-in-Motion

[^1]: Included in this effort should be all vehicle classification and
    WIM counts, since these counts should also provide total volume
    data.

[^2]: This and the previous bullet are intended to ensure that
    sufficient measurements of truck volumes are taken on each important
    route with a bare minimum of a single count

[^3]: These are strictly examples. Each State highway agency should
    select the appropriate number and definition of truck groups based
    on its economic and trucking characteristics.

[^4]: It is also possible to account for seasonal variation by counting
    multiple times during the year at a single location and this is
    appropriate for sites where a high degree of accuracy is needed or
    where truck adjustment factors are not considered highly reliable.

[^5]: The exception would be for a specialized road. Just as
    "recreational" routes are often monitored with a single permanent
    counter location for volume factoring, a "specialty truck weight
    road" like that leading to a port may be monitored with a single WIM
    site.

[^6]: If the data collection shows that a group of roads has a very
    stable seasonal pattern, then relatively few continuous counters are
    needed to monitor the pattern. However, if the State has little data
    on seasonal weight patterns or if previous data collection has shown
    the pattern to be inconsistent within that group of roads, a larger
    number of continuous counters may be needed.