Traffic Monitoring Guide
Traffic Monitoring Guide
Traffic Monitoring Guide
May1, 2001
TRAFFIC
MONITORING
GUIDE
May 1, 2001
U.S. Department of Transportation
Federal Highway Administration
Office of Highway Policy Information
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
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special needs program represents many different operations and may include the
following:
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 trave l. To achieve this goal, the recommended traffic
monitoring program consists of two basic components, a continuous count program and a
short duration count program.
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, count s 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,
includ ing 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:
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.
Included in this effort should be all vehicle classification and WIM counts, since these counts should
also provide total volume data.
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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.
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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.
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
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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.
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8.00%
7.00%
6.00%
5.00%
4.00%
Rural Cars
Business Day Trucks
3.00%
Through Trucks
Urban Cars
2.00%
1.00%
0.00%
1
9 1 0 1 1 1 2 13 14 1 5 1 6 17 18 19 2 0 2 1 22 23 2 4
Hour of Day
1.3
1.2
1.1
0.9
Traditional Car
Recreational Car
0.8
Typical Truck
Through Truck
0.7
0.6
0.5
0.4
0.3
Sunday
Monday
Tuesday
Wednesday
Thursday
Friday
Saturday
Day of Week
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passenger vehicles
single- unit trucks
single combination trucks (trucks and tractors with a single trailer)
multi- trailer trucks.
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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
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
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.
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.
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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.
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.
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.
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
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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).
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.
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.
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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.
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
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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.
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ACRONYMS
Acronym
Meaning
3S2
ADT
AADT
AADTT
AASHTO
ADUS
ARTS
ASTM
ATIS
ATMS
ATR
AVC
BMS
CAAA
CMS
CVC
CVO
DVDT
EAL
EPA
ESAL
FHWA
GIS
GPS
HPMS
IRI
ISTEA
ITS
LTPP
MADT
MPO
NHS
OFE
OPA
PMS
PSR
PTR
SHRP
TEA21
TMG
TVT
TWS
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VDT
VMT
VTRIS
WIM
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