Estimating Cumulative Traffic Loads, Volume II:
Traffic Data Assessment and Axle Load Projection for The Sites With Acceptable Axle Weight Data, Final Report for Phase 2
CHAPTER 4. COMPUTED PARAMETER TABLES FOR PROJECTED TRAFFIC DATA
This chapter describes computational procedures used to generate
projected annual axle load spectra, explains how the projected data
are stored in the computed parameter tables in the IMS Traffic
Module, and describes relationships between computed traffic
parameters and other data elements in the IMS database. The
computational procedure described here applies only to sites with
site–specific annual axle load data.
Description of Computed Parameter Tables
The projection of traffic loads for all in–service years for the
LTPP traffic sites resulted in the five computed parameter tables.
These tables contain both the projected annual axle load spectra
and the intermediate variables. The intermediate variables carry
important information documenting how the projected axle load
spectra were calculated. The following list represents a set of the
projected traffic computed parameter tables. Each table is named
twice: the descriptive name is first, and the proposed IMS name
follows in bold type.
- Projected Annual Axle Load Spectra
- Normalized Base Annual Axle Load Spectra
- Base Annual Axle Load Summary
- Annual Projection Factors
- Projection Summary Table
Figure 25 presents an overview of the IMS Traffic Module that
includes historical, monitoring, and projected traffic data. The
relationship between the projected and other traffic tables stored
in IMS is shown in figure 26. Figure 26 also provides a flowchart
used for the calculation of variables stored in the computed
parameter tables. The main computed parameter table is highlighted
by heavy borderlines. Below the main table, in the oval shape, is
the provision for calculating "cumulative axle loads." The
cumulative axle load spectra are not available in the IMS database;
they can be obtained by adding up annual axle load spectra for any
combination of in–service years.
Figure 25. Overview of the IMS Traffic
showing the proposed addition of projected traffic data.
Generation of Projected Annual Axle Load Spectra
This section describes the computational procedure used to
generate projected annual axle load spectra for all in–service
years, and the relationships among the variables in the computed
parameter tables. The description builds on the outline of the
traffic projection procedure provided in chapter 2. Since only
traffic projections for Category 1 and 2 sites were carried out
during the study, the description herein is based on methodology
developed for Category 1 and 2 projections (see p. 8 of this report for descriptions of the
Computation of the projected annual axle load spectra for all
in–service years and preparation of the computed parameter tables
involves the following major steps:
- Computation of normalized base annual axle load spectra (table
- Computation of base annual axle load summary (table
- Computation of annual projection factors (table
- Computation of projected annual axle load spectra (table
- Reporting projection summary (table
These steps are described in more detail in the following
Figure 26. Flowchart used for calculating
computed parameter tables.
Computation of Normalized Base Annual Axle Load Spectra
Normalized base annual axle load spectra for single, tandem,
triple, and quad axle groups are given in table
TRF_PRJ_BAS_ANL_PCT_AXLE. The normalized
base annual axle load spectrum is the base annual load spectrum
with the axle load counts for a given weight range (and axle type)
expressed as percentages of the total axle counts for the given
axle type. This spectrum is computed for the same set of weight
ranges that are used in table
TRF_MONITOR_AXLE_DISTRIB. Normalized spectrum
provides a characteristic shape of axle weight distribution for
each LTPP traffic site.
To develop normalized base annual spectrum, available monitoring
annual spectra from the IMS table
TRF_MONITOR_AXLE_DISTRIB were critically assessed
and base annual spectrum was computed by averaging annual axle load
data for selected years as described in chapter 2. Then, the
computed base annual spectrum was normalized with respect to the
total annual axle load counts for each axle type. The computational
procedure for obtaining normalized base annual axle load spectrum
is shown in figure 27.
Figure 27. Flowchart for computation of
the normalized base annual load spectra.
Computation of Base Annual Axle Load Summary
The base total annual numbers of axles for single, tandem, and
triple axle groups contained in the base annual spectrum are given
in table TRF_PRJ_BAS_ANL_AXLE_SUM (base annual
axle load summary). The base total annual number of axles is
associated with normalized base number of axles from
TRF_PRJ_BAS_ANL_PCT_AXLE and is used to calculate
projected annual axle load spectra. The base total annual number of
axles provides information about overall truck volume (in terms of
total axle counts) that is representative for each LTPP traffic
The total base annual number of axles was computed (see figure
28) by summing axles from the computed base annual spectrum across
all weight ranges. This summation was done separately for each axle
Figure 28. Flowchart for computation of
the base annual axle load summary.
Computation of Annual Projection Factors
Annual projection factors, calculated for each in–service year
for LTPP sites, are given in table
TRF_PRJ_YR_MULTIPLIER. These factors define an
annual truck traffic growth pattern and adjust the base total
annual axle counts up or down to fit the selected projection model.
Annual projection factors are used to compute projected annual axle
The annual projection factors are selected based on critical
review of the available historical and monitoring traffic data for
all available years from the following IMS tables:
- MONITOR_BASIC INFO.
The computational procedure for the annual projection factors is
shown in figure 29.
Figure 29. Flowchart for computation of
the annual projection factors.
Computation of Projected Annual Axle Load Spectra
Projected annual axle load spectra (for single, tandem, and
triple axle groups) are the main product of the LTPP traffic
projection process. These results are presented in the table
TRF_PRJ_YR_AXLE_DISTRIB. Projected annual load
spectra for each year are computed (see figure 30) by multiplying
the base annual load spectrum with annual traffic projection
factors from table TRF_PRJ_YR_MULTIPLIER. The base
annual load spectrum is computed by multiplying the normalized base
annual axle load spectrum for single, tandem, and triple axle
groups from the table TRF_PRJ_BAS_ANL_PCT_AXLE
with the total base annual number of axles for single, tandem, and
triple axle groups from the table
TRF_PRJ_BAS_ANL_AXLE_SUM. The projected load
spectra are reported in terms of annual axle counts for the same
set of load ranges used in table
Figure 30. Flowchart for computation of
projected annual axle load spectra.
Reporting Projection Summary
The summaries of projection results (including applicable
traffic projection interval; information about traffic projection
category used for each LTPP traffic site; the assigned traffic
projection codes; and any specific traffic characteristic for each
LTPP site) are stored in the table
To assign projection confidence codes and for QA purposes,the
projected annual axle load spectra were used to calculate ESALs.
Calculated ESALs were then compared with the available historical
ESAL trends. Also, TFs were computed and compared with historical
TFs and with typical TFs based on FHWA functional highway
classification. Analysts assigned initial projection confidence
codes to the results using the guidelines described in chapter 2.
After the traffic projection results were reviewed by the
participating highway agencies, the changes were made to the
initial projections, and reviewed projection confidence codes were
assigned to each site.