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Publication Number: FHWA-RD-03-094
Date: March 2005

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 2. PROCEDURES FOR TRAFFIC DATA ASSESSMENT AND PROJECTION

This chapter describes the procedures used to carry out the assessment of the quantity and quality of traffic data, and to estimate traffic loads for all years the LTPP sites were in service. The process of estimating traffic loads is referred to as traffic projection.

The assessment of the data quality and quantity was carried out for all LTPP sites. The projection of truck volumes was carried out for all sites with appropriate monitoring or historical truck volumes, and the projection of axle load spectra was done for all LTPP sites that had appropriate site-specific monitoring axle load data in the Level E release of the IMS database (first quarter of 2000).

The assessment of traffic data and the development of traffic projections have been carried out in two phases over the course of 30 months; they involved eight main activities (see figure 1).

Figure 1. Overview of main traffic data assessment and projection activities.
Phase 1
  1. Preliminary assessment of LTPP traffic data.
  2. Development of LTPP traffic projection procedure.
  3. Validation of LTPP traffic projection procedure using case studies.
Phase 2
  1. Development of LTPP Traffic Feedback and Resolution Package.
    1. Development of the content of the package.
    2. Evaluation and testing of the package.
      1. Review by RCO representatives and other parties.
      2. Review and enhancement by pilot studies.
  2. Preparation of LTPP Traffic Feedback and Resolution Packages for all participating agencies.
    1. Assessment of traffic data.
    2. Projection of truck volumes.
    3. Development of base annual spectra.
    4. Assignment of initial projection codes.
    5. Computation of annual axle load spectra.
  3. Review of LTPP Traffic Feedback and Resolution Packages by RCOs.
  4. Review of LTPP Traffic Feedback and Resolution Packages by participating agencies.
  5. Implementation of review comments received from participating agencies.
    1. Responding to agencies.
    2. Preparation of revised traffic projections.
    3. Placing projected traffic data into computed parameter tables.
Phase 3 Recommended phase
Recommendations for Phase 3 are provided in chapter 6.

Of the eight traffic data assessment and projection activities, three were carried out in Phase 1 and five in Phase 2. In this chapter, the activities are described in the order they are listed in figure 1 as steps 1 to 8. To enable the reader to follow the traffic data assessment and projection process better, a general outline of the eight activities is provided first, followed by a more detailed description of the activities.

The main traffic data assessment and projection activities carried out in Phase 1, and described in Phase 1 report were:[1]

  1. Preliminary assessment of LTPP traffic data - The objective of this activity was to obtain a basic understanding of the overall quantity and quality of traffic data. This understanding was necessary for the development of the traffic projection procedure.
  2. Development of LTPP traffic projection procedure-The projection procedure developed in Phase 1 was designed to utilize fully all available historical and monitoring traffic data and to incorporate additional traffic information obtained from local agencies.
  3. Validation of LTPP traffic projection procedure using case studies -The projection procedure was used to estimate axle loads for all in-service years of 12 LTPP sites with different amounts of historical and monitoring data. The objective was to validate the projection procedure using realistic examples.

The main traffic data assessment and traffic projection activities carried out in Phase 2, and described in here, were:

  1. Development of LTPP Traffic Feedback and Resolution Package-The purpose of the LTPP Traffic Feedback and Resolution Package was to summarize and present historical and monitoring traffic data and the results of traffic data projections in a structured and user-friendly manner. The package, developed using a systematic procedure applied consistently to all LTPP sites, was the main means of communication and interaction with the participating agencies. The package contains traffic data summaries for the LTPP sites, easy-to-follow results of traffic projections, background information, and guidelines on how to assess traffic data and review traffic projections. During the development of the package, input and comments were received from the representatives of RCOs, members of the Expert Task Group (ETG) on Traffic Data Collection and Analysis, and others. To further evaluate suitability and ease of use, and to provide an opportunity for the RCOs to become familiar with the content and the purpose of the package, pilot studies involving four participating agencies were also carried out.
  1. Preparation of the LTPP Traffic Feedback and Resolution Packages for all participating agencies -Altogether, 62 LTPP Traffic Feedback and Resolution Packages were produced (one for each participating agency). Each agency package addresses all LTPP traffic sites that belong to the agency, and each LTPP site was processed in terms of: (a) the assessment of the amount and quality of historical and monitoring traffic data, (b) the development of traffic projections, provided that appropriate traffic data were available, and (c) the assignment of confidence codes to the initial traffic projections.
  2. Review of LTPP Traffic Feedback and Resolution Packages by RCOs-The responsibilities of RCOs include initial traffic data quality checks, as well as processing and storage of traffic data supplied by the participating agencies. Because of a long-standing involvement of the RCOs with traffic data issues on the local level and the knowledge of local traffic issues, the packages were sent to the RCOs for review prior to being sent to the participating agencies.
  1. Review of Traffic Feedback and Resolution Packages by participating agencies-After the concerns of the RCOs were addressed, the updated packages were sent to the participating agencies for review. The involvement of the participating agencies has been crucial; many data problems cannot be resolved without local involvement.
  1. Implementation of review comments received from participating agencies-To facilitate communication with the participating agencies on an individual section basis, a standardized questionnaire (Feedback and Data Resolution Sheet) for each LTPP site was developed, and the participating agencies were asked to complete it. Based on the responses we received from the participating agencies, the initial traffic projections were revised and the initial projection confidence codes were changed to reviewed projection confidence codes.

Step 1-Preliminary Assessment of LTPP Traffic Data

The Phase 1 report contains the results of the evaluation of LTPP traffic characteristics.[1] The results include examples of spatial and temporal variation in truck volumes; comparison of trends in historical and monitoring annual truck volumes and annual ESALs; the evaluation of the potential of Class 9 vehicles for prediction of axle loads; and the evaluation of axle load distribution characteristics: The Phase 1 report contains a review of several previous studies that used LTPP traffic data to: characterize seasonal variation in truck volumes;[8] evaluate the consequences of alternative sampling plans on the estimated annual traffic loads;[9] and assess the potential of LTPP traffic data to develop models for predicting axle load distributions.[10] The results of an exploratory analysis of trends in traffic data were presented in an interim report that preceded the Phase 1 report. [11]

The review of LTPP traffic data carried out during Phase 1 identified the following major issues, concerns, and recommendations regarding quality and quantity of traffic data and the projection of traffic loads.

  • Trends in historical and monitoring data-The relationship between annual historical and monitoring ESALs, and between monitoring ESALs obtained for different years, investigated for all sites in the North Atlantic and North Central Regions, showed considerable variation. The differences between historical and monitoring ESALs, and between ESALs obtained for consecutive monitoring years, frequently exceeded 100 percent. Because truck volumes and axle loads on interstates and other major highways are expected to show relatively steady growth over the years, the observed differences indicated important data concerns requiring resolution.
  • Involvement of participating agencies-Considering the available historical and monitoring data and the challenges involved in carrying out traffic projection, the involvement of participating agencies in the traffic projection process was identified as being essential for knowledge-based traffic data assessment and projection of traffic loads.
  • Unavailability of monitoring data-Typically, only 3 or 4 years of monitoring axle load data are available for LTPP sites. Many sites do not have any axle load (WIM) or truck class distribution (AVC) data.
  • Calibration of AVC and WIM equipment-AVC and WIM scale calibration concerns have not been studied in sufficient detail. For example, the CTDB does not contain any data on quadruple axles even though such axles regularly occur in several participating agencies, including Michigan and Ontario. For many sites, axle load spectra exhibited unexpectedly large year-to-year variations.
  • Quality assurance -The QA techniques employed previously were characterized as an automated review to detect common equipment problems, rather than a comprehensive and detailed QA process. To ensure the integrity of the data used for traffic projections and for any subsequent analysis, the Phase 1 study recommended that a comprehensive QA process of underlying traffic data be carried out.
  • Development of PLG -The data QA process would greatly benefit from the development of a knowledge base documenting typical values and ranges of traffic variables, particularly truck class and axle load distributions.

The knowledge gained during the preliminary assessment of the amount and quality of available historical and monitoring data was instrumental in the development of the LTPP traffic projection procedure.

Step 2-Development of LTPP Traffic Projection Procedure

This section outlines the LTPP traffic projection procedure. The detailed description of the procedure, including numerical examples in the form of case studies, is contained in the Phase 1 report.[1] To satisfy the following design requirements, the projection procedure was developed to ensure:

  • The ability to estimate annual axle loads for the majority of LTPP sites and for all years the sites were in service.
  • Compatibility with the available historical and monitoring traffic data and their full utilization.
  • Transparency (to be understandable to the users of projected traffic data) and modularity (for ease of future enhancements of the projection procedure, and for ease of updating the projections if more data become available).

The procedure for projecting axle load spectra for the individual LTPP sites includes the following activities:

  • All available annual historical and monitoring data were used to establish a model for estimating annual truck volumes for all years the site was in service.
  • The model for estimating annual truck volumes was used to obtain annual projection factors.
  • A base annual axle load spectrum, representing a typical axle load spectrum for the site, was established.
  • The projected annual axle load spectra for all in-service years were obtained by multiplying the base annual spectrum by the annual projection factors.

Mathematically, the LTPP traffic load projection procedure is based on the following formula:

(1)

Annual Axle Load Spectrumy = Base Annual Spectrum * Annual Projections Factor y

Where:

Annual Axle Load Spectrumy = Projected annual distribution of axle weights by load ranges for year y. Axle weights are reported separately for single, tandem, and triple axles.
Base Annual Spectrum = A typical annual axle load spectrum chosen to represent traffic loads on the site.
Annual Projection Factory = Annual truck volume adjustment factor for year y used to scale the Base Annual Spectrum according to the total volume of trucks in year y.

The above formula means that once the base annual spectrum was established, it was scaled using the annual projection factor corresponding to the annual truck volume for the given year. Consequently, it was assumed that the truck class distribution remained constant over the years and that only the amount of trucks changed.

Specific techniques, described in Phase 1 report, were developed to obtain base annual spectra for the five projection categories.[1] The five projection categories, classified by the amount of available monitoring data, are defined in chapter 1 of this report. Annual axle load spectra were projected also for the years with monitoring (measured) annual axle load spectra. For these years, both the monitoring and the projected axle load spectra will be available in the IMS database.

To obtain cumulative axle loads, annual axle spectra could be summed as shown in equation 2.

(2)
Cumulative Load Axle Spectrum = y=n Annual Axle Load Spectrum
S
y=1

Where:

Cumulative Axle Load Spectrum = Total amount of axle weights during n years. Axle weights are reported separately for single, tandem, and triple axles.
n = The number of years from the opening of highway to traffic through (and including) 1998.
Projections for Sites in Category 1 and 2

Category 1 and 2 sites have monitoring annual axle load spectra in the IMS database; these spectra were used to obtain the base annual spectrum. Traffic load projections carried out in Phase 2 were done for Category 1 and 2 sites only, and were in terms of annual axle load spectra. Typically, the base annual spectrum was an average of all acceptable annual spectra for the given site. Techniques used to establish base annual spectra for Category 1 and 2 sites are summarized as part of step 5 (figure 1) in the section titled "Development of Base Annual Spectra."

The Phase 1 report also contains the description of procedures developed for estimating base annual spectra for Category 3 and 4 sites, as well as numerical examples of traffic projections for these sites.[1] Even though these procedures were not used in this report, they are outlined below because they are important for the understanding of the subsequent recommendation to develop the LTPP PLG (discussed in chapter 5 of this report).

Projections for Sites in Category 3

Category 3 includes sites with site-specific monitoring truck class distribution data but without acceptable site-specific monitoring axle load distribution data. Because these sites do not have annual axle load spectra in the database, the base annual spectrum must be estimated. According to the Phase 1 report, it is proposed to accomplish this by combining the following two inputs:[1]

  • Site-specific monitoring truck class distribution.
  • Surrogate axle load spectra for individual truck types.
Site-Specific Monitoring Truck Class Distribution

If there are several years for which the site-specific monitoring truck class distributions are available, a typical truck volume distribution (called the base truck distribution) is established by plotting and assessing all annual truck class distributions using similar techniques as those used to obtain the base annual spectrum. The objective is to obtain the truck class distribution that best represents the given site.

Surrogate Axle Load Spectra for Individual Truck Types

The sources of surrogate axle load spectra include site-related data, regional data, and generic or typical data.

Projections for Sites in Category 4

Category 4 includes sites with annual truck volume data but without site-specific truck class and axle load distributions. Because these sites do not have annual axle load distribution in the database, the base annual axle load spectrum must be constructed. According to the Phase 1 report, it is proposed to accomplish this by combining the following three inputs:[1]

  • Site-specific total truck volume.
  • Surrogate monitoring truck class distribution.
  • Surrogate axle load spectra for individual truck types.

The sources of surrogate truck class and axle load distribution data include site-related data, regional data, and generic or typical data.

Step 3-Validation of the LTPP Projection Procedure Using Case Studies

The methodology for projecting axle load spectra was evaluated and demonstrated in Phase 1 using case studies for selected LTPP sites. Case studies are numerical example applications of the traffic projection procedure for actual LTPP sites. All together, 12 case studies were carried out (3 case studies in each projection category except Category 5). The following observations were based on the results of the case studies:[1]

  • The procedure for projecting traffic loads developed in the course of the Phase 1 study can be used to estimate axle load spectra for all LTPP sites.
  • The involvement of participating agencies in the projection process is essential.
  • The selection of truck class and axle load distributions required for Category 3 and 4 projections must be done judiciously and should be supported by a reference database summarizing characteristic truck class and axle load distribution data .
  • The QA process would greatly benefit from developing a knowledge base or a catalog documenting typical or expected values and ranges of traffic variables, particularly axle load spectra for individual vehicle classes.

Step 4-Development of LTPP Traffic Feedback and Resolution Package

To meet the objectives of the Phase 2 study, it was necessary to work with all LTPP sites and with all participating agencies. The LTPP traffic database contains 890 unique traffic sites located in 62 agencies. Reflecting the objectives of the Phase 2 study, the objectives of the LTPP Traffic Feedback and Resolution Package were:

  • To report back to the participating agencies the quantity and quality of the traffic data available for all LTPP traffic sites in their respective jurisdictions, using easy-to-understand graphic displays. Although the participating agencies had received traffic feedback reports in the past, the previous reports were limited in scope and did not contain all relevant traffic data for all historical and monitoring years, nor did they include long-term trends in traffic volumes and loads.
  • To present traffic data in the format that would:
    • Facilitate the assessment of the quality and quantity of traffic data by the participating agencies, RCOs, and the project team.
    • Facilitate the understanding of the projection process, and the assessment of the initial traffic projections by the participating agencies and RCOs.

To efficiently assess traffic data and carry out traffic projections for all 890 LTPP sites, and to communicate the traffic projection results to participating agencies effectively, we developed a standardized package-the LTPP Traffic Feedback and Resolution Package (also referred to here as "the package"). The package combines graphic traffic data displays, information on how to interpret the data displays, and questionnaires addressing overall and site-specific traffic data issues. A separate package was prepared for each participating agency.

The rest of this section describes the content of the package and the involvement of the representatives of RCOs and others in its development and validation.

Content of the LTPP Traffic Feedback and Resolution Package

The package consisted of five items:

  • Introductory letter.
  • Outline of the LTPP traffic projection procedure.
  • Initial overall feedback and resolution report.
  • Initial site-specific feedback and resolution report.
  • Site-specific reports for each LTPP site within the agency.

These five items are briefly described below. The package was quite bulky, particularly for agencies with many LTPP sites.

Introductory Letter

The introductory letter was prepared and signed by a representative of the RCO and was addressed to the State or Province LTPP contact engineer or other official. Typically, the letter outlined the purpose of the package and asked that a person within the agency who was familiar with the collection of traffic data for LTPP sites review the package and respond to the issues raised. Some letters also included additional information and comments regarding the traffic data availability for specific sites (e.g., if data submitted previously by the participating agencies were not yet included in the package).

Outline of the LTPP Traffic Projection Procedure

The outline of the LTPP traffic projection procedure was a 10-page report that explained the traffic projection procedure used to estimate traffic loads in straightforward language. The objective of the outline was to enable the reviewer to understand the relative importance of the different issues, potential discrepancies, and questions posed in the package. The outlined discussed:

  • Objectives of the traffic projection procedure.
  • Definition of key technical terms used in the outline.
  • The need to estimate traffic loads in terms of axle load spectra. The outline also included a typical example of axle load spectra in graphic and tabular forms.
  • Reasons for the involvement of participating agencies in the traffic estimation process.
  • LTPP traffic projection procedure.
Initial Overall Feedback and Resolution Report

The initial overall feedback and resolution report was typically a 4- or 5-page report that contained a summary of traffic data assessment and traffic projection issues concerning more than one LTPP site. The report was called "initial" because it was concerned with the first traffic projections carried out for LTPP sites. The initial overall feedback and resolution report served two purposes:

  • To summarize issues identified during the initial traffic data assessment and projection effort that required input from agency representatives.
  • To seek additional traffic data and information from the agency representatives to improve traffic projections.

The issues and questions were grouped under the following headings:

  • Overall review.
  • Missing data.
  • Location of sections.
  • Traffic volumes.
  • Vehicle classification-operation of AVC equipment.
  • Axle weights-operation of WIM scales.

Overall review -The overall review included a table listing all LTPP sites for the agency and their corresponding LTPP experiment numbers and the initial projection confidence codes. The initial projection confidence codes were used to characterize the level of confidence associated with initial traffic projections. The assignment of the projection codes is described later ("Assignment of Initial Traffic Projection Codes" in step 5).

Missing data -This section provided a comprehensive listing of traffic data that were missing from the IMS database.

Location of sections -Particular attention was paid to the nearby sites located on the same highway, particularly if the sites were also located in the same direction of travel. The objective was to ascertain the existence of expected relationships between truck volumes and axle loads on related sites. Any potential discrepancies were brought to the attention of the agency representatives.

An example of the data assessment carried out under the heading of "Location of sections" is provided in table 3. As expected, nearby sites on the same highway in the same direction have similar truck volumes, truck percentages, and truck growth rates. The exception appears to be the two northbound sites on U.S. Route 93 in Arizona that have quite different truck percentages (15 versus 27) and quite different recent truck growth rates (1.0 versus 10.0 percent).

Traffic volumes -The objective of this part of the report was to emphasize the importance of trends in annual historical and monitoring truck volumes for the development of the traffic load projections. Sections with unexpected variation in annual truck volumes were identified and reviewers were asked to address truck volume discrepancies on a section-specific basis.

Vehicle classification-operation of AVC equipment-Typically, the main concern with vehicle classification was the number of vehicles that were not properly classified by the traffic monitoring equipment. These vehicles are identified in the IMS as Class 14 vehicles. Some agencies reported that the percentage of Class 14 vehicles in the total truck flow was, for the majority of sites and years, above 10 percent. Some agencies did not report any Class 14 vehicles. Questions posed to the agencies regarding vehicle classification were regarding: (a) distribution of Class 14 vehicles into "legitimate" vehicle classes; (b) reporting of vehicles that were not properly classified by AVC equipment; and (c) type of procedures used to ensure that the vehicles are properly classified.

Axle weights-operation of WIM scales-This segment of the overall report contained two parts.

The first part identified the LTPP sites with questionable axle load data and inquired about the procedures used to calibrate WIM scales and to review axle load data prior to their submission to RCOs. Questionable axle load distributions, for at least some sections and some monitoring years, were reported by all agencies that reported monitoring axle loads. Typically, the following questions were asked: What type of procedure is used to ensure that WIM scales are calibrated? Is WIM calibration done routinely? Do you have resources to review traffic data prior to their submission to LTPP? If so, what procedures do you use?

The second provided the results of analysis carried out to ascertain basic traffic loading patterns for all LTPP sites within the agency. The objective was to provide a summary of axle load characteristics and to identify sites that do not fit overall patterns. An example of such a summary is shown in tables 4 and 5.

Table 3. Arizona LTPP sites near other sites on the same highway and in the same direction.
Location, Hwy No. and Direction Nearby Sites 1998 AADT Truck Volume 1998 Truck Percentage Recent Truck Growth, % Year or Years for which Axle Load Data are Available, Comments
93 N 0100 900 15 1.0 1997, axle load spectra are identical
1036 800 27 10.0
10 W 1001 2620 31 7.4 1995 and 1996
1003 3400 52 7.0 1994
1006 3160 45 6.7 1995
1007 3530 44 7.4 1995, 1996, 1997
7614 3490 35 9.0 1996
19 S 1015 620 7 7.0 None
1016 470 8 6.0 1994
1018 450 8 6.0 1993
6054 570 8 2.0 1993
19 N 1017 540 10 4.0 1997
6060 590 13 7.0 1996, 1997, 1999
40 W 1002 2210 49 8.0 1997, 1999
1021 2240 50 11.0 None
1022 2240 50 11.0 None
1025 2130 47 7.0 1993, 1994
1062 2190 58 9.0 1993
40 E 1024 2080 46 7.0 1993
1065 2140 54 9.0 1993

The results in tables 4 and 5 indicate a similarity of traffic load characteristics with a few exceptions. For example, all TFs in table 4 are in the range of 0.9 to 1.3, except the TF of 0.6 for Mississippi site 1001. Consequently, the axle load data on this site need to be assessed to ascertain the reason for lower-than-expected axle loads.

Initial Site-Specific Feedback and Resolution Report

The purpose of this report was to:

  • Describe the standardized displays of traffic data and the initial traffic projection results for individual LTPP sites.
  • Provide guidelines for the assessment of traffic data and for the interpretation and evaluation of traffic projection results.
Table 4. Traffic loading parameters for Mississippi LTPP sites with axle weight projections.
Highway Functional Class Site No. (Prefix of 28) Flexible Pavement Rigid Pavement
1998 Truck % Average Unloaded / Loaded Peaks of Tandem Axle Load Spectra (kips)** Average Truck Factor (ESALs /Truck) 1998 Truck % Average Unloaded / Loaded Peaks of Tandem Axle Load Spectra (kips)** Average Truck Factor (ESALs / Truck)
Urban Principal Arterial 1016 9 9/31 1.0 - - -
5805* - - - 15 11/27 1.1
Rural Principal Arterial 0500* n/a 11/33 1.3 - - -
1001 9 9/29 0.6 - - -
1802 22 11/31 0.9 - - -
2807 12 11/33 1.1 - - -
3018 - - - 24 9/29 1.2
3019 - - - 24 9/29 1.2
3081 31 11/32 1.1 - - -
3082 n/a 13/33 1.2 - - -
3087 8.5 11/36 1.3 - - -
3089 12 11/33 1.1 - - -
3091 13 11/33 1.3 - - -
3093* 20 11/31 0.9 - - -
3094* 20 11/31 0.9 - - -
3097* - - - 14 11/27 1.7
3099* - - - 41 -/31 1.7
5006 - - - 28 11/29 1.1
5025 - - - 16 11/33 1.9
5803 - - - 36 11/33 1.8
7012* - - - 29 -/31 1.8
9030* 32 13/31 1.0 - - -

*Interstate; **1 kip = 4450 newtons

Table 5. Traffic loading parameters for Mississippi sites without axle weight projections.
Highway Functional Class Site No. (Prefix of 28) 1998 Projected AADT Truck Volume 1998 Truck % 1998 Annual Growth Rate % Historical TF (ESALs/Truck)
Rural Principal Arterial 3083 25 8 1.5 0.6-1.5
3085 25 8 1.5 0.6-1.5
3090 40 7 2.0 0.3-1.0
4024 130 4 3.0 0.5-1.4

An example of the Initial Site-Specific Feedback and Resolution Report, prepared for the representatives of participating agencies assigned the task to review the initial traffic projections, is provided in figure 2. Specifically, figure 2 includes the entire site-specific feedback and resolution report prepared for the Mississippi Department of Transportation (DOT); the reports prepared for other agencies were similar.

The report in figure 2 refers to 26 site-specific reports (one for each of the 26 Mississippi LTPP sites that have traffic data in the IMS database). One of the 26 site-specific reports for Mississippi, report for site 285805 is presented in figures 3 through 10. To assist the reader in making the connection between the reports, figure 2 contains references to figures 3 through 10. The first data sheet of the site-specific report (figure 3) was on blue paper and is referred to as a "Blue Sheet."

Site-Specific Reports

The site-specific report for site 285805 (figures 3 through 10) consists of eight data sheets:

The procedures used to develop site-specific reports are described in the Phase 1 report, and the content and purpose of the report are described in figure 2.[1] Subsequent sections will provide a description of techniques used to develop the initial projections shown in figures 6 and 10, and to assign projection confidence codes.

The Blue Sheet in figure 3 is shown as having been completed by the Mississippi Department of Transportation (MDOT) to avoid the need to show separate examples of uncompleted and completed Blue Sheets. The completion of Blue Sheets by participating agencies will be discussed in step 7 (Review of LTPP Traffic Feedback and Resolution Packages by Participating Agencies).

Figure 2. Initial site-specific report for Mississippi (page 1).

INITIAL SITE-SPECIFIC FEEDBACK AND RESOLUTION REPORT FOR MISSISSIPPI

November 2000

ERES LTPP DATS Traffic Analysis Team

Baltimore-Washington DC Area Office
Phone: **********
Fax: **********
e-mail: **********

INTRODUCTION

This report is about site-specific traffic projection issues concerning individual LTPP sites. The traffic projection issues that are applicable to more than one site are addressed in the Initial Overall Feedback and Resolution Report for Mississippi. The report is called "initial" because it comments on the first, or initial, traffic projections carried out for Mississippi sites.

The purpose of this report is twofold:

  • To describe the standardized display of the results obtained during the initial traffic projection process for individual LTPP sites.
  • To provide guidelines for the interpretation and evaluation of results.

Attached to this report are site-specific reports prepared for the 26 Mississippi LTPP sites that contain traffic monitoring data in the LTPP traffic database. Each report starts with a blue "Feedback and Data Resolution Sheet." Please review each report according to the guidelines provided herein.

In order for the reviewer to understand the relative importance of the different issues, potential discrepancies, and questions posed in this report, it is recommended that the reviewer become familiar with the overall traffic projection methodology. For this reason, an outline of the traffic projection methodology, entitled, An Outline of the LTPP Traffic Projection Procedure, is attached to this mailing.

How To Communicate with Us

Please use the blue Feedback and Data Resolution Sheet in front of the packages prepared for the individual sites. Handwritten notes are certainly sufficient. The sheets contain a number of questions and comments on a variety of issues, and seek input on these issues from the representatives of Mississippi DOT. In some situations, it may be appropriate to respond, in addition to using the blue sheet, by other means including telephone and e-mail.

Thank you very much for your help.

INSTRUCTIONS FOR THE REVIEWER

In order to facilitate the review of the projected traffic data, and the identification of site-specific issues (that need to be brought to the attention of the reviewer), a standardized format was developed to report and present traffic data (historical, monitoring, and projected) for the individual LTPP sites. Traffic data for each LTPP site that has WIM scale monitoring data are presented using a set of eight sheets. The eight-sheet set constitutes a standard package for the display and presentation of the projected traffic data and for their review by the representatives of State highway administrations (SHAs). The set contains the following sheets.

  • Feedback and Data Resolution Sheet (figure 3).
  • Site Map (figure 4).
  • Annual Traffic Projection Sheet (figure 5).
  • Projected AADT Truck Volumes (figure 6).
  • Annual Vehicle Class Distribution (figure 7).
  • Annual Load Spectra (figure 8).
  • Average Annual Load Spectrum (figure 9).
  • Projected Annual ESALs (figure 10).

The following briefly describes the purpose of each sheet and highlights typical features and concerns that usually require the review and assessment by SHA representatives.

Feedback and Data Resolution Sheet (Blue Sheet), figure 3

The purpose of this sheet is to summarize all major site-specific features that may influence traffic projection. This is the principal communication tool between the traffic team and the SHA reviewer.

It is expected that the reviewer will complete the blue sheet for each LTPP site under review. Additional comments and suggestions are encouraged. Please use additional sheets or the reverse side if needed.

Site Map, figure 4

The objective is to clearly identify the location of the site in question, as well as the location of nearby sites (which may serve as surrogate source of traffic data).

The location of each site should be verified.

Annual Traffic Projection Sheet, figure 5

The annual traffic projection sheet is used to summarize trends in historical and monitoring traffic data (AADT volumes, AADT truck volumes, average ESALs per day, and TFs).

It is expected that traffic volumes and ESALs will exhibit an increasing trend.
TFs (ESALs per truck) should be at a level or perhaps increasing (to reflect the increased cost-efficiency of the trucking industry).

Projected AADT Truck Volumes, figure 6

This sheet shows historical and monitoring truck volumes and the suggested projection model-typically a smooth line or a curve-used to estimate truck volumes for all in-service years. This model is also used to project (estimate) axle load spectra. If needed, the model can exactly duplicate the reported historical data trends.

Are additional data available from other State sources?

Can a better model be developed and used? If so, please sketch the new model on the sheet.

Annual Vehicle Class Distribution, figure 7

The top half of the sheet shows actual axle counts for different truck classes. The bottom half of the sheet shows the distribution of trucks as a percentage of the total truck count. All available years are plotted to spot outliers and questionable results.

The unexpected truck distributions should be identified. This activity contributes to the judicious selection of the base spectra. Usually, for the same site, annual vehicle class distributions should be similar from year to year. It is expected that the number of Class 9 vehicles (5-axle tractor-semi-trailers) will predominate on rural interstates while, on the other hand, the number of 2-axle single-unit trucks will become significant on urban and semi-urban roads.

Annual Load Spectra, figure 8

Annual axle load spectra are plotted for all available years (the years with monitoring WIM scale data). The annual spectra are used to select/calculate a typical "base" spectrum. The base spectrum is then used to estimate axle load spectra (particularly for years without monitoring WIM data). In some instances, two base spectra are used for the projection: the first spectrum and the last spectrum.

If the site has reliable annual spectra for many years, two base annual spectra may be used for the projection. For example, the first base spectrum is used to represent traffic during the years before the installation of a WIM scale, and during the initial operation of the scale. The second spectrum is used to represent traffic levels for the most recent years, with and without WIM scale data. Both the first and the last spectra can be the averages of several annual spectra.

At most sites, and there are exceptions, most tandem axle loads are caused by Class 9 trucks (5-axle tractor-semi-trailers). Consequently, the distribution of tandem axles (shown in the middle of the sheet) usually has two peaks. The first peak corresponds to unloaded tandems, and the second peak to the fully loaded tandems. These two peaks are usually at 10 to 11 kips and 31 to 33 kips, respectively. (Federal regulations limit tandem axle weight to 34,000 lb.)

It is important to identify all incorrect and suspicious spectra, and to recommend which spectra should be used (averaged) to obtain the base spectrum. Usually, annual load spectra for the same site should be similar. It may be difficult, without inside knowledge, to judge the validity of spectra if there are just a few spectra available to define the expected pattern.

Average Annual Load Spectrum, figure 9 The average annual load spectrum is obtained by calculating the average of the annual spectra, presented in figure 8, that are considered to be valid. It is also the base spectrum used for the projection.

Projected Annual ESALs, figure 10

This sheet provides a summary of equivalent single axle loads (ESALs) projected for all in-service years. ESALs are calculated using the site-specific pavement structure (type and thickness). It is recognized that the purpose of traffic collection and analysis is to obtain axle load spectra (and not ESALs). ESALs are used mainly for comparison and QA purposes.

Are the projected annual ESALs and the cumulative ESALs reasonable? Only pavement professionals can typically answer this question.

Figure 3. Feedback and Data Resolution Sheet for site 285805.
GPS-5 285805 Feedback and Data Resolution Sheet
State / Province Mississippi Direction West
Road Number 10 Total No. of Lanes 4
County / Town Harrison Pavement Structure D=8.2
Functional Class Urban Principal Arterial - State Open to Traffic 1975
Please review the attached sheets (and any other information that you feel is applicable), and respond to the following questions. Please use the reverse side of this form if you need more space.
  1. Were there any sudden changes that could have significantly and suddenly affected the amount of truck traffic on this site (e.g., opening or closing of parallel highways or the arrival of new industry)?
    [   ] Yes [?] No
    If yes, please describe the changes and provide the dates when they occurred
    __________________________________________________
    __________________________________________________
  2. Is there predominant single-commodity traffic on this site (e.g., lumber, grain, gravel)?
    [   ] Yes [?] No
    If yes, what is the commodity and which truck type is used to haul it?
    __________________________________________________
    __________________________________________________
  3. Are there any additional non-LTPP traffic data which could be useful for this site?
    [   ] Yes [?] No
    If yes, please explain
    __________________________________________________
    __________________________________________________
  4. Do you find the following projected numbers to be reasonable? Yes No Comments
    Recent truck traffic volume growth rate 4.5% ?  
    1998 AADT truck volume (LTPP lane) 2430 ?  
    1998 Truck percentage 15% ?  
    Average number of ESALs per truck 1.1 ?  
  5. Please comment below on the following site-specific observations and questions.

    Truck factor has increased from about 1.0 in 1992 to 1.5 in 1996. Is this expected? The average axle load spectra for 1992 to 1995 was used for the projection. Tandem loaded peak is lower than expected. Should we use 1996 axle loads to improve the projection?
    __________________________________________________
    __________________________________________________
  6. Using the rating below, please indicate your opinion about the accuracy of traffic data available for this site.

    Rating: 1. Excellent, 2. Very Good, 3. Good, 4. Fair, 5. Poor
    Historical Truck Volumes [ 3 ] Monitoring truck volumes [ 3 ]
    Historical Truck Loads (ESAL / truck) [ 3 ] Monitoring truck loads / axle load spectra [ 3 ]
  7. What, in your opinion, should be done to improve the accuracy of traffic data for this site (e.g., the type of additional traffic monitoring, use of surrogate data, a more detailed look at past data)?
    __________________________________________________
    __________________________________________________
Completed by: Date: Tel. No.: Email:
Thank you very much for your help.
ERES LTPP DATS Traffic Analysis Team

Figure 4. Site map for site 285805.

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Figure 5. Annual Traffic Projection Sheet for Site 285805.

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Data Type Availability of Monitoring Data
1990 1991 1992 1993 1994 1995 1996 1997 1998 Total
AVC Days - - 266 361 351 363 160 - - 1,501
Month - - 9 12 12 12 - - - 45
WIM Days - - 177 333 351 361 161 - - 1,383
Month - - 6 11 12 12 - - - 41

Figure 6. Projected AADT volumes for site 285805.

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Year AADT Truck Volumes Projected Growth
Historical Monitoring Projected Percentage Factor
1975 970 - 885 - 0.45
1976 1060 - 923 4.2 0.47
1977 1160 - 962 4.2 0.49
1978 1310 - 1003 4.3 0.51
1979 1300 - 1046 4.3 0.53
1980 1330 - 1091 4.3 0.55
1981 1440 - 1139 4.4 0.58
1982 1480 - 1189 4.4 0.60
1983 1530 - 1241 4.4 0.63
1984 1660 - 1296 4.4 0.66
1985 1720 - 1354 4.5 0.68
1986 1790 - 1415 4.5 0.72
1987 1830 - 1479 4.5 0.75
1988 2070 - 1545 4.5 0.78
1989 2190 - 1616 4.5 0.82
1990 1544 - 1690 4.6 0.85
1991 1524 - 1767 4.6 0.89
1992 - 1789 1848 4.6 0.93
1993 - 1937 1934 4.6 0.98
1994 - 2079 2023 4.6 1.02
1995 - 2105 2118 4.7 1.07
1996 - 2307 2217 4.7 1.12
1997 - - 2320 4.7 1.17
1998 - - 2429 4.7 1.23

Figure 7. Annual vehicle class distribution for site 285805.

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Figure 8. Annual load spectra for site 285805.

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Figure 9. Average annual load spectrum for site 285805.

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Figure 10. Projected annual ESALs for site 285805.

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Year Annual ESALs
Historical Monitoring Projected
1975 330000 - 359237
1976 371000 - 374473
1977 414000 - 390386
1978 479000 - 407112
1979 486000 - 424689
1980 509000 - 443146
1981 565000 - 462405
1982 595000 - 482762
1983 631000 - 504029
1984 702000 - 526419
1985 748000 - 549829
1986 799000 - 574507
1987 840000 - 600428
1988 978000 - 627632
1989 1063000 - 656134
1990 837000 - 686160
1991 827000 - 717567
1992 - 618395 750656
1993 - 744923 785370
1994 - 833212 822124
1995 - 1017005 860362
1996 - 1256965 900519
1997 - - 942662
1998 - - 986994
Cumulative - - 14835602

Evaluation and Testing of LTPP Traffic Feedback and Resolution Package

The package described in the preceding section was the final version of the one that was distributed to the majority of the participating agencies. It was developed and refined over the course of several months through an interactive process involving reviews by interested parties and by representatives of several participating agencies. The two main review activities were:

  • Review of the package by representatives of RCOs and by other interested parties.
  • Review and enhancement of the package by pilot studies.

In addition, throughout the course of Phase 2 work, several small changes and enhancements to the package were instituted.

Review by Representatives of RCOs and Other Interested Parties

The important role of the RCOs in the projection process was recognized from the beginning of the study. Representatives of RCOs reviewed the first version of the LTPP Traffic Feedback and Resolution Package and subsequently reviewed agency-specific packages for all participating agencies in their respective regions.

The first package also was submitted to all members of the LTPP ETG on Traffic Data Collection and Analysis for review and comments, and a later version was presented during an ETG meeting in the spring of 2000. Reviews of an early version of the Package were also obtained from the representatives of the Technical Support Services Contractor and from the Minnesota Department of Transportation (Mn/DOT).

All reviewers provided valuable comments and suggestions on how to make the package more effective and user-friendly.

Review and Enhancement of the Package by Pilot Studies

During Phase 1 traffic projection work, it became apparent that it was necessary to involve representatives from the participating agencies in the traffic projection process because many data problems cannot be resolved without their involvement and help. The primary contact between the participating agencies and the LTPP program has been through RCOs. RCOs provide general technical support to the agencies regarding traffic data collection and analysis issues and are responsible for initial traffic data quality checks and for processing traffic data collected by the agencies. Consequently, the involvement of both the participating agencies and RCOs in traffic data assessment and projection is important. Pilot studies brought together the representatives of the participating agencies, RCOs, the project team, and others.

The main feature of the pilot studies was a 1-day meeting that was held at the participating agencies and attended by representatives of the participating agency (typically including an LTPP contact engineer, personnel responsible for traffic data management, field traffic data collection personnel, and others), RCOs (personnel responsible for traffic data), FHWA (representing the LTPP program, and FHWA Division Office), and two members of the project team.

The purpose of the pilot studies was to review and enhance the process of traffic data assessment and projection of traffic loads, including the content of the LTPP Traffic Feedback and Resolution Package, and to discuss specific issues concerning the agencies' LTPP sites. The pilot studies also provided an opportunity for the representatives of the RCOs to become better acquainted with the package and traffic data assessment and projection issues. For this reason, one pilot study took place in each of the four LTPP regions:

Agency Meeting Place LTPP Region
California DOT Sacramento, CA Western Region
Florida DOT Tallahassee, FL Southern Region
New Jersey DOT Trenton, NJ North Atlantic Region
Indiana DOT Indianapolis, IN North Central Region

During the course of the meeting, attendees discussed the LTPP package prepared for the agency. Agency representatives provided comments on specific LTPP sections following the format of the Feedback and Data Resolution Sheet (figure 3).

The pilot studies resulted in several improvements to the package, and in better communication among all interested parties, particularly between the representatives of the RCOs and the project team.

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The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT).
The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT). Provide leadership and technology for the delivery of long life pavements that meet our customers needs and are safe, cost effective, and can be effectively maintained. Federal Highway Administration's (FHWA) R&T Web site portal, which provides access to or information about the Agency’s R&T program, projects, partnerships, publications, and results.
FHWA
United States Department of Transportation - Federal Highway Administration