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 (cont'd)

Step 6–Review of LTPP Traffic Feedback and Resolution Packages by RCOs

The collection of traffic data on LTPP sites has been the responsibility of the participating agencies. However, throughout the LTPP program, RCOs have been responsible for many activities concerning traffic data, such as:

• Communicating the LTPP traffic data collection requirements to the participating agencies.
• Providing technical assistance to the agencies regarding traffic data collection and traffic data handling and reporting.
• Providing QA for traffic data supplied by the participating agencies.
• Inputting and storing traffic data in the regional traffic database.

Figure 19. Projected AADT truck volumes (initial) for site 473104.

Because RCOs are responsible for communicating with participating agencies and have detailed knowledge of local traffic data and issues acquired through long–standing cooperation with local agencies, the LTPP Traffic Feedback and Resolution Packages were first sent to RCOs for review and comments. Representatives of RCOs reviewed the packages, often providing written comments to the project team, and chose one of the following alternatives:

• Sending the package to the agency with a standard introductory letter that asked the agency to review the package and respond to questions. The introductory letter became part of the package as outlined in the section describing the content of the package. About 60 percent of the packages were sent this way.
• Sending the package to the agency using an introductory letter that provided additional information to the agency regarding traffic data. Typically, this information was about traffic data that were not included in site–specific reports (because the data had not yet reached the IMS database). Some RCOs also included handwritten notes on the packages.
• Asking the project team for explanation or for modifications in the package before sending it to the agencies. Typically, modifications requested by RCOs concerned the initial overall feedback and resolution report. About 15 percent of the packages were modified by the project team after initial submission to the RCOs. Once the explanation and modifications were provided to the satisfaction of the RCOs, the RCOs sent the packages to the participating agencies for review.

Step 7– Review of LTPP Traffic Feedback and Resolution Packages by Participating Agencies

It was recognized early in Phase 1 that the involvement of the participating agencies in the traffic data assessment and traffic load projection process would be essential for meeting the project objectives. Specific issues requiring local agency involvement included trends in annual historical and monitoring truck volumes, changes in local traffic patterns due to changes in highway network, influence of predominant single–commodity traffic, existence of local truck weight and size regulations and truck operating permits, and the availability of local QA/QC information.

All LTPP Traffic Feedback and Resolution Packages were sent to the participating agencies by the end of March 2001. By the end of May 2001, the project team received responses from about 40 percent of participating agencies. These responses varied considerably. Some reviews may not have been as useful as was originally anticipated, while others were very useful, insightful, and even included additional supplemental traffic data.

Representatives of the participating agencies were asked specifically to respond to two items of the LTPP Traffic Feedback and Resolution Package: the initial overall feedback and resolution report and the Blue Sheet (figure 3) that was part of the initial site–specific report.

The initial overall feedback and resolution report contained general questions regarding the procedures used to classify vehicles and to measure axle loads; responses varied considerably. Regarding vehicle classification, some agencies assigned unclassified vehicles proportionally to all vehicle classes unless there was a clear indication that a Class 14 vehicle belonged to a particular class. Other agencies simply reported the existence of Class 14 vehicles. Regarding WIM calibration procedures, some agencies attempt to carry out periodic verification of the accuracy of WIM scales, while others had not calibrated their scales beyond the initial calibration. Some agencies reviewed traffic data before their submission to RCOs and some did not.

The Blue Sheet was the main means of communication between the participating agencies and the project team; it contains seven parts (figure 3):

• The first part solicits additional information on truck volumes.
• The second part concerns the existence of single commodity traffic on the LTPP site. With very few exceptions, the LTPP sites were not exposed to single or predominant commodity traffic.
• The third part enquires about the existence of additional traffic data, other than the one submitted to LTPP, that may be useful for traffic data assessment and projection of traffic loads. This question was probably not specific enough because most respondents answered that no additional data existed. However, most agencies have extensive traffic databases that may yield useful information.
• The fourth part asked the agencies to comment on four specific traffic data characteristics. These characteristics were selected to provide an overall assessment of traffic loads and to motivate the reviewer to compare the reported LTPP data with data from other sources. The question regarding the 1998 truck percentage was included to obtain verification of truck volumes based on AADT volumes.
• The fifth part was used to solicit an agency's response to specific observations and questions. For some sites, the responses to the observations and questions were used to change the initial projection confidence codes.
• The sixth part asked the reviewer to provide an opinion regarding the accuracy of historical and monitoring traffic data. The respondent for site 285805 (figure 3) felt that the accuracy of both the historical and monitoring traffic data were similar (good).
• The seventh part solicited the reviewer's opinion for improving the quality of traffic data. Typical responses included the installation of new traffic data collection equipment and the calibration of WIM scales.

Step 8–Implementation of Review Comments Received from Participating Agencies

This last activity consisted of three tasks:

• Responding to agencies.
• Preparation of revised projections.
• Placing traffic data into computed parameter tables.

Responding to Agencies

A response letter was prepared for each agency that provided a review of the LTPP Traffic Feedback and Resolution Package. The response letter included a listing of specific changes, if any, that were made to the initial projections based on the agency's recommendations.

Preparation of Revised Projections

The preparation of revised projections also included the assignment of revised projection confidence codes. The revised projection confidence codes were assigned using the same guidelines as those used for the assignment of the initial projection confidence codes, but also utilized information supplied by the participating agencies.

Based on the comments received from the agencies, about 10 percent of the initial projections were revised and about 15 percent of the initial projection confidence codes were changed. Typically, the change was from "questionable" to "acceptable" code.

Placing Projected Traffic Data into Computed Parameter Tables

The projected traffic data that were placed into the computed parameter tables (these tables will become a part of the IMS database) included traffic projections with both acceptable and questionable projection confidence codes, whether or not they had been reviewed by the participating agencies. The projection confidence code and the review status information were included in the database. The description of computed parameter tables used to store the projected traffic data is provided in chapter 4.