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Publication Number: FHWA-RD-06-067
Date: March 2006

Guidelines for The Collection of Long-Term Pavement Performance Data

Chapter 10. Traffic Data

10.1 Introduction to Traffic Data

Traffic data are collected separately for the lanes (or lane) being monitored. Because each lane experiences different traffic, each should be considered as a separate test section. For LTPP, data collection is planned only for the outside lane in one direction.

The traffic data include distribution of traffic by vehicle classes; days of data collected; and distribution of axle loads for single, tandem, and tridem axles by vehicle class. For locations where traffic data have been submitted for all lanes, the data may include average annual daily traffic (AADT) and percent trucks.

Data forms have been developed to capture information on sections inservice before the start of monitoring in 1990. Data collected from 1990 forward been submitted in one of the electronic record formats documented in FHWA's Traffic Monitoring Guide (TMG), second through fourth editions. Data are submitted between twice a month and annually depending on the practices of the individual agencies. Except for special cases, monthly submittal is preferred, and quarterly is typical. In the absence of electronic data on a section, a set of data forms has been provided for recording estimates of traffic during that timeframe.

Data for individual vehicles require considerable computer storage, so these are expected to be stored offline rather than directly in the LTPP IMS. Appropriate summary data such as numbers of axles in certain weight categories, vehicles in certain classes, equivalent axle loads, etc., are to be calculated from the raw data and stored in the LTPP IMS. The raw data will be available for use when needed.

The traffic data file will include both historical data before initiation of the monitoring activity and data collected throughout the monitoring period. Data have been submitted using the TMG classification schemes (Truck Weight Study 6-digit and 13-bin) as well as agency-defined schemes. All offline data and daily summaries are provided in the classification scheme used for submission. All data provided from online sources summarized at the monthly or annual level are in the TMG 13-bin classification scheme. The database does not contain information on the algorithms used or the changes that may have been made to the algorithms over time. Thus, for example, the impact of large pickups cannot be explicitly traced in the data.

Traffic data reporting for LTPP is separated into two categories: historical and monitoring. Historical traffic data are defined to cover the period from the dates the pavement sections were initially opened to traffic (or from the date of the most recent overlay or rehabilitation project) through 1989. The overall purpose of historical traffic data collection is to obtain the best estimate of annual traffic levels on each test section before the time monitoring began on that section. The monitored traffic data cover traffic data collection activities initiated for monitoring the LTPP test section.

10.2 References Used in Collecting Traffic Data

10.2.1 Traffic Data Collection Plans

The DCG provides a detailed description for collecting historical data and a general description for collecting monitored traffic data.

The following document provided a revised and more detailed plan for the collection and processing of traffic data to prepare the data for entry into the IMS.

Additionally, a separate document was issued providing a procedure to check the equipment used in traffic data collection.

The Revised Data Collection Plan and the Protocol for Calibrating Traffic Data Collection Equipment were subsequently combined into a single document along with all the additional required information used in collecting traffic data for the LTPP program.

A separate set of documents was prepared to provide guidelines for collecting data at SPS projects. The following references provide specific requirements for performance of weigh-in-motion (WIM) equipment at these locations and collection of data at SPS projects.

The following two documents provide details on equipment installation requirements for bending plate WIM equipment.

The following document, which will codify a range of operational practices, is under development.

10.2.2 Software Guides

Software to perform quality control checks and process/accumulate the desired data to be stored in the IMS has been a very important part of traffic data collection.

The first set of software guides was issued in 1997.

The software for performing quality control checks and data analysis has been revised and reissued. The documents referenced below provide the guidelines for use with this revised software.

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