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WIM Data Analysts Manual


Section 1. Introduction

Weigh-in-Motion (WIM) systems meeting the Type I requirements of ASTM E 1318 have the capability of producing continuous high quality traffic data for multilane roadway locations. These WIM systems produce various data elements for each vehicle passing through the site, including:

  • Time and Date
  • Lane
  • Speed
  • Vehicle Classification
  • Wheel Load
  • Axle Load
  • Axle Group Load
  • Gross Vehicle Weight
  • Individual Axle Spacings
  • Overall Vehicle Length
  • Violation Code

It is noted that some of the listed data elements may not actually be stored by the system as raw data onsite, but is instead generated during the data processing session following transfer of the raw data to the Office Computer (discussed below). The stored data element "Lane" is typically the "WIM" lane number as determined by a particular system's sensor configuration and sensor inputs to the controller unit. However, for reporting purposes, the vendor's or agency's application software is programmed to display the data for a lane or lanes based upon the agency's lane designation (e.g. "Northbound No. 1").

1.1. OVERVIEW OF WIM DATA

A WIM system's controller typically stores both summary (binned) data and vehicle record data for each day.

  • Binned data
    • All of a day's vehicles are typically binned by count for hour of day, lane, classification, and speed range (see previous explanation of "lane" as stored by the controller).
    • Contain no individual vehicle data elements.
  • Individual vehicle record data
    • Include data elements for individual vehicles.
    • Typically the system allows the user to define parameters, such as classification or front axle weight threshold, which determine whether a record is stored for a particular vehicle or whether the vehicle is simply counted in bins.
    • These individual vehicle records are sometimes referred to as "Per Vehicle Records" (PVRs) or "Truck Records".

It is the function of the WIM system's onsite controller to process inputs from the in-road sensors and to create and temporarily store the binned data and the individual records, typically in binary format. This raw data is routinely downloaded or otherwise transferred to the analyst's Office Computer (sometimes referred to as Host Computer). An application software program provided by the WIM system vendor is then utilized to process the raw data, including the generation of reports and ASCII files and the view of individual vehicle records. Some agencies utilize their own custom application software to process the raw data. Also, some agencies utilize their own or third party software to automate the raw data downloads and/or perform data validation checks.

1.2. SIGNIFICANCE OF HIGH QUALITY WIM DATA

Truck wheel loading data is of particular interest to determine inputs to the Mechanistic-Empirical Pavement Design Guide (M-E PDG) software. However, for loading data to be considered of high quality, such data must meet the ASTM E 1318 Type I requirements for accuracy displayed in Figure 1.

Figure 1. List. ASTM E 1318 Type I WIM Systems Requirements.  For loading data produced by a Type I WIM system to be considered of high quality, 95 percent of such data must be within tolerance of the listed accuracy requirements when compared to reference values obtained by weighing and measuring static vehicles.  First, wheel loads must be within plus or minus 25 percent. Next, axle loads must be within plus or minus 20 percent. Also, axle-group loads must be within plus or minus 15 percent. Last, gross-vehicle weights must be within plus or minus 10 percent.
Figure 1. List. ASTM E 1318 Type I WIM Systems Requirements.

Note that high quality WIM equipment properly installed in structurally sound and smooth pavement, at a site with proper roadway geometry and traffic operating characteristics, has the capability to produce loading data with much higher accuracy than those required by ASTM E 1318. However, to produce high quality data, the WIM system must be properly monitored and maintained.

1.3. OVERVIEW OF THE MANUAL

This Manual describes recommended procedures to be followed by an agency's WIM Office Data Analyst in performing data quality checks of WIM traffic data. This section (Section 1) provides an overview of a WIM system's function and its data output, describes the quality of WIM data that are addressed in this manual, and provides an overview of the manual itself.

SECTION 2 provides information on "WIM Basics" which may be helpful to the novice analyst.

SECTION 3 provides guidance and recommendations on performing data validation and performing system monitoring remotely from the office, including remote real time checks of traffic, reviewing reports generated by the office computer's WIM application software, and follow-up procedures to be performed when questionable data is identified.

SECTION 4 discusses procedures for performing extensive analyses of individual vehicle records by importing the WIM data into spreadsheet or database programs.

SECTION 5 discusses procedures for monitoring a system's calibration over time and procedures that may be taken to fine-tune a system's calibration factors in order to provide the most accurate size and weight data possible.

Contact

Mike Moravec
Office of Transportation Performance Management
202-366-3982
E-mail Mike

 
 
Updated: 01/04/2013
 

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United States Department of Transportation - Federal Highway Administration