High-Speed Weigh-in-Motion System Calibration Practices (2008)

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5Weigh-in-motion (WIM) is the process of weighing vehicle tires or axles at normal roadway speeds ranging up to 130 km/h (80 mph). WIM systems also are capable of measuring and calculating various other traffic data elements, such as axle spacing, vehicle classification, gross vehicle weight (GVW), and equivalent single-axle loads (ESALs). WIM systems con- sist of sensors embedded into the pavement surface and a data acquisition system equipped with software capable of pro- cessing sensor signals into weight, computing additional traf- fic data elements, and summarizing them into various database formats. Since their inception, the performance of WIM systems in capturing truck weight data has been the focus of considerable investigation. There has been a multitude of reports docu- menting WIM system calibration methods and practices in the United States and internationally (1–3), whereas considerable research has taken place to improve these methods (4,5). Through this process, two standards of on-site calibration have emerged, one developed by ASTM (6) and the other developed by the European Cooperation in the Field of Science and Technology (COST) 323 study (7). Recently, the deployment of WIM systems has prolifer- ated through initiatives such as the Long Term Pavement Performance (LTPP) and the Commercial Vehicle Informa- tion Systems and Networks (CVISN) programs. Addition- ally, WIM data are being used extensively for other purposes, such as pavement design, bridge design, monitoring highway usage, and highway cost allocation. High-quality WIM data are essential to these applications. The new Mechanistic- Empirical (ME) Pavement Design Guide developed under NCHRP Study 1-37A, for example, requires WIM data for predicting performance in terms of the number of years it takes for pavement distresses to become critical. Poor-quality WIM data may lead to significant overestimation of this per- formance period and, hence, lead to premature functional failures. Similarly, high-quality WIM data are essential to research. On-going bridge design research under NCHRP Project 12-76, Protocols for Collecting and Using Traffic Data in Bridge Design, for example, relies on WIM data to revise design truck loading configurations for bridges. In addition, accurate WIM data are essential in establishing the level of pavement utilization in design-built projects, where premature pavement failures become the subject of litigation. These examples demonstrate the urgent need for ensuring WIM data accuracy. This is accomplished through routine WIM system calibration and periodic WIM data quality con- trol (QC). The way these tasks are carried out varies widely among agencies. The goal of this study is to synthesize the state-of-the-practice in high-speed WIM system calibration. Practice relates to the operational procedures used by state agencies to evaluate the in situ performance of WIM systems in terms of their load measuring accuracy, rather than the equipment-specific technical details used by WIM suppliers and installers for obtaining these measurements. OBJECTIVES The objective of this synthesis is to assemble state-of-the- practice information on the methodologies used by state agencies in evaluating and calibrating high-speed WIM sys- tems, as well as in monitoring WIM calibration over time. This objective was addressed through a thorough literature review and a survey questionnaire. The literature review covered U.S. and international sources on standards, prac- tices, and recent research efforts on WIM calibration. The survey questionnaire was used as the main instrument for collecting state-of-the-art practice information from state agencies in the United States. It was addressed to the managers of the state agencies that administer WIM systems, including those used for data collection and/or load limit enforcement screening. DEFINITIONS The following definitions clarify some of the main terms used throughout this report and the survey questionnaire. High speed—highway speeds of up to 130 km/h (80 mph) and, as such, includes permanently installed WIM sys- tems on mainline and is used for continuous data collec- tion or enforcement screening. Therefore, this excludes portable WIM systems used for temporary data collec- tion and enforcement screening WIM systems installed on approach ramps to truck inspection stations. WIM system refers to one controller, its computer and associated electronics, and all roadway sensors for all lanes for which traffic data are being processed by the controller and at least one lane is instrumented with weigh-in-motion sensors. WIM site—a specific roadway location at which a WIM system has been or will be installed. Such a site includes CHAPTER ONE INTRODUCTION

all WIM in-road components, the WIM controller and its electronics, the power and communication facilities, all wiring, conduits, pull boxes, and cabinets necessary to make the WIM system functional, the pavement sec- tion in which the roadway components are installed, and the pavement approach and departure from the in-road sensors. WIM lane—any lane that is instrumented with weigh-in- motion sensors. MS-WIM or multi-sensor WIM system refers to WIM systems using multiple piezoelectric sensors. Type I and Type II WIM—definitions given by ASTM E1318-02. Type I systems weigh the right- and left- hand side axles individually, whereas Type II systems weigh entire axles. ASTM E1318-02 accuracy toler- ances are more stringent for Type I systems than for Type II systems. Type I systems are typically equipped with bending plates, load cell plates, or quartz piezo- electric sensors. Type II systems are typically equipped with 1.8-m (6-ft) or 3.6-m (12-ft) long ceramic or poly- mer piezoelectric sensors in various configurations. Piezoelectric (also referred to as piezo) sensor manu- facturers typically rate their sensors as Class 1, which are designed for weighing, and Class 2, which are designed to act only for axle detection. Site assessment encompasses on-site activities preceding either an on-site evaluation or calibration to document that a WIM system is operational, the sensors have no visible problems, and the pavement condition shows no apparent deterioration. Evaluation/validation—on-site activities related to ascer- taining compliance of WIM systems to error tolerances. This involves test trucks or samples of trucks from the traffic stream. It includes on-site testing for initial sys- 6 tem acceptance, routine checks for calibration mainte- nance, and conformance to warranty requirements. Calibration involves adjusting the system’s calibration factors by setting the mean error measurements to zero. The data for determining such errors are typically obtained from test trucks or traffic stream trucks of known static weights. Calibration factor—a user-defined value that is imple- mented by a WIM system to convert raw sensor output into weights. Calibration factor speed point (also referred to as speed “bin”)—a user-defined speed for which a weighing sensor’s calibration factor can be entered. Certain WIM systems provide for three or more calibration factor speed points that allow the user to determine appropri- ate calibration factors over a range of vehicle speed that will best compensate for the effect of speed. WIM calibration monitoring—data analysis that typi- cally involves comparisons of representative traffic stream values to known load trends (e.g., the weight of steering axles of five-axle semi-trailers varies within a fairly narrow range and their gross vehicle weight exhibits a distinct double-peak pattern). Autocalibration—a mechanism built into WIM software effecting automatic calibration adjustments when cer- tain measurements fall outside prescribed limits. Class 9 vehicles, under FHWA Scheme F, include all five-axle vehicles consisting of two units, one of which is a tractor or straight truck powered unit. 3S2 trucks are Class 9 vehicles consisting of three-axle tractors and two-axle semi-trailers. Class 5 vehicle refers to the FHWA vehicle classification system and encompasses all two-axle, six-tired single- unit trucks.