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50 change from vendor to vendor and from site to site.) However, listed in Table 5.2 has been developed for LTPP. A brief dis- when budgeting for new sites, initial costs should also include cussion of the method was presented in Appendix 2 of the any necessary pavement rehabilitation costs (although those States' Successful Practices Weigh-in-Motion Handbook. costs are often paid out of other funding sources). Pavement The LTPP calculation allows inclusion of specified pave- rehabilitation to achieve necessary smoothness levels is not ment rehabilitation and maintenance. The spreadsheet used by a function of the equipment technology selected. Accuracy LTPP to compute WIM cost estimates is available through the degrades for all types of WIM equipment when they are LTPP web site at http://www.tfhrc.gov/pavement/ltpp/spstraf- placed in rough pavement. Other initial costs include vehi- fic/index.htm. While now several years old, the spreadsheet cle presence and weight sensors, roadside electronics, road- allows input of up-to-date cost components (including pave- side cabinets, and installation. Annual recurring costs include ment rehabilitation costs), as well as the costs and character- site maintenance, system maintenance, calibration, and per- istics of new WIM equipment. formance evaluation. Site design life and expected sensor life can be combined to predict the estimated initial cost per lane and the estimated 5.5 DEVELOP, USE, AND MAINTAIN average cost per lane over the selected site design life. For A QUALITY ASSURANCE PROGRAM example, Table 5.2 provides an estimate of system perfor- No matter how much money is budgeted and spent for mance, initial cost per lane, and average annual cost per lane the initial purchase and installation of a WIM site, all WIM (not including pavement rehabilitation costs). This compari- equipment requires continual care and attention. Without son of performance and cost is based on the information ini- ongoing attention to equipment performance and data col- tially provided in the States' Successful Practices Weigh-in- lection site conditions, equipment performance will degrade Motion Handbook, dated December 1997. The performance over time. While vehicle classification equipment tends to be of the systems is given as a percent error on gross vehicle more robust (it is less sensitive to calibration drift), it requires weight (GVW) at highway speed and is contingent on the periodic attention and continuous monitoring. site's meeting ASTM E 1318 standards. The estimated initial Consequently, another key practice is for highway agen- cost per lane includes the equipment and installation costs, cal- cies to implement and use a quality assurance program that ibration, and initial performance checks. It does not include the monitors data being collected and reported. cost of traffic control. The estimated average cost per lane is It is poor practice to simply place equipment and hope that based on a 12-year site design life and includes expected an autocalibration function will soon bring the system into maintenance and the cost of periodic calibration and valida- calibration. While autocalibration has some important uses, tion checks. The system maintenance is based on a service all autocalibration functions have significant shortcomings. contract with the system provider. Each relies on the concept that some particular traffic value A more detailed method (including a simple cost-calculation will remain constant over time, and that constant value can spreadsheet) that includes all the site cost considerations be used to tune the calibration of the data collection device. TABLE 5.2 WIM system accuracy and cost comparison Estimated Initial Cost Estimated Average Cost Performance Per Lane Per Lane Per Year1 (Percent Error on GVW (Equipment and (12-Year Life Span2 WIM System at Highway Speed) Installation Only) 1 Including Maintenance) Piezoelectric Sensor ± 10% $22,600 $7,350 Bending-Plate Scale ± 5% $37,600 $7,900 Piezoquartz Sensor ± 5% $43,600 $10,100 Single Load Cell ± 3% $73,900 $8,800 Notes: 1. Pavement rehabilitation costs are not incorporated in this estimate or the average annual cost. 2. Some of these systems are unlikely to reach a 12-year life span due to early sensor failure, failure of the pavement/sensor bond, or deterioration of the pavement condition itself.
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51 (The most common value used is the mean front-axle weight classification count that can be compared with reported traf- of FHWA Class 9 trucks.) Unfortunately, these values often fic counts. For WIM equipment, it is often necessary to val- are not constant. Even more importantly, there often are site- idate the calibration setting for the site. specific variations in the values of these variables. Thus, unless The following types of data checks are often used in the the autocalibration function is first independently measured quality assurance process: and tracked at a site, the equipment controlled entirely by an automatic self-calibration function will be miscalibrated, pro- · Has the location of either the loaded or unloaded peak ducing biases in the data collected. in the GVW distribution for the FHWA Class 9 trucks Calibration problems identified by a quality assurance pro- changed since calibrated data were last collected at this gram may also not be solved through simple adjustment of site? (See Figure 5.1.) Other vehicle classes that exhibit the calibration factor for the scale. In many cases, calibration a common loading characteristic at the site in question drift is a symptom of a larger problem (pavement deteriora- can also be used in this data review. tion, sensor degradation, etc.) that requires a site visit and · Has the mean front-axle weight for loaded FHWA equipment or site maintenance action. Class 9 trucks changed since calibrated data were last Quality assurance programs are designed to review col- collected at this site? lected data and report unusual or unexpected results. In many · Has the percentage of all weekday trucks that are clas- ways, this is similar to how many autocalibration systems sified as FHWA Class 9 changed significantly from pre- work. Where they differ is that quality assurance programs vious counts at this site? Did percentages increase in should not result in automatic changes to the data collection classifications that indicate malfunctioning classification equipment or collected data. Instead, problems identified by equipment (e.g., an increase in FHWA Class 8 would the quality assurance process should result in an independent indicate a missed axle)? review of the operation of the equipment. Only after this · Did the number of unclassified vehicles increase to unex- independent check takes place should data and equipment be pected levels? discarded or adjusted. · Did the number of counting errors reported by the For permanently installed sensors, unusual data flagged by equipment increase to unexpected levels? the quality assurance process normally means that a site visit · Are the left and right wheel path sensors (for those scales should occur to check the performance of sensors and their with multiple sensors) reporting similar axle weights? connected electronics. Such a site visit should include a visual · Has the measured distance between axles for tractor drive review of pavement and sensor condition and a short, manual tandem axles changed? 14 Original Calibration 12 Shift Indicating Calibration Drift 10 Percent of Trucks 8 6 4 2 0 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80 84 88 92 96 100 104 108 112 GVW (thousands of pounds) Figure 5.1. Use of GVW of FHWA Class 9 trucks to detect scale calibration drift.