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2.6 Weight Data Collection The truck weight data-collection program is somewhat different from the classification data- collection effort. Truck weight data are primarily collected using WIM technology, although a few state highway agencies also use weight data collected from static scales. In an ideal world, vehicle (and axle) weights would be collected continuously at a limited number of per- manent locations as well as at a large number of sites where only short weighing sessions would be performed. Unfortunately, both the cost of WIM data-collection and functional limitations in WIM sensor technology restrict the number and location of data-collection points at which state highway agencies can collect WIM data. WIM equipment only works accurately on flat, smooth pavements that are in good condition.9 In addition, each time a WIM scale is placed in or on a pavement, the effects of road profile and roughness on vehicle dynamics mean that the scale must be recali- brated in order to collect data accurate enough to be used as input to the pavement design process. Because calibration of a WIM scale is both time consuming and expensive, the cost of using accurate portable equipment is very high. Similarly, the cost of permanently installing WIM sensors is also high. The result is that state highway agencies generally operate relatively few WIM sites. The design of the WIM data-collection program is intended to obtain the best weight data for traffic load estimation, given these limitations. The first recommendation of the Traffic Monitoring Guide is to make sure that the data being collected are accurate. This often means that the number of WIM sites must be reduced in order to ensure that the sites that are used are supplying accurate data. The NCHRP 139 proj- ect team fully endorses this recommendation. Use of weight data from poorly calibrated WIM scales can create significant biases in the pavement design process,10 leading to unreliable pavement performance. Given a limited number of WIM locations within each state, it is recommended that those sites be distributed across the state in such a way as to discover and measure truck weight patterns that differ by geographic region and/or by type of road. Thus, in a state such as Kentucky, some scales should be placed on roads that carry significant volumes of coal trucks, while other scales should be placed on roads that carry little or no coal traffic. Data from these dif- ferent locations are then summarized to create regional load estimates that can be used within the TrafLoad and Pavement Design Guide design software. When deteriorating pavement conditions at a WIM site cause the data to become unreliable, the WIM equipment should be moved to a new location where little is known about truck weights. This allows a state to slowly expand the geographic coverage of its weight data-collection effort. Slowly expanding the geographic coverage of the WIM program helps an agency learn about 9 See the equipment descriptions in Cambridge Systematics, Inc., and Washington State Transportation Center, op. cit. 10 WIM Calibration, a Vital Activity, FHWA Publication Number FHWA-RD-98-104, July 1998. 2-21
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the variations in trucking characteristics that occur in the state, while staying within available data-collection budgets. Where resources exist to collect and analyze the data collected, WIM sites should operate as permanent, continuous data-collection sites. (Note that these sites also provide continuous classification data as well as continuous volume data and thus take the place of ATRs and per- manent vehicle classifiers.) Analyses of these continuous data sources allow states to learn if truck weights are changing over time or if they change by season of year or even by time of day. These data can also be used to direct the timing of enforcement actions intended to pre- vent overloaded trucks from using the roadway system. Where resources for analyzing WIM data are extremely limited, discontinuous data may be col- lected, even from permanently mounted sensors. Limiting the data collected from permanently mounted sensors simply allows the state highway agency to focus its available resources on productive data-collection efforts. Where possible, even in cases of limited resources, sufficient data should be collected to measure possible changes in vehicle and axle weights over time. 2-22