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48 If the count is moved, it should be placed so that truck traf- tions can be removed intact from the roadway and reused. fic being measured is as closely related to the actual project (This does not include technologies such as bending plates, traffic as possible. If the project location, for example, is on where the sensor itself can be removed, but the frames into I-80 in Wyoming, a valid data collection location could be sev- which the plates are set are not removable.) Thus, it makes eral miles away. However, if the project location is on I-95 in little sense to design a 5-year WIM site for a pavement that New Jersey, the count most likely needs to be taken within will be repaved in 3 years. the same set of interchanges as the pavement project. For WIM data collection, site failure is often the result of Selecting data collection locations for pavement design failure of the pavement condition around the site, not just the purposes can also be affected by the need to coordinate with failure of sensors themselves. Thus, site design life is a func- other data collection needs. A highway agency may be will- tion of the fatigue life of the sensor itself, the installation ing to accept some minor error in the traffic loading estimate quality of the sensor, the initial site condition and design, and in order to reduce the total counting burden of the state, and the expected wear on the pavement. the agency thus may choose to use an existing count that is Sensor fatigue life is usually a function of the sensor slightly removed from the project location rather than go to design and the traffic loadings. Vendors normally warranty the expense of collecting newer, more precise information. their sensors for a specified period, and obtaining a warranty Even broader flexibility is available to highway agencies is itself a recommended best practice. Sensors with longer as they select those locations where data are collected to fatigue lives are usually more expensive than shorter-lived compute the TWRGs. The primary goal of the TWRG is to sensors. provide an accurate measure of average conditions for a However, many WIM systems become inoperable not given set of roads. Given the lack of weight data available to because of sensor failure, but because of the failure of pave- most highway agencies and the cost and difficulty of col- ment around them. This includes both when the pavement/ lecting accurate weight data, most agencies know relatively sensor bond fails and when pavement deterioration such as little about the vehicle weights present on specific roads. rutting exposes the sensor to impact loads (e.g., snowplow Thus, considerable latitude is available in the selection of blades) that cause catastrophic failure. A primary cause of data collection sites that are included in the TWRG compu- premature pavement/sensor bond failure is poor initial instal- tations because most agencies have little information upon lation quality. This includes such errors as poor mixing of which to judge alternative locations and any valid data are adhesives, poorly cleaned or dried pavement cuts, incompat- better than no data. ibility of sealants and pavement, and inappropriate tempera- The first criterion of TWRG formation is that the sites be ture conditions. similar in characteristics to the other roads they represent. Site condition and site design are key areas that successful (For example, the shape of the axle load distribution associ- programs examine as part of WIM site design and imple- ated with FHWA Class 9 trucks should be similar at all sites mentation. Where remaining pavement life is only modest, within the TWRG.) The second criterion for data collection strong consideration should be given to rehabilitating the is that the sites selected be conducive to accurate weight data pavement prior to WIM sensor installation if an extended collection. This means that the pavement should be in good design life for the site is desired. Unfortunately, pavement condition. It should be flat, with no ruts. The pavement should rehabilitation is a costly addition to WIM installation. How- be strong enough to support weight sensors effectively under ever, if a scale site is expected to have a long life, life-cycle whatever environmental conditions are present when weight costs are far lower if the pavement at the site is rehabilitated data are being collected. prior to initial sensor installation. It is recommended that, at least initially, data collection for In many cases, highway agencies have found it to be a TWRG development be oriented toward sites at which accu- wise investment to build 300-foot concrete pavement sec- rate data can most confidently be collected. As budgets permit, tions into which WIM scales are placed. This gives agencies the weight data collection program should then be expanded smooth, strong, maintainable platforms in which to place sen- or moved to other locations around the state (where WIM sors. Strong concrete pavements generally do not change equipment can be accurately operated) in order to gain a more structural strength with changing temperatures and tend to complete picture of truck weights around the state. deteriorate slowly. Thus, strong concrete pavements are gen- erally considered to be good locations for scale sensors. A 5.3 DETERMINE DESIGN LIFE pavement with high-durability characteristics provides for a AND ACCURACY REQUIREMENTS long design life and low maintenance costs for the scale sys- tem. (However, it is important to note that the pavement must Another key to efficient expenditure of data collection be smooth as well as durable to be good for vehicle weighing.) resources is to match the design life of equipment to the life Not all WIM installations are intended to last many years. of pavement and select the equipment accordingly. It is rarely In many cases, an agency only wishes to collect data for a year a wise decision to select a WIM sensor that is expected to out- or two at a location before moving the agency's scarce WIM live the pavement in which it is placed. Few WIM installa- resources to another location. In such a situation, the design