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18 do not. The primary advantages of these three technologies As a consequence, data collection efforts at permanently are that they are relatively inexpensive to purchase, are easy placed, continuous count locations tend to be far more capi- and inexpensive to place, and are capable of providing the tal intensive than are those of short-duration counts. Contin- information required for most uses. The technologies' biggest uous counts usually use sensors that require traffic control drawback is that they are generally designed to operate in low- or heavy equipment (such as a bucket truck and a trenching and moderate-volume rural settings. In congested conditions, machine) for placement and are made by counting devices where vehicles are accelerating or decelerating while crossing that are stored in installed, locked cabinets rather than chained the sensors, or where vehicles are tailgating each other, these to nearby utility poles. However, once these devices are sensors often have accuracy problems caused by an inability placed, they are designed to operate with relatively little staff to measure axle spacings correctly or to distinguish between intervention except for periodic maintenance. closely spaced vehicles. (For example, in congested condi- The most common data collection technologies for con- tions, two closely spaced cars are often reported incorrectly as tinuous classification data collection are in-pavement sensors a single, four-axle, combination truck.) In addition, on higher- based on dual-inductance loops or piezoelectric (ceramic) volume roadways, even the most quickly installed sensors cables. Limitations in these two technologies, and the recog- require the presence of full traffic control in order to protect nition that more classification data are needed, have led to a the staff placing the sensors. The need for traffic control sig- significant increase in the number of technologies available nificantly increases the cost of portable data collection and for conducting continuous vehicle classification counts. In can entirely prevent short-duration classification data collec- particular, considerable advances have been made in the devel- tion where staff are not able to safely place sensors. opment of non-intrusive technologies, which use sensors that Research is currently being performed on the development are not physically placed in the roadway itself but which mon- of non-intrusive sensors specifically designed for collecting itor traffic from above or beside the road. Non-intrusive sen- truck volume information on high-volume urban roadways. sors have the advantage of allowing sensor placement with no The Minnesota Department of Transportation has recently lane closure (for roadside sensors) or with a less disruptive begun testing these devices. closure (for overhead-mounted sensors). They also have the In order to increase staff safety, eliminate the need for traf- advantage of not being subject to the impact of traffic loads fic control for each count, and allow data collection on high- or to the stresses that result from pavement interaction with volume roadways, some highway agencies place sensors per- the environment. manently in the ground at high-volume locations, but only However, non-intrusive sensors have limitations. The fore- collect data at these locations periodically. In these cases, the most limitation is that it is more difficult to detect and count data collection electronics usually "rove" from sensor location the axles on passing vehicles with non-intrusive sensors than to sensor location. This allows short-duration counts to be with intrusive sensors such as the piezo cable. Because axle made quickly and inexpensively by simply connecting the rov- counts by type of axle are generally required for accurately ing electronics to existing permanently mounted sensors. This estimating pavement loads, data collected with non-intrusive option reduces the cost and danger of placing sensors when- sensors usually require at least one extra data manipulation ever counts are required, but it entails a high capital cost for step (based on assumptions) when used for pavement load initial purchase and installation of a large number of sensors. determination. This step involves converting the vehicle classes collected with the non-intrusive technologies into a vehicle classification scheme compatible with the vehicle 2.1.2 Continuous Classification Counts classes that are collected using available WIM technologies. Finally, even the newest technologies have difficulty cor- Equipment that works well for short-duration classifica- rectly classifying vehicles in stop-and-go traffic and when tion counting often is a poor choice for continuous data col- vehicle separation is small. These conditions make it extremely lection over longer periods of time. Technologies that use difficult to separate tailgating cars from multi-unit trucks and sensors mounted on the surface of a roadway usually are not make it very difficult to measure vehicle length and axle spac- able to operate for extended periods of time without having ing correctly. These limitations are a primary reason why most the sensors reinstalled because the traffic has loosened them states have only modest amounts of classification data for from their original placements. Continuous counts require a urban roadways. long-lived sensor installation. In addition, continuous count devices require power and communications capabilities that are far different from portable devices. Portable counts nor- 2.2 WIM DATA mally are collected using battery power, with the counts down- loaded manually from the data collection electronics to a lap- 2.2.1 Short-Duration WIM top computer or data transfer device. Long-duration counts, however, require electrical power, usually from electric power Two technologies, capacitance mats and BL-style piezo- service or from solar cells, as well as telephone communica- electric sensors, are commonly used in the United States for tions for downloading data. high-speed (i.e., on-highway) portable WIM data collection.
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19 Both technologies involve mounting a sensor on top of exist- they are being used for weight enforcement. Hence, these col- ing pavement. This action requires a temporary lane closure lection locations may be avoided by illegally overloaded and often work by more than one person. trucks, resulting in biased results. However, these technologies While the basic technique of placing sensors on top of the are acceptable for truck weight data collection where truck roadway is essential for collecting WIM data in a truly portable volumes are light, where only a small sample is required, and mode (i.e., at any site that meets the physical requirements for where truck evasion is difficult because of limited opportu- acceptable sensor operation), there is a system performance nity for trucks to by-pass the scale site. problem that limits the accuracy of high-speed portable WIM The second method uses portable electronics with perma- scales. nently mounted WIM sensors that allow weight sensors to be Because the sensor is physically on top of the roadway sur- flush mounted with the roadway. This eliminates the bump face, a bump is created as the tire of each axle mounts the that occurs with surface-mounted sensors and results in a bet- weight sensor. This bump causes two physical effects, each ter environment for collecting accurate axle weights, but it of which is detrimental to WIM system accuracy. The first does not ensure accurate WIM data. Even in this type of por- effect is the additional dynamic motion imparted on the vehi- table operation, calibration is required prior to starting data cle being weighed. This motion makes it much harder for the collection, and care should be taken to ensure that pavement WIM system to accurately estimate the static weight applied deterioration over time has not created bumps at the joint by each axle. The second physical effect is that the need to between sensors and roadways. This type of site is less costly climb over this bump causes the tire itself to flex, absorbing to operate than a continuously operated WIM site (because some of the horizontal force from impact with the bump. This one set of data collection electronics is used for several data tire flex force is transmitted to the weight sensor, causing addi- collection sites and because permanent power and commu- tional bias and noise in the measurement process. nications are not needed and therefore do not need to be con- The result of these physical phenomena is that portable structed). However, the initial capital cost is higher than for WIM rarely achieves the same level of accuracy as a correctly truly portable WIM--a factor that the highway agency con- placed permanent scale. This does not mean that weights col- siders when deciding where to collect WIM data. lected using portable scales are not useful in the traffic load estimation process, but it does mean that highway agencies 2.2.2 Continuous WIM must be particularly careful to calibrate portable scales each time they are placed on the roadway and to monitor the data Because of the physics problem noted above for portable produced after scales have been calibrated to ensure that the equipment, the majority of research and development in WIM system is producing reliable results. has been done for permanently installed weight sensors. Five The need to calibrate every time portable sensors are placed technologies are currently in common use throughout the also reduces the difference in the total costs associated with United States. Other sensor designs are under active develop- data collection using permanently mounted sensors and using ment. The most common permanently mounted weight sen- portable sensors. Without calibration, data collected by por- sors are table scales will be significantly less accurate than data pro- duced by permanent scales. · Bending plates, Because of the limitations in truly portable WIM systems, · Hydraulic load cells, some state highway agencies use one of two methods for col- · Piezoceramic cables, lecting short-duration WIM data. One method involves the · Piezopolymer cables, and use of low-speed (off-highway) WIM scales or portable · Piezoquartz sensors. static scales. The other method relies on permanently mounted weight sensors and portable data collection electronics. Other sensor technologies that are either in more limited In the first method, conventional, portable static scales use or are still under development include (loadometers) or low-speed portable WIM scales (usually bending plates or capacitance pads) are used for portable · Permanently mounted capacitance mats, weight data collection. These traditional technologies require · Permanently mounted capacitance strips, flat areas (such as a parking area of a rest stop) where the · Fiber-optic cables, scales can be laid out and trucks diverted over the scales. · Subsurface strain-gauge frame, and Trucks are either stopped on these scales or driven at slow · Bridge or culvert WIM. speeds over the scales. These data collection techniques tend to be labor intensive (because trucks must be directed over All of the systems are designed to have sensors perma- the scales), and they result in fairly small datasets in com- nently installed in or under the roadway. This results in less parison with high-speed WIM data collection. Also, they dis- dynamic vehicle motion and less impact force on sensors than rupt the truck traffic stream (which must be diverted off the for surface-mounted sensors, which in turn results in more roadway and over the scales), and drivers are likely to assume accurate weighing conditions and longer sensor life.
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20 The various sensor technologies were developed either to areas with rapid temperature changes, such as those experi- take advantage of particular material properties (to reduce enced in mountain passes and in the Southwestern deserts. the cost of the sensor and/or installation) or to provide a spe- Environmental and site conditions (pavement condition, cific advantage to the signal-processing algorithm that con- temperature, wind, grades, etc.) play a large role in the per- verts sensor output into an estimate of axle weight. Each sen- formance of any WIM system, regardless of sensor technol- sor technology has its own strengths and weaknesses. No one ogy. A high-speed WIM system will not work accurately if sensor is best for every WIM application. the site selected for weighing is not conducive to weight data For example, both the piezoelectric cable and fiber-optic collection. ASTM specification E 13181 provides specific cable sensors are specifically designed to require a relatively guidance on the pavement conditions needed for accurate small pavement cut for sensor installation. This results in a WIM system performance. This guidance stipulates a pave- fast and relatively low-cost sensor installation. However, these ment that is sensors are so small that at no time during the weighing process is the entire tire (axle) that is being weighed isolated · Flat (no horizontal or vertical curves), on the sensor. Thus, both of these technologies suffer from · Smooth (no bumps or other surface conditions that cre- signal noise because of the fact that, during the weighing process, the axle weight is partially supported by the pave- ate vehicle dynamics), · Strong (to reduce pavement flex underneath the WIM ment that surrounds the sensor. Each vendor takes into account the selected sensor's sensor), and strengths and weaknesses when designing a WIM system. The · In good condition. means for accounting for specific weaknesses has a great deal to do with how well specific sensors work in given installa- WIM sites should also be sites where vehicles are travel- tions. Because vendors often take different approaches to ing at fairly constant speeds (i.e., not accelerating or decel- sensor installation design and signal processing, the perfor- erating), are not changing lanes frequently, and have good mance of a specific sensor technology can vary widely from lane discipline. If these conditions are met, then the trucks vendor to vendor. In some cases, the conditions at a specific being weighed are likely to have relatively modest dynamic WIM site directly (and negatively) coincide with the partic- motion. They will tend to track correctly in their lanes (and ular weakness of a given sensor technology. In these cases, will hit the weight sensors as expected), and the speeds mea- even the best vendor responses to handling those weaknesses sured and used in various signal-processing algorithms will may not allow sensors to work correctly. be accurate. All of these factors improve the performance of A good example is temperature sensitivity. Temperature- any WIM system, regardless of sensor technology. sensitive WIM sensors are not good choices for WIM sites where temperatures change rapidly. Although such sensors are used with temperature compensation algorithms, often based 1 American Society of Testing Materials, Annual Book of ASTM Standards 2000, Sec- tion 4, Construction, Volume 04.03, Designation: E 1318--Standard Specification for on some type of autocalibration technique, these adjustments Highway Weigh-In-Motion (WIM) Systems with User Requirements and Test Method, cannot be made fast enough to maintain scale accuracy in ASTM, 100 Barr Harbor Drive, West Conshohocken, Pennsylvania, 19428-2959.