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33 Active Infrared than the normal road surface energy returns, a vehicle is pres- ent. Signal analysis allows determination of vehicle presence Active infrared sensors differ from passive sensors in that and occupancy. Using two closely spaced beams aimed a a low-power laser beam is directed from the data collection known distance apart allows for computation of vehicle speed device to the road surface. Measurement of the time lapsed and consequently vehicle length. Pulse ultrasonic devices are until the reflected signal returns to the device is used to deter- capable of high count accuracy when optimally mounted. An mine the presence of a vehicle. By splitting the laser beam overhead mounting location provides a perpendicular reflec- into two separate signals from a single sensor, it is possible to tive surface, offering the best signal return. compute vehicle speed and overall length. This allows length- Tests indicate that great changes in temperature and extreme based classification from a single active infrared sensor. air turbulence may inhibit accuracy of ultrasonic devices. Active infrared systems are also capable of measuring Such devices are not commonly used in the United States at vehicle height and can thus create two- and three-dimensional this time. images of passing vehicles. This allows even more compre- hensive vehicle classification capability. Infrared sensors do have signal degradation during weather 3.2 WIM conditions that reduce visibility. A good rule of thumb recom- mended by the Vehicle Detector Clearinghouse12 is that if vis- This section discusses sensor technologies that provide the ibility drops to the point where the human eye does not see an ability to weigh vehicles. The systems must be capable of object clearly, then infrared sensors are also likely to experi- supplying axle weights and classifying vehicles into at least ence difficulties. the 13 FHWA vehicle classification categories. All WIM sensors currently used in the United States mea- sure transient forces applied by tires to sensors as vehicles Passive Acoustic pass over. They use the measured force to predict the weight applied by the tire (axle) when the vehicle is at rest. The sen- Passive acoustic devices consist of an array of microphones sors used to perform this measurement include very thin, nar- aimed at the traffic stream. The devices are passive in that row sensors placed directly in the pavement (fiber-optics, they are listening for the sound energy of passing vehicles. piezo cables); large plates resting in frames that are in turn The primary source of sound is the noise generated by the imbedded in the pavement (bending plates, hydraulic load contact between tires and road surface. At slower vehicle cells); instrumented roadway structures (bridge and culvert speeds, the sound of the vehicle's engine is more prominent. WIM); and flat sensors placed on top of the road surface Passive acoustic devices are best used in a side-fired position, (capacitance pads). Selecting a specific technology requires pointed at the tire track in a lane of traffic. considering the following factors: Acoustic detectors physically measure the changes in sound energy radiating from the roadway. Increases in energy indi- Cost of the sensors and their installation, cate the arrival of a vehicle, and decreases in energy indicate Locations where a given technology can be successfully its departure. From these data, it is possible to determine lane occupancy. By using multiple detection zones, it is possible installed, to estimate vehicle speed and length, thus allowing vehicle Sensitivity of sensors to various factors (temperature, classification by length. vehicle dynamics, traffic volume, and speed), Some models of acoustic sensors have been shown to be Expected life span of sensors, and sensitive to undercounting in cold temperatures. In addition, Robustness of sensor installation (e.g., the ability to some acoustic sensors have a loss of accuracy when vehicles continue to collect data if one or more sensors fail or to are stopped or moving very slowly. These sensors are not com- compare output of one sensor against another). monly used for classification purposes in the United States at this time. The WIM task is heavily complicated by the dynamic motion of trucks being weighed. As trucks move, they bounce. The degree to which each truck bounces is a function of Ultrasonic pavement roughness, vehicle load, environmental conditions such as wind, and each vehicle's design and suspension sys- Pulse ultrasonic devices emit pulses of ultrasonic sound tems. The greater the amount of vertical motion exhibited by energy and measure the time lapsed until the signal returns trucks, the more difficult the task for WIM systems to accu- to the device. When the sound energy returns more quickly rately estimate static axle loads. Thus, for all WIM technologies, a key issue for collecting accurate weight data is to select locations for data collection 12 Mimbela and Klein, A Summary of Vehicle Detection and Surveillance Technolo- gies Used in Intelligent Transportation Systems, prepared by the Vehicle Detector that minimize the dynamic motion of trucks being weighed. Clearinghouse, for FHWA, Fall 2000. The lower the vertical dynamic motion of passing trucks, the

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34 more accurate the WIM scale, regardless of the technology (Capacitance mats are difficult to use on inside lanes because selected. the lead wires and sensor connections must be exposed to The other step required to account for truck dynamics is to traffic in those positions.) calibrate the WIM scale to the unique traffic characteristics Portable capacitance mats have significant shortcomings of each data collection site. While it is possible to calibrate in terms of overall system accuracy. The primary ones are each sensor in the laboratory, it is not possible to account for that (1) the system only weighs one side of passing axles and the dynamic motion of trucks at a specific roadway site with- (2) the sensor itself is fairly thick, creating a "bump" in the out measuring those forces in the field at the location where road that both increases vehicle dynamics and causes an the sensor is being placed. Only direct comparison of WIM impact load on the sensor that degrades system accuracy. system output against known axle weights for specific vehi- These shortcomings cause accuracy from portable systems to cles allows the calibration needed to enable a WIM system fall below that of flush-mounted, full-lane width, permanent to accurately predict static axle weights. While many vendors WIM systems. supply autocalibration features with their WIM systems, auto- Accuracy limitations are also inherent in the placement of calibration depends on one or more key assumptions that also mats on the roadway. While mats can be initially calibrated must be calibrated to each specific site. at a control location, the effects of vehicle dynamics at each given data collection location can only be determined by site- specific calibration efforts. The cost of these efforts often far 3.2.1 Portable WIM Operations exceeds the cost of placing and retrieving the data collection sensors and greatly increases the cost of collecting accurate There are only two technologies commonly used for por- weight data with these systems. While many states limit the table WIM data collection in the United States: capacitance amount of site-specific calibration done with their portable mats and piezoelectric (BL-style) sensors, although a number mat systems, the lack of site-specific calibration significantly of states used bridge WIM systems in a portable fashion in the affects the mat's ability to accurately estimate the static axle late 1980s and early 1990s. These three technologies are dis- weights needed for the pavement design process. cussed in this section. Finally, some states perform portable operations by moving electronics from one set of perma- nently mounted sensors to another. This style of "portable" data collection will be treated as permanent operations sim- Piezoelectric Sensors (BL and Ceramic Cable) ply because the sensors themselves are permanently placed in the roadway. The primary alternative to capacitance mats currently used by state highway agencies for high-speed portable WIM data collection is thin-strip piezo sensors. There are two basic Capacitance Mats styles of thin-strip piezo sensors: a flat plate configuration (the BL sensor) and unmounted piezoceramic coaxial cable. A capacitance mat consists of two metal sheets separated Both systems operate on the same basic principle. When a by dielectric material. An outer surface layer surrounds the mechanical force is applied to a piezoelectric device, it gen- sensor, protects the steel plates, and allows the sensor to be erates a voltage by causing electrical charges of opposite placed on the pavement. A voltage is applied across the two polarity to appear at the parallel faces of the piezoelectric metal plates. When a vehicle crosses over the plate system, crystalline material. The measured voltage is proportional to it causes the distance between the two plates to decrease, the force or weight of the wheel or axle. The piezoelectric which increases the capacitance of the system. Measure- effect is dynamic (i.e., charge is generated only when the ments of the resonance frequency of the circuit allow the esti- forces are changing); piezoelectric sensor systems can only mation of axle weight as it is applied to the sensor system. be used in applications where vehicles are moving at speeds A typical portable capacitance mat system covers one-half not less than 10 mph. Piezoelectric sensor systems cannot be of a lane and measures one side of each passing axle. It is used in applications having either slow-moving traffic or stop- usually secured to the roadway surface using a combination and-go traffic. of asphalt nails and tape. Portable loops are usually also For portable weighing operations, sensors (each sensor is placed as part of the system installation in order to provide roughly one lane width in length) are taped to the roadway, measures of vehicle presence and vehicle speed. perpendicular to traffic. Normally, two sensors are placed a Capacitance mats are moderately priced (each pad is about measured distance apart. The time difference between axle $10,000, not counting data collection electronics) and light- contact on the two cables is used to determine vehicle speed, weight. Installation requires several people, however, both to which is then used to determine the axle spacings needed for help place the sensors and to provide traffic control and cal- vehicle classification. ibration assistance. Use of portable capacitance mats allows These systems are relatively easy to set up, although, like WIM data collection to take place on the outside lane of capacitance mats, they are routinely placed only in the out- almost any level roadway that has a reasonable shoulder. side lane of traffic in order to allow the lead wires to be placed

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35 on the roadway shoulder. The sensors themselves are less this technology may increase the number of structures suitable expensive than individual capacitance mats. for use as weight sensors. However, like the capacitance mat systems, piezo cables used in portable operations suffer from significant limitations in accuracy for the following reasons: Conventional Static (Loadometer) Scales Sensors are temperature sensitive, making it difficult to For low-volume roadways, it is also possible to collect keep them in calibration when temperature changes dur- truck weight data with portable static scales used for truck ing the day. weight law enforcement. Any scale that meets Handbook 44 The sensors' narrowness allows tires being weighed to standards13 is acceptable for truck weight data collection. "fold" over them, meaning that at no time during the Collecting data with static scales requires either a perma- axle weighing process is an entire tire isolated on the nent scale facility or a level, paved surface where trucks can sensor. Therefore, changes in tire pressure or tire-tread be pulled off the road safely. This requirement significantly patterns affect the force measured by this type of sensor limits the locations at which data can be collected. more significantly than many other WIM technologies. Significant numbers of workers are needed to perform this The same site-specific calibration problems that affect task. Weighing vehicles statically is a slow process and results all portable WIM systems also affect these systems (cal- in a dataset of limited size. On high-volume roads, this small ibration of the sensors to site-specific vehicle dynamics dataset can easily represent a biased estimate of actual traffic is necessary to obtain the level of accuracy needed for loads, especially if by-pass routes exist, that might bias the direct inclusion of these data into the pavement design weights of trucks being sampled. However, on low-volume process). roads with little bypass opportunity, this approach to weight Sensors have a relatively high signal-to-noise ratio. data collection can provide an accurate and complete mea- sure of truck traffic for the periods for which the highway agency can afford to collect data. Bridge WIM A number of low-speed WIM systems can be used to speed In the 1980s and early 1990s, a number of states instru- up this process, while maintaining good data quality. Infor- mented bridges for use as WIM platforms. The technology mation on such systems can be found on the FHWA Demon- works by measuring the response to traffic loads as measured stration Project 121 web site http://www.ornl.gov/dp121/. by strain gauges attached to girders under the bridge. A num- ber of European countries are still strong supporters of bridge 3.2.2 Permanent WIM Operations WIM, and Australia extensively uses a similar system based on the deflection of culverts. The majority of WIM data collection is now done with Portable operations were achieved by attaching the strain permanently installed weight sensors, although many states gauges, either with C-clamps or permanently, and then con- do not collect data continuously at these sites. Instead, they necting portable roadside electronics to those gauges when attach data collection electronics to previously mounted sen- data collection was desired. sors when data collection is desired. The scale sensors are Bridge WIM has the advantage of having a very large then calibrated (or should be calibrated), and data are col- weighing platform: the bridge deck itself. This helps limit the lected for the desired time period. effects of vehicle dynamics. Unfortunately, various other fac- Permanently mounting WIM sensors allows them to be tors degrade the signal from the strain gauges and limit the installed flush with the roadway surface. When done prop- accuracy of data from bridge WIM. The most significant of these factors are the presence of other traffic on the bridge at erly, this eliminates the bump that vehicles experience when the same time a truck is being weighed (which significantly crossing surface-mounted sensors. The removal of impact increases the noise in the weight signal) and the fact that states loads on sensors and the elimination of extra vertical motion could not adequately define the expected response of many caused by bumps result in improved system accuracy. bridges to given loading conditions (which limits the accuracy Permanently mounting sensors flush with the pavement sur- of the computation of loads based on bridge response). face also decreases the impact loads on sensors themselves, Currently, without extensive site-specific set-up, calibra- which in turn increases sensor life. One common cause of sen- tion, and testing, bridge WIM is considered reliable only on sor failure is when sensors become directly exposed to hori- short-span, simply supported, steel girder bridges where trucks zontal forces from tire contact. This exposure often leads to can be isolated on the span during the weighing process. How- early fatigue failure for both sensors and the bonds between ever, the Australians use a similar WIM technology called "CULWAY" that works on a similar principle (measuring the 13 Tina G. Butcher et al., Specifications, Tolerances, and Other Technical Require- strain of the underside of a structure), but is attached to the ments for Weighing and Measuring Devices: As Adopted by the 83rd National Con- underside of large culverts rather than to bridge girders. Use of ference on Weights and Measures, 1998, ISBN #0-16-049825-2.

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36 sensors and pavement. Exposed sensors are also highly sus- Piezoceramic sensors, ceptible to damage from contact with snowplow blades. Piezopolymer sensors, As noted in Section 2.2.1, a variety of other factors play Piezoquartz sensors, important roles in both the output accuracy and the life of Bending plates, permanently mounted sensors. Pavement at permanent sen- Hydraulic load cells, sor locations should be Bridge and culvert WIM systems, Capacitance mats, and Flat (no horizontal or vertical curves), Other WIM technologies (fiber-optic, subsurface strain Smooth (no bumps or other surface conditions that gauge, multi-sensor). increase dynamic vehicle motion), Strong (to reduce pavement flex underneath the WIM Each of these technologies is introduced briefly below. sensor), and In good condition. Piezoceramic Sensors Flat, smooth pavement reduces vehicle dynamic motion and As noted above, piezoelectric WIM sensors come in vari- increases the accuracy of all WIM sensors. Strong pavement ous forms, but all systems operate on the same basic princi- results in longer lived pavement, which in turn increases sen- ple. When a mechanical force is applied to a piezoelectric sor life. Strong pavement is especially important for strip device, it generates a voltage by causing electrical charges of sensors that are embedded directly into the pavement. The opposite polarity to appear at the parallel faces of the piezo- output from these sensors depends on the performance of the electric material. The measured voltage is proportional to the pavement itself. If pavement strength varies significantly force or weight of the wheel or axle and is transmitted by the over time (e.g., with environmental conditions), sensor out- sensor to electronics that measure and interpret the voltage put will also vary, and this greatly decreases the likelihood signal. of accurate sensor calibration. Some researchers have sug- The first piezo traffic sensor marketed in the United States gested criteria for the pavement strength required for installing uses a ceramic powder compressed between a solid core and WIM equipment based on falling weight deflectometer (FWD) an outer sheath of copper. The cable is about the size of con- measurements. These criteria stipulate a maximum deflec- ventional coaxial cable. When used as permanent WIM scale tion under the center of the applied load and a minimum sensors, the cable is most commonly placed in aluminum deflection basin area. channels filled with epoxy resin or another substance. The Good condition pavement reduces vehicle dynamics and channel is then placed so that the top of the sensor is flush makes the bond between sensors and pavement more likely to with the road surface, in a slot cut into the pavement, less last. A common cause of sensor failure is the failure of sensor/ than 2 inches wide. (Different vendors use slightly different pavement bonds, which is often traced to poor pavement con- sensor mounting techniques.) Routine site installations can dition. Poor installation is another common cause of this fail- consist of two piezoceramic sensors, two sensors plus an ure. Poor cleaning or drying of pavement cuts results in a weak inductance loop, or one piezo sensor and two inductance loops. bond that allows moisture intrusion and further deterioration Each of these configurations allows for the computation of of the bond. vehicle speed and, consequently, axle spacing, which in turn Moisture is also a common cause of equipment failure permits vehicle classification as well as axle weighing. The because of intrusion into either the sensor itself or the com- installations using two piezo sensors tend to provide better munication lines connecting the sensor to the data collection estimates of static axle weights because each sensor provides electronics. an independent measure of axle weight during a different Each vendor and each state highway agency has its own time period associated with the vertical motion of the vehicle procedures for fighting moisture intrusion. Similarly, agencies being weighed. Combining the two independent weight esti- and vendors have equipment and procedures for protecting mates generally improves the accuracy of the static weight permanent equipment from lightning strikes, other environ- estimate. mental effects (extreme temperatures, humidity, dust), insects, The piezoelectric effect generated by the sensor is dynamic. power surges, and various other causes of equipment or com- That is, the charge is generated only when the forces applied munications failure. No single document exists that lists best to the sensor are changing. As a result, piezoelectric sensor practices for protecting equipment from these common prob- systems can only be used in applications where vehicles are lems. The U.S. Department of Transportation has recently moving at speeds not less than 10 mph; they are not reliable started promoting information exchanges between state high- in slow-moving or stop-and-go traffic. way agencies in order to increase the sharing of knowledge In addition, it is difficult to construct a cable for these sen- in these areas. sors that has uniform response across its entire length. Strict Specific WIM system technologies that can be used for laboratory testing is done to ensure that cables used for weigh- permanent, continuous weight data collection are ing meet uniformity standards. Cables that successfully pass

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37 uniformity tests are called "Class 1" sensors. Sensors that Piezoquartz Sensors function correctly but do not meet the highest signal unifor- mity standards are designated as "Class 2" sensors and can be The piezoquartz sensor was recently introduced in the used for vehicle classification purposes, but not for weighing. United States. It differs from the other piezoelectric sensors Piezoceramic sensors produce weight estimates of average both in the piezoelectric material used and in the design of quality. They suffer from three significant limitations in sys- the sensor itself, although it still fits into a pavement cut gen- tem accuracy: temperature sensitivity, reliance on the pave- erally less than 2 inches wide. ment itself for structural support, and narrow sensor design. While it is more expensive per sensor than the other piezo- Because the piezoceramic sensor is temperature sensitive, style sensors, the quartz sensor has the distinct advantage of piezoceramic WIM systems must include various algorithms being insensitive to changes in temperature. It is therefore and/or additional sensor inputs that allow the WIM system to generally more accurate than other piezo sensors. However, account for temperature changes when estimating weights. because the sensor still relies on structural support from the Each equipment vendor tends to approach this problem dif- pavement, if the pavement structure is sensitive to tempera- ferently, and different levels of success are achieved. The ture, the sensor will show some change in response to a given technical process is further complicated by the fact that sen- axle load simply as a result of the change in pavement strength sors are placed directly in the pavement structure and, because with changing environmental conditions. This sensor is not many pavement structures have structural responses that are sensitive to changes in temperature or soil moisture if placed in a thick portland cement concrete pavement. However, out- also temperature dependent, this also affects the piezo's signal put from this sensor is likely to be sensitive to changes in strength for a given load. Consequently, the sensor response is temperature, although not as much as other piezo sensors affected by two independent (but related) sources of varia- would be, if placed in a moderately thin asphalt pavement. tion in signal strength, and these lead to errors in estimated Like other piezo sensors, this sensor is placed into a rela- axle weights. tively small slot cut into the pavement. Each sensor is roughly The narrow-sensor design is an advantage when it comes 1 meter (3 feet) long, so four sensors are placed in an end-to- to the time and cost required for installation. However, the end arrangement to instrument an entire 12-foot traffic lane. narrow sensor also means that tires being weighed are never The site installation can consist of two lines (eight sensors) isolated on the sensor. That is, during all points in the weigh- of piezoquartz sensors, two lines plus an inductance loop, or ing process, at least part of the tire is being supported by the one line of piezo sensors and two inductance loops. pavement surrounding the sensor and not the sensor itself. As with other piezo installations, each of these configura- Thus, the sensor never senses the entire force applied by a tions allows for the computation of vehicle speed and, conse- tire. This effect is exacerbated by some tire tread designs that quently, axle spacing, which in turn allows vehicle classifica- can concentrate forces on small surface areas, and those sur- tion. The installations using two piezo lines tend to provide faces may or may not be directly on the sensor itself. The better estimates of static axle weights, because each line pro- combination of these effects is that the sensor can sense a vides an independent measure of axle weight, and the aver- variety of different forces, and this results in a larger error aged weight estimate can be used to account for the dynamic when estimating static weights than with some other WIM motion of the vehicle more effectively than a single line of technologies. sensors. Real-world experience with piezoquartz sensors is still being gained in the United States, but the sensor appears to Piezopolymer Sensors offer accuracy on a par with bending-plate systems when installed in structurally strong pavements. The second common piezo technology uses a piezoelectric polymer surrounded by a flat brass casing. This sensor, com- monly called the BL sensor, is placed directly on the road for Bending Plates portable weighing but, like the piezoceramic cable, is com- monly placed into an aluminum channel filled with epoxy Bending-plate WIM systems use plates with strain gauges resin when being used as a permanent WIM sensor. bonded to the underside. As axles pass over the bending This sensor is used exactly as the piezoceramic cable is plate, the system measures the strain on the plate and calcu- used and has essentially the same benefits and drawbacks. lates the load required to induce that level of strain. The BL sensor is also temperature sensitive, and the piezo- Individual bending plates are generally 6 feet long and electric effect it generates is dynamic. It is not a reliable sen- roughly 2 feet wide. One bending plate is generally installed sor in slow or stop-and-go conditions, and additional steps are in each wheel path. In some cases they are installed aligned, needed when processing sensor output to account for changes while in other cases the right and left wheel path plates are in sensitivity because of changing temperatures. Finally, like staggered in order to measure tire loads at two different points the piezoceramic cable, it comes in Class 1 and Class 2 con- in the vehicle's dynamic path. A typical bending-plate site also figurations, which indicates the degree of sensor uniformity. includes two inductance loops used to detect approaching

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38 vehicles, to differentiate between closely spaced vehicles, and systems. It is generally insensitive to changes in temperature to measure speed. and can weigh vehicles at both low and high speeds. It is, how- Bending plates are mounted flush with the roadway into ever, the most expensive WIM system to purchase and install. steel frames placed in the pavement. The use of steel frames The term "deep-pit scale" comes from the fact that this load separates the plate sensor from the roadway structure and cell itself requires a significant excavation in the roadway for increases the accuracy of the weight measurement in com- installation. This means long lane closures are required for parison with strip sensors. In addition, the weighing platform sensor installation. Heavy construction equipment is needed is large enough to isolate each tire as it is weighed. This also to dig installation pits and place sensors and associated elec- negates the bridging effect from which strip sensors suffer, as tronics. However, the fact that considerable construction is well as limiting the effect that different tire pressures and involved normally means that sensors are only placed at loca- tread designs have on the forces exerted on the scale platform. tions with smooth pavements (or pavements are made smooth Tests of system performance generally indicate that bend- at the time of sensor installation). Thus, hydraulic load cells ing plates are more accurate than traditional piezo cable and tend to be correlated with "expensive" installations, which in capacitance mat WIM systems and are roughly equivalent in turn result in better system performance. accuracy to piezoquartz sensors, but are less accurate than Load cells are contained in a steel frame that is indepen- hydraulic load cells. However, differences in weighing accu- dent of the pavement. This makes the load cell's response to racy that result from technological differences between WIM axle weights insensitive to changes in pavement strength systems are often overshadowed by problems inherent with caused by changes in environmental conditions (i.e., temper- specific weighing installations. (For example, a load cell ature and moisture content). In addition, the weighing plat- placed in rough pavement will provide less accurate data form is large enough to isolate each tire as it is weighed. This than a bending-plate system placed in smooth pavement.) again eliminates the negative effect pavement strength has on The cost and installation time required to place bending- strip sensors, as well as limiting the effect different tire pres- plate systems also falls between that of piezo and load-cell sures and tread designs have on the forces exerted on the systems. Because placement of the steel frame involves a scale platform. more substantial pavement cut than is required for the strip sensor installation, the duration of the lane closure required for system installation is far longer than for piezo systems. Bridge and Culvert WIM Systems However, the time required for bending-plate installation is considerably less than that required for load-cell installation. In bridge WIM systems, strain gauges are placed on the underside of bridges or on the girders of bridges. Strain- gauge output is analyzed to determine the loads on specific Hydraulic Load Cells vehicle axles. While the number of bridge WIM installations has declined in the United States since the late 1990s, con- As with most of the WIM technologies, there is more than siderable research on this subject is still being performed in one high-speed hydraulic load-cell WIM system design in Europe. Information on this research is available at http:// the United States. The most common versions operate by wim.zag.si/wave/download/wp12_report.html. transferring wheel weights applied to the weighing platform Culvert WIM is a variation of bridge WIM and is exten- to one or more hydraulic cylinders containing oil. Changes sively used in Australia. In this system, strain gauges are in the hydraulic pressure are correlated with axle weights. attached to the underside of large culverts, and the strain The most common load cell design uses two in-line scale measurements obtained are used to estimate truck axle loads. platforms that operate independently and provides weight The short span of the concrete culvert and the relatively sim- estimates for the right and left tires of each axle. The system plistic design of the culvert make the analysis of the strain records the weights measured by each scale and sums them signal straightforward, thus eliminating several of the prob- to obtain the axle weight. Off-scale detectors are frequently lems experienced by bridge WIM systems used in the United integrated into the scale design to detect any vehicles off the States. weighing surface. In addition, at least one inductive loop and While the culvert-based system has been marketed in the one axle sensor are usually included as part of the system United States, it is not widely used at this time. design. The inductive loop is placed upstream of the load cell to detect vehicles and alert the system of an approaching vehicle. The axle sensor is usually placed downstream of the Capacitance Mats load cell to determine axle spacings and vehicle speed. If a second inductive loop is used in place of the second axle sen- Capacitance mats consist of two metal sheets separated by sor, it is placed downstream of the load cell to determine a dielectric material. An outer surface layer surrounds the vehicle speed, which is needed to determine axle spacings. sensor, protects the steel plates, and allows the sensor to be The deep-pit load-cell system is generally considered the placed on the pavement or in a mounting frame. A voltage is most accurate of the available conventional high-speed WIM applied across the two metal plates. When a vehicle crosses

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39 over the plate system, it causes the distance between the two roadbed.) The scale sensor is placed by removing the plates to decrease, which increases the capacitance of the existing pavement at the site and repaving once the scale system. Measurement of the resonance frequency of this cir- is correctly positioned. The sensor's strain gauges reg- cuit allows the estimation of the weight of each tire as it is ister the strain transmitted through the pavement to the applied to the sensor system. steel frame. A neural network computing algorithm then Permanently mounted capacitance mats differ from portable converts these signals to estimates of vehicle and axle mats in that the former mats are placed in steel frames that weights. The system uses the pavement structure to are installed in the pavement surface. This allows the surface dampen the effect of vehicle dynamics and to increase of the mats to be flush with the roadway and improves the sensor life by limiting the fatigue problems associated accuracy of the system. It also reduces the impact load on the with repetitive tire contact and pavement maintenance sensor itself, both increasing sensor life and decreasing the activities. The manufacturer claims that the sensor system potential for the sensor to be dislodged from the roadway. is maintenance free. The testing being performed will Most capacitance mat systems rely on weighing only one determine whether the neural network processing algo- wheel path. This limitation makes them slightly less accurate rithm is able to accurately estimate weights given the than other potential WIM system alternatives. However, ven- mitigating effects of the overlying pavement structure. dors do sell permanent capacitance mat systems that use mats Multi-sensor WIM is one of the bigger research issues in both wheel paths. in Europe's WAVE ("WIM of Axles and Vehicles for Europe") program. The concept is to use a larger num- ber of moderately priced sensors to weigh a given vehi- Other WIM Technologies cle multiple times during a single pass. By stretching these sensors over many meters, it is possible to deter- New WIM technologies continue to be developed and mine a vehicle's dynamic motion and thus significantly brought to the market. Many of the new technologies have improve the estimate of vehicle weight. The use of mul- been developed specifically to address limitations in the cost, tiple sensors also provides multiple independent mea- performance, and flexibility of current technologies. The sys- tems discussed below are either in active use elsewhere in the sures of the same basic quantity. While this technique world or in active development in the United States: shows considerable promise, it is unclear if it is eco- nomically feasible or if the improvements in accuracy Fiber-optic sensors detect the presence of a load by achieved warrant the cost of additional sensors and their measuring the decrease in optical transmission caused placement. by constriction of the fibers when vehicles pass over sensors. Fiber-optic sensor systems contain light trans- A number of states and vendors have moved to take advan- mitters (usually a light-emitting diode), photon detec- tage of the concept of multi-sensor WIM without taking the tors, and signal analysis hardware and software in addi- approach the European WAVE program tested. In Europe, tion to the fiber sensor itself. The potential advantages multi-sensor WIM systems deployed a large number of sen- of fiber-optic sensors are relative insensitivity to road sors (10 or more). In the United States, vendors and states have temperature and low cost. Fiber-optic sensor systems both increased the number of sensors deployed and changed are not fully developed and are not in field operational the location of sensors in order to improve the measurement of use. System accuracy and life have not been established. vehicle dynamics. However, they have not increased the num- Capacitance strip sensors have been used in the United ber of sensors to the extent examined in the European tests. Kingdom for a number of years. These sensors use the The increase in sensors allows a more accurate measurement same basic principle as capacitance mats (described of (and accounting for) the variation in axle weight caused by above), but use a thin sensor (instead of the larger mat) vehicle motion. However, by limiting the number of sen- designed for in-pavement installation similar to piezo- sors added, the increase in capital cost and installation time sensor deployment. The capacitance sensor material was required to build the WIM site is moderated. selected to avoid the temperature sensitivity problems One fairly common approach to multi-sensor WIM in the associated with piezoceramic and piezopolymer sen- United States has been to use three half-lane bending plate sors. However, only limited testing of this sensor has scales (rather than the traditional two sensors) and to stagger been done in the United States, and the sensor is not the left wheel path and right wheel path sensors (rather than actively marketed in the United States. placing them side by side). This allows measurement of both Subsurface strain-gauge frame technology is currently sides of the vehicle and provides measurements at three dif- being tested at Virginia Polytechnic Institute and State ferent points in the dynamic spectrum while only increasing University. This technology places a steel frame fitted the sensor cost by 50 percent. with a large number of strain gauges underneath the pave- Another common approach is to place four staggered sets ment. (The 2-ton frame is installed at least 2 inches under of half-lane piezo sensors. The concept is the same for this the pavement surface and can be completely below the system as for the bending plate system, in that staggering the

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40 sensors yields more information on the dynamic variation of In the case of the bending-plate system described above, the axles, while, in this case, there is no actual increase in the num- loss of one of the two right bending plates actually leaves the ber of sensors required when compared with a conventional site as being equivalent to a conventional bending-plate piezo-based layout (i.e., two full lanes' worth of sensors). WIM site in terms of sensor accuracy. Both of these designs also have the advantage of providing Note that before such an approach is adopted, the highway an extra layer of site reliability. This is because the extra sen- agency must make sure that the vendor's data collection elec- sors allow "graceful degradation" of the WIM system. That tronics can both handle any additional sensor inputs and cor- is, the loss of one sensor does not make the WIM data unus- rectly interpret the signals coming from sensors placed in a able; it simply degrades the accuracy of the system somewhat. staggered position.