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21 CHAPTER 3 TECHNOLOGY DESCRIPTIONS Tables 3.1 and 3.2 list the most commonly used technolo- should serve primarily as a starting point to the selection and gies for vehicle classification and WIM, respectively, together operation of vehicle classification and WIM equipment. with their primary strengths and related concerns. Strengths and concerns are summaries of material found in the literature. 3.1 VEHICLE CLASSIFICATION Opinions of the strengths, weaknesses, or level of expected performance for any given technology or piece of equipment The descriptions of technologies for vehicle classification often differ from one expert to another, usually based on the are grouped on the basis of whether they use intrusive or experience that individual has had with a specific piece of non-intrusive sensors. Technologies using temporary, sur- equipment. The performance of any specific device may differ face-mounted sensors are considered intrusive technologies, from these summaries. This chapter provides further informa- because they involve access to the roadway structure. tion about 3.1.1 Intrusive Technologies How these technologies work, The types of data they can provide, This section covers sensor technologies that are placed either Installation conditions required for accurate performance, in or on top of the pavement and, at a minimum, provide the Specific weaknesses, and ability to classify vehicles into passenger vehicles and trucks. Typical uses (e.g., portable versus permanent data col- lection). Portable Operations As noted earlier, sensor technology is constantly under Portable sensor technologies used for classification include development. This chapter includes summaries of published research. For more current information, readers should consult Road tubes, resources such as the Vehicle Detector Clearinghouse at New Piezoelectric sensors (BL [brass linguini], ceramic cable, Mexico State University (http://www.nmsu.edu/~traffic/), and quartz), the FHWA's Demonstration Project 121 web site on WIM Fiber-optic cable, Technology (http://www.ornl.gov/dp121/) maintained by Oak Other pressure sensors, Ridge National Laboratories, and the European WIM of Axles Preformed inductance loops, and Vehicles for Europe (WAVE) project web site (http:// Magnetometers, and wim.zag.si/wave/).1 In addition, excellent written documen- Side-fired radar and other non-intrusive sensors. tation exists that should be used when learning about equip- ment attributes and selection. Useful documents include the Road tubes, piezoelectric sensors, and fiber-optic cable FHWA's States Successful Practices WIM Handbook,2 the technologies are pressure sensitive. That is, they deflect as Traffic Detector Handbook,3 the FHWA's Traffic Monitoring vehicle tires pass over them, and the deflection causes a sig- Guide,4 and the ASTM E 1318 WIM5 standard. This report nal that is detected and interpreted. Inductance loop and mag- netometer technologies are presence detectors that detect the presence of a vehicle (by changes in the sensor's inductance 1 All web sites referenced in this report were active as of June 20, 2003. 2 B. McCall and W.C. Vodrazka Jr., States Successful Practices WIM Handbook, Cen- or the earth's magnetic field) as a result of the presence of ter for Transportation Research and Education (CTRE), Iowa State University, Decem- metal in the vehicle. ber 15, 1997, http://www.ctre.iastate.edu/research/wim_pdf/index.htm. 3 J.H. Kell and I.J. Fullerton, Traffic Detector Handbook, Second Edition, U.S. Depart- Pressure-sensitive technologies have several strengths and ment of Transportation, Federal Highway Administration, Washington, D.C., 1992. weaknesses. These technologies count vehicle axles and mea- 4 Traffic Monitoring Guide, U.S. Department of Transportation, Federal Highway Administration, Office of Highway Policy, January 2001, http://www.fhwa.dot.gov/ sure axle spacings. Most classification systems that use intru- ohim/tmguide/index.htm. sive sensors base their classification on these variables. Hence, 5 American Society of Testing Materials, Annual Book of ASTM Standards 2000, Sec- tion 4, Construction, Volume 04.03, Designation: E 1318-02--Standard Specification the performance of the equipment is a function of how accu- for Highway WIM (WIM) Systems with User Requirements and Test Method. rately these measurements are made and how well they assign

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22 TABLE 3.1 Sensors commonly used for vehicle classification Type of Sensor Strengths Concerns Portable Vehicle Classification Sensors Road Tubes Inexpensive Inaccurate under high volumes (axle-based classification) Very common Difficult to install on multi-lane facilities Easy to use Conventional road tubes can only measure classifications in lanes next to shoulders or medians. Multi-lane road tube technology is relatively new to the market Inductance Loops (preformed) Inexpensive More difficult to place than road tubes (total length-based classification) Difficult to install under high-volume conditions Accuracy degrades with tight headways Magnetometer Ease of deployment Difficult to deploy in high-volume (total length-based classification) Simple installation conditions Little reliability information published Accuracy degrades with tight headways Eight length-based classification bins Conventional Pressure Sensors Well supported by vendor community Can be difficult to place in high-volume includes various piezo Ease of deployment in low-volume conditions (may require traffic control) technologies and tape switches conditions and when measurement lane Meticulous installation required (axle-based classification) is accessible from a shoulder Easy to break sensor/wiring connection Reliable if used for lanes not bordering on shoulders Susceptible to lightning Fiber-Optic Cable Can monitor multiple lanes Relatively new technology with little (axle-based classification) Not susceptible to lightning performance history Often requires traffic control to install if deployed across multiple lanes Side-Fired Radar Non-intrusive sensor Use in a portable configuration is (total length-based classification) relatively uncommon vehicles to the desired classes. Differentiation between closely Poor lane discipline, resulting in vehicles changing lanes spaced vehicles is often improved by using pressure sensors in as they cross sensors or traveling with one tire outside of conjunction with an inductive loop. the established lane lines (and striking sensors in adja- Where traffic, geometric, or environmental conditions make cent lanes). it difficult to count axles and measure axle spacings cor- rectly, pressure-sensitive sensors do not work effectively. The Traffic signals, major interchanges, and congestion can three most common problems associated with the use of this cause the last two conditions. As a result, it is difficult to use type of sensor are these technologies for collecting classification counts at many urban locations or at rural locations immediately adjacent to Very rough pavement (which causes axles to bounce major interchanges. over the sensors); Another problem with equipment accuracy is a poor cor- Roadway conditions that cause braking or vehicle accel- respondence between the variables measured and the vehicle eration while vehicles are crossing sensors (interfering classes of interest. Pressure-sensitive technologies, by them- with the estimation of axle spacing); and selves, have difficulty distinguishing between vehicles in the

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23 TABLE 3.1 (Continued) Type of Sensor Strengths Concerns Permanent Vehicle Classification Sensors Intrusive Sensors Sensors installed in the pavement tend to be Axle sensor-based systems allow use of FHWA (General Comments) adversely impacted by poor pavement condition 13-category system and similar state Poor lane discipline limits accuracy classification systems Must be reinstalled if channelization changes When traffic flow conditions are unstable, as often occurs in urban areas, simpler, more aggregated, Snow can badly degrade lane discipline and length-based classification schemes often work consequently classification count accuracy more accurately than the more complex, axle- based classification systems Inductive Loop Widely supported technology Length classification not as detailed as axle- (conventional) Inexpensive based classifications (total length-based Loses accuracy in areas with closely spaced classification) vehicles Inductive Loop New technology Relatively new technology with little (undercarriage profile) performance history Higher traffic volumes deteriorate accuracy Requires well-tuned loops Piezo Cable Widely used and supported Requires regular maintenance (ceramic, polymer [film], Best practices information available Difficult to maintain in areas of high traffic or quartz) volumes Ease of deployment Can work well in areas of high volume, if speeds are stable Fiber-Optic Promising new technology Little data available for accuracy and Immune to lightning reliability Inexpensive if amortized for moderate period of time Other Pressure Sensors Sensors are generally immune to lighting Not widely deployed Technology is generally well understood Requires new interfaces from several Used frequently in toll applications along manufacturers with loops, which allows accuracy in low- speed, unstable (stop-and-go) conditions Magnetometer Ease of deployment Limited classification bins based on length Little reliability data available Data retrieval from some models can require wireless communications (continued on next page) FHWA Classes 2 (cars), 3 (light-duty trucks), and 5 (six-tire, A related problem is differentiating between two closely two-axle, single-unit trucks). Many of these vehicles have following vehicles (often two cars) and a truck pulling a trailer. axle spacings that overlap the boundaries that are commonly Traffic signals tend to create platoons of closely spaced vehi- used to distinguish vehicles in these classes. Various types of cles. These vehicle platoons are often miscounted as multi-unit recreational vehicles are also difficult to distinguish based on trucks. These types of errors have more significant impacts on their axle configurations. In some cases, these errors are irrel- traffic load estimates. evant in terms of traffic load estimation (e.g., misclassifica- Presence detectors have some of these same problems. In tion of cars as light duty trucks). particular, presence detectors rely on constant vehicle speeds

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24 TABLE 3.1 (Continued) Type of Sensor Strengths Concerns Permanent Vehicle Classification Sensors (Continued) Non-Intrusive Sensors Easily adjusts to new channelization Normally cannot provide FHWA 13-category (General Comments) Accuracy normally not affected by classification information deteriorating pavement conditions Requires mounting structure (bridge, sign bridge, pole) Accuracy tends to be significantly affected by mounting height and angle of view Stability of mounting platform affects accuracy Video Allows multiple lanes of data collection Affected by visibility problems (snow, fog, from a single camera heavy mist/rain) Easy to deploy Camera lenses must be protected from the Widely accepted technology elements Well supported Less accurate in multi-lane environment Generally, only performs length-based classification accurately Microwave Radar Accuracy not affected by weather or Under good conditions is generally less poor pavement conditions accurate in multi-lane environment than Allows multiple lanes of data collection traditional sensors from a single device Only performs length-based classification Easy to deploy Widely accepted technology Well supported Infrared New technology appears promising Affected by visibility problems (snow, fog, Multiple lanes can be measured by one heavy mist/rain) device Requires regular maintenance Not as accurate in multi-lane environment Little reliability data available Ultrasonic New technology - appears promising Little reliability data available Requires multiple sensor installation Accuracy deteriorates as traffic volumes increase Some environmental conditions (air turbulence) can decrease system accuracy Acoustic New technology Little reliability data available Accuracy deteriorates with increasing variability in traffic speeds in order to accurately measure vehicle length (and correctly The other major limitation of most presence detectors is classify vehicles). Acceleration and deceleration interfere with that they are not capable of detecting axles,6 so they cannot this measurement. Presence detectors also have difficulty sep- be used to classify vehicles into the axle-configuration cate- arating closely spaced vehicles and differentiating between tailgating vehicles and vehicles pulling trailers. However, by limiting the number of length classes used, overall accuracy 6 One new loop-based technology, "Undercarriage Profile Loops," currently under from presence detectors tends to be higher than with axle development for use at permanent sites, is designed to detect axles. This technology is detectors in areas with only modest changes in vehicle speed. discussed in the next subsection.

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25 TABLE 3.2 Sensors commonly used for WIM Type of Sensor Strengths Concerns Permanent WIM Sensors General Comments Permanent sensors are placed flush with the The accuracy of all WIM sensors decreases with road surface, increasing the accuracy of the decreasing pavement conditions sensor outputs Unstable speeds, which are common in urban areas, result in significant decreases in WIM accuracy, regardless of the technology chosen Piezoceramic Cable Easier, faster installation than most Sensitive to temperature changes other WIM systems Accuracy affected by structural response of Generally lower cost than most other roadway WIM systems Susceptible to lightning Well supported by industry Meticulous installation required Low cost and ease of installation often result in placement in slightly rutted pavements, resulting in loss of accuracy Piezopolymer Easier, faster installation than most Sensitive to temperature changes other WIM systems Accuracy affected by structural response of Generally lower cost than most other roadway WIM systems Susceptible to lightning Well supported by industry Meticulous installation required Low cost and ease of installation often result in placement in slightly rutted pavements, resulting in loss of accuracy Piezoquartz Easier, faster installation than many More expensive than other piezo other WIM systems technologies May be more cost-effective (long term) if Requires multiple sensors per lane sensors prove to be long lived Above average maintenance requirement Very accurate sensor Sensor longevity data not available Sensor is not temperature sensitive Accuracy affected by structural response of Growing support by industry roadway Bending Plate Frame separates sensor from pavement Longer installation time required than piezo structure systems Entire tire fits onto sensor Some systems have experienced premature Moderate sensor cost failure, while others have been very long lived Sensor is not temperature sensitive Extensive industry experience with the technology (continued on next page) gories used by most WIM systems. Instead, length-based lel, a measured distance apart, perpendicular to and within a classes are used, producing somewhat less accurate estimates single lane of traffic. The time differential between these two of axle loads experienced by pavements. known sensor positions allows the computation of vehicle Additional details about these technologies follow. speed and, consequently, the spacing between axles. Road tubes are air switches. As an axle crosses each tube, Road tubes. Road tubes are by far the most frequently used the tube collapses and pushes air through a switch at the portable classification sensors. Like most pressure sensors, the counter. The air switch generates an electrical signal that is most common configuration is two road tubes placed in paral- used to record the time each axle crosses the sensor.

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26 TABLE 3.2 (Continued) Type of Sensor Strengths Concerns Permanent WIM Sensors (Continued) Load Cell Entire tire fits onto sensor Most expensive WIM system Frequently considered the "most Requires significant construction effort accurate" of conventional WIM to install technologies Becomes cost effective if constructed Some systems have demonstrated very and maintained for a long life span long life spans Fiber-Optic Promising technology New technology, no longevity history Not susceptible to lightning Not well supported yet by industry Accuracy affected by structural response of roadway Subsurface Frame System designed to eliminate impact Very new technology, currently Strain-Gauge System loads on sensor, increasing expected undergoing testing in the United States design life No data on longevity of system, or Buried design increases "time on accuracy of output using current sensor" for an axle software design Unclear if variation in structural response of pavement will affect system accuracy Expensive, long-duration installation Multiple Sensor Systems Increasing the number of sensors used Increase in the number of sensors (piezo, bending plate) increases accuracy, everything else held increases the chance that at least one constant sensor will fail System performance only somewhat Higher number of sensors increases degraded if one sensor fails, thus installation time and maintenance costs increasing system reliability Bridge WIM Bridge platform limits the effect of Only proven to work consistently on a (includes CULWAY) vehicle dynamics limited set of bridge designs (mostly Recent European advances offer short-span girder bridges) significant improvements over previous Needs truck isolated on bridge to weigh U.S. versions accurately Not actively marketed in the United States Capacitance Mats Modest sensor cost Most common configuration only Frame separates sensor from pavement measures one wheel path structure Tubes used for classification purposes must be placed par- Traditional road tubes were limited to outside travel lanes allel to each other and perpendicular to the direction of travel. for classification purposes. This is because placing a single (If the tube is not placed perpendicular to the direction of tube across more than one lane of travel generates signals travel, a single axle may generate more than one air pulse, from each lane. Several tube makers have solved this prob- resulting in an inaccurate count of axles.) Both tubes must be lem by making road tubes that have only a limited section of the same length, or the timing of the air pulse at the air switches tubing that produces air pulses. These tubes are lane sensi- will not be equal, and the time differential between the first and tive and can be used in multi-lane applications. Also, it is second sensors will be inaccurate, resulting in inaccurate esti- possible to use a multi-tube configuration with certain detec- mation of speed and, consequently, axle spacing. tor products to obtain classification and lane volumes across

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27 TABLE 3.2 (Continued) Type of Sensor Strengths Concerns Portable WIM Sensors General Comments Sensors placed on top of the pavement create a "bump" that decreases the accuracy of static weight estimates Placement of portable sensors without performing new, "in place" calibration effort is likely to lead to unreliable weight estimates Bridge WIM In most "portable" configurations, only Only proven to work consistently on a (includes CULWAY) axle counting sensors must be placed on limited subset of bridge designs (mostly the roadway, leading to easy, short- short-span girder bridges) duration equipment set up Generally needs truck isolated on Recent European advances offer bridge to weigh accurately significant improvements over previous Not actively marketed in the United U.S. versions States Piezo Ease of deployment Susceptible to variations in temperature (ceramic cable, BL-polymer film) Inexpensive sensor cost More accurate if used in permanent installation Capacitance Mats Ease of deployment Only measures one wheel path Modest sensor cost Creates the largest "bump" of the portable technologies multiple lanes in the same direction. However, multi-lane The piezoelectric effect is dynamic; i.e., charge is gener- installations are prone to error because it is difficult to anchor ated only when the forces applied to the sensor are changing. them tightly enough to keep them from bowing in the middle, Thus, piezoelectric sensor systems can only be used in appli- violating the requirement that they remain perpendicular to cations where vehicles are moving at speeds above 10 mph. traffic. Piezoelectric sensor systems cannot be used at locations with The primary advantages of road tubes are that they are very either slow-moving or stop-and-go traffic. inexpensive to purchase and are easy to install. They also are Some piezoelectric materials (and sensors) are sensitive to frequently used for traditional volume counting. temperature and do not perform well in very cold temperatures. As with road tubes, the most common portable piezo sen- Piezoelectric sensors (BL and ceramic). Piezoelectric sen- sor installations consist of two sensors, parallel to each other sors come in a variety of shapes and materials. For classifica- and perpendicular to the roadway, a measured distance apart. tion purposes, each of the most common sensor styles has Unlike conventional road tubes, piezoelectric sensors are fairly similar properties. When a mechanical force is applied lane specific. Thus, they can be used to monitor inner lanes; to a piezoelectric device, it generates a voltage by causing however, for portable operations, lead wires to the sensors electrical charges of opposite polarity to appear at the paral- must be placed across the outer lane(s). This increases the lel faces of the piezoelectric material. An electronic compo- potential for damage to sensor connections to lead wires, one nent of the counter detects this signal and uses it to indicate of the more common causes of sensor failure. the passage of an axle. The measured voltage from the sen- sor is proportional to the force or weight of the wheel or axle Fiber-optic cable. Use of fiber-optic sensor technology for as it is applied to the sensor. This allows the piezo sensor to axle detection is fairly new and relatively uncommon in com- be used as a scale. Sophisticated vehicle classifiers use this parison with other intrusive technologies. Fiber-optic sensors measure of axle weight to improve the accuracy of the vehi- detect the presence of a load by measuring the decrease in cle classification algorithm; however, many classifiers use optical transmission caused by constriction of the fibers when the strength of the sensor output signal only to separate sig- tires pass over the sensors. Fiber-optic sensor systems contain nal noise from the passage of an axle. a light transmitter (usually a light-emitting diode), a photon

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28 detector, and signal analysis hardware and software in addi- Limitations in the accuracy of the overall length measure- tion to the fiber sensor itself. ment restrict how many vehicle categories are normally col- Fiber-optic sensors are used in the same way as piezo- lected. In addition, considerable error exists in the correlation electric sensors. The sensor itself is normally the width of a between overall vehicle length and the FHWA's 13-category lane. Like road tubes, sensor manufacturers have also designed classification system (or the state-specific variations of that specific sensors that allow for collection of data on all lanes system) used by most WIM scales. As a result, most dual-loop of a multi-lane facility. systems normally classify traffic into only three or four broad Fiber-optic sensors are more responsive than road tubes, length categories. This reduces the number of classification theoretically making them more accurate under both very errors, while still providing an excellent measure of the num- slow speed conditions and very high volume conditions. The ber of large trucks versus the number of smaller trucks and advantage of fiber-optic sensors over piezo sensors is that the passenger cars. However, it does not provide other potentially former are not temperature sensitive and the sensors them- useful information, such as distinctions between the number selves do not conduct electricity, thus making devices using of heavy single-unit trucks with three or more axles and the these sensors less susceptible to lightning strikes. number of usually less-damaging two-axle trucks. Other pressure sensors. A variety of other pressure sen- Magnetometers. Magnetometers measure changes in the sors have been used at one time or another as portable axle magnetic field surrounding sensors to determine the presence sensors. All share the basic functionality of producing an elec- of passing vehicles. Like dual-inductance loop technology, trical signal when the pressure from a passing axle closes a cir- magnetometers use estimates of vehicle speed and the dura- cuit. The most common of these is probably the tape switch. tion of the signal to determine the length of vehicles. Vehi- Most portable pressure sensors, like the tape switch, are laid cle length is then used to classify vehicles into defined length on top of the travel lane and held in place by asphalt tape. categories. Portable magnetometers are commonly used throughout Preformed inductance loops. Inductance loops are used the United States for volume counting and, to a lesser extent, in traffic signal operations, making them the most common vehicle classification. They are placed on top of the pave- permanent vehicle sensors. When two loops are placed in ment in the center of each traffic lane, much like portable series, they allow passing vehicles to be classified on the inductance loops. However, they are much smaller, making basis of their overall length. This is done by determining the them easier and faster to place. In other respects, their char- difference in time between activation of the first and second acteristics are similar to those of inductance loops. loops. This time difference, and the distance between loops, allows for the computation of vehicle speed. Using vehicle Side-fired radar and other non-intrusive sensors. speed and the total time one of the loops stays active allows Because intrusive sensors cannot be placed in many locations overall vehicle length to be derived. due to high traffic volumes, a variety of non-intrusive sensors It is possible to use preformed inductance loops (most com- have been developed. These sensor technologies are described monly, wire loops attached to a thin solid frame) as portable in Section 3.1.2. The vast majority of these technologies are sensors. Preformed loops are taped to the road surface a pre- currently designed strictly for permanent operation. A number determined distance apart in order to create the required sen- sor configuration. Lead wires can also be taped to the road of vendors are currently working on developing portable ver- surface, allowing preformed loops to be placed on multi-lane sions of their existing non-intrusive detectors. facilities. In addition, a number of enterprising efforts have already Loops have the advantage of being placed in the center of been undertaken to create portable devices using these sen- the lane and so are not subject to the same level of impact sor technologies. For example, Ohio Department of Trans- loading as pressure-sensitive portable sensors. Thus, they are portation (DOT) has developed the ability to use a side-fired less likely to be knocked loose by passing traffic, and they can microwave radar system as a portable traffic counter. In this frequently be used for longer counting periods than pressure- case, the radar sensor is mounted on an extendable pole that sensitive detectors. is mounted on a trailer. The trailer can be parked in a safe Dual-loop installations, however, are limited in the accu- location beside a roadway. The pole is then raised, and the racy of the data they can provide. Because inductance loops radar system aimed and operated. Power for the system is actually measure the presence of metal, and signal strength is supplied by batteries. a function of the amount and proximity of the metal, not all vehicles are detected at the same distance from the loop. Vehi- Permanent Operations cles that contain large amounts of metal tend to be detected for a longer time period than vehicles with little metal. This Except for road tubes, the portable sensor technologies means that inductance loops tend to overestimate the length described above can also be permanently installed in the of vehicles with a lot of metal and underestimate the length pavement and used for continuous data collection. For this of vehicles with less metal. purpose, the sensors are placed in a pavement cut, which is

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29 then sealed with an epoxy or tar and used for data collection piezo sensor in between (to count axles and determine the over extended periods of time. Road tubes, by design, must spacing between those axles). Also, a four-sensor layout can be placed on top of the pavement, where they do not have a be used (two loops and two piezo sensors) in order to allow long enough fatigue life to be used as permanent sensors. for loss of one sensor (either a loop or piezo) without loss of Sensors placed in the pavement for long-duration count- classification capability. ing have particular attributes. The primary advantage of in- The differences in piezo sensor operating characteristics pavement sensors is that the impact loads associated with are more important for weighing accuracy than they are for surface-mounted sensors are no longer present. This greatly classification capabilities. In general, BL sensors require the increases sensor life. smallest pavement cut. Quartz sensors are the least affected However, placing sensors in the roadway has some disad- by temperature change and forces (stresses) that move hori- vantages. A road closure is needed to initially place the sen- zontally through the pavement. Quartz sensors are also the sor, as well as every time the sensor needs to be examined or most expensive and are primarily used as WIM sensors, rather maintained. Road closures are both expensive and publicly than simply for classification. unpopular, particularly on high-volume roads. Piezo sensors can often be paved over and still function Once placed, in-pavement sensors normally cannot be correctly. That is, most piezo sensors are sensitive enough moved. Thus, if channelization changes (i.e., the lane lines that they can be covered by an asphalt overlay and still be are moved), the sensors are no longer correctly located in the used to detect passing axles (so long as the sensor and its lead lanes and new sensors must be installed. This makes intru- wire and connections are not damaged in the process of lay- sive sensors a poor choice for those locations where lane ing the new pavement). lines will be moved in the near future. Permanent sensors can fail because of fatigue or because Other pressure sensors. There are a variety of other pres- of environmental effects such as moisture getting into the sure sensors available for use as permanent classification sensor or a nearby lighting strike that shorts out the sensor or sensors. Fiber-optic cable and older pressure switch tech- its electronics. Also, failure of the surrounding pavement can nologies belong to this category. destroy a sensor or render its output unusable. Like the piezo sensors, other pressure sensors are typically Successful practices designed to limit failures and extend placed into small saw cuts in existing pavement and held in sensor life are discussed in Chapter 5. In summary, initial site place by some type of epoxy or other bonding agent. How- selection and installation are the key to achieving long sen- ever, unlike piezo sensors, most of these pressure sensors are sor life. Placing an intrusive sensor in pavement that is in not sensitive enough to function correctly underneath an poor condition is likely to result in poor sensor performance asphalt overlay layer. and short sensor life, regardless of the technology chosen. Other pressure sensors generally are less expensive to pur- Similarly, haphazard sensor installation (e.g., poorly cleaned chase than piezo sensors, though installation time and effort or dried pavement cuts) can also lead to early sensor failure. tends to be very similar. Placement of sensors in pavement that is badly deterio- rated also leads to inaccurate results. Vehicle axles that are bouncing badly "jump" over pressure sensors. Concrete slabs Dual-inductance loops. Dual-inductance loops were the that rock because of joint failure cause pressure sensors to first mechanism used to collect long-duration classification pick up spurious signals and report "ghost axles." In these data. While the number of these systems in rural areas has cases, the sensors are actually working correctly; they are just been declining in favor of axle sensor-based systems (in functioning in an operating environment that prevents them order to collect data using the FHWA's 13-category classifi- from counting axles accurately. cation scheme), they are still commonly used in urban areas. Descriptions of intrusive sensor technologies that can be Because urban environments often involve congested traffic permanently installed follow. conditions, many agencies are unwilling to spend the money needed to place the more expensive sensors required to per- Piezoelectric sensors. The various types of permanent form axle-based classification. At the same time, in many piezoelectric sensors have similar layouts and slightly differ- urban areas, volume, speed, and lane occupancy data are ent operating characteristics but different installation require- needed to operate modern traffic control systems. By plac- ments and performance history. The minimum layout is two ing dual loops in the roadway, these data can be obtained. parallel sensors. An inductance loop can be added to this Loop systems also offer the potential for collecting length- basic installation (usually placed mid-way between the two based classification data. parallel sensors), which is used to help separate vehicles. Loops have an advantage over pressure-sensitive technolo- (That is, the loop presence is used to tell the data collection gies in that they do not involve contact with vehicle axles and equipment when one vehicle ends and the next begins.) An so are not subject to the impact loading that leads to sensor alternative to this sensor layout is to place two inductance failure. Sensor failure for loops is more commonly tied to loops (to measure vehicle speed and presence) with a single freeze-thaw conditions that result in pavement movements

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30 sufficient to "cut" the wire placed in the pavement. They also Non-intrusive technologies have been available for vehi- fail as a result of failing roadside amplifiers. cle detection and volume counting for a number of years,7 Given the weaknesses inherent in the collection of length- and improvements in computer processing power have based vehicle classes, the primary drawback to loop systems allowed these technologies to be extended to the more com- (other than their susceptibility to freeze-thaw failure) is the fact plex task of vehicle classification. In addition, with both the that classification accuracy degrades significantly under con- reduction in computer costs and the increased production of gested conditions. Thus, significant quality assurance efforts non-intrusive sensors resulting in economies of scale for their are needed before data collected at congested, urban sites are manufacture, the cost of many of these technologies has accepted as accurate measurements of truck volumes. declined considerably in the last 10 years. Non-intrusive technologies have a number of distinct advan- Undercarriage profile loops. A new technology has tages over technologies that must be placed in or on the road- recently been released by several manufacturers that uses the way surface, including the following: shape of the inductance signature of passing vehicles to clas- sify the vehicle. While the specific technical approaches used Increased staff safety (as staff do not need to be in the by the different manufacturers appear to be somewhat dif- roadway in order to place the sensors), ferent, the overriding concepts appear to be similar. In one Less traffic disruption during sensor installation (as sen- approach, additional loops are used to help detect axles (by sors can be placed with little or no traffic disruption, detecting the change in inductance caused by presence of the even on high-volume roadways), metal in the axles), while in another approach, the shape of The ability to reorient the sensor to adjust for changing the primary inductance pattern itself is matched against the lane configurations or other geometric changes without known shape of specific vehicle types. having to physically replace sensors, This approach shows more promise to allow sophisticated The capability of some non-intrusive sensors of collect- classification capabilities than previously available using loop ing data on more than one lane at a time from a single technology. At this time, however, these systems are rela- sensor (e.g., camera), tively new, and little practical experience is available to deter- Ease of maintenance and repair of above-ground sensors mine their accuracy and reliability. in comparison with sensors that are placed in ground, and Not being subjected to many types of environmental Magnetometers. As with undercarriage profile loop clas- damage that commonly reduce the sensor life of intru- sifiers, several different versions of permanent magnetome- sive sensors (e.g., freeze-thaw damage, tire impacts on ters are being marketed currently. Some are placed directly exposed sensors, and pavement failure around sensors). in the pavement, and others are inserted into conduits placed underneath the pavement. Both styles of magnetometers mea- sure vehicle presence by monitoring changes in the earth's Non-intrusive sensors also have weaknesses. The biggest magnetic field. The sensors are capable of estimating vehicle drawback is the difficulty for non-intrusive sensors to count speed and use that measure along with the duration of vehicle vehicle axles accurately, which is a key aspect of traffic load detection to estimate vehicle length. This length estimate is estimation for pavement design. Some non-intrusive sensors used to classify vehicles. do count vehicle axles, but these systems are limited in their Note that the conduit style of magnetometer is frequently application and have either installation problems similar to considered to be "non-intrusive" because the conduit can be intrusive sensors (i.e., they can only measure one lane of traf- placed (by drilling under the pavement from the roadway fic without being placed at roadway level on the lane lines) shoulder) without closing the lane of travel. The sensor can or suffer from occlusion that occurs when a system cannot be placed in the conduit without disrupting traffic, and the "see" one vehicle or axle because the sensor's "view" of that sensor can be repositioned within the conduit if lane geome- vehicle is blocked by an intervening vehicle. try is changed. As a result of their inability to easily count axles, most non-intrusive sensors classify vehicles by overall vehicle length, similar to dual-inductance loop technology. While this does not correlate directly with the vehicle classes com- 3.1.2 Non-Intrusive Technologies monly collected by WIM systems, it does provide useful data for Classification for pavement design purposes. Use of vehicle classifications based on overall vehicle length does require an additional While the majority of vehicle classification counting is data manipulation step for correlating these classes to those performed with intrusive or surface-mounted sensors, an used by an agency's WIM equipment. The staff time and the increasing percentage of classification counting is being per- formed with non-intrusive sensors. Non-intrusive technolo- gies include sensors that can be mounted overhead or to the 7 Field Test of Monitoring of Urban Vehicle Operations Using Non-Intrusive Tech- side of the roadway. nologies, FHWA, May 1997.

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31 potential for error associated with this extra data processing The accuracy of specific implementations of these tech- step must be traded off against the benefits obtained from use nologies was recently studied by a project jointly sponsored of non-intrusive technologies. by the U.S. DOT and the Minnesota DOT.10 The first round of Currently, most non-intrusive classification counting is field tests was completed in 2001, and the second round was done with permanently mounted sensors. Some vendors and completed in September 2002. While the study focused on the several highway agencies have been exploring the develop- collection of volume data using non-intrusive devices, the ment of portable versions of non-intrusive devices. These results of these tests can be of considerable use to agencies devices usually consist of one of two designs. In one design, interested in using non-intrusive data collection equipment. A sensor arrays are mounted to poles, which are in turn mounted pooled fund study specifically looking at portable use of non- onto trailers fitted with a power source.8 The trailer is then intrusive devices has been proposed and is actively being pur- towed to the desired roadside location, and the pole is lifted sued. Information on the completed and ongoing non-intrusive into position. This allows side-fired detection systems to detector tests can be obtained from http://www.dot.state.mn. operate. The second style of system is designed to be tem- us/guidestar/projects/nitd.html. porarily mounted on existing highway infrastructure, usually light standards or highway signs.9 These portable systems are not actively marketed in the United States. Video As with intrusive sensors, the accuracy of non-intrusive classification systems is a function of the quality of the clas- Video detection is the most widely used of the non-intrusive sifier's sensing system, the proper installation of the sensor, detection technologies. Video devices convert camera images the placement of the sensor in an environment that is con- into digital representations (pixel images) and then use micro- ducive to the proper operation of that specific technology, and processors to analyze those representations. There are two the vendor's algorithm used to process the raw sensor data. primary video image analysis techniques, trip line and image The placement of the sensor in a location where it will tracking, with the trip line approach being the oldest and most work correctly is the most important variable that is within commonly used. the control of the data collection agency. The starting point In the trip line technique, a specific portion of the video for this process is the ability to place the sensors where they image is defined as a "zone." Pixels within this zone are mon- can properly sense the vehicles they are intended to classify. itored for change, and changes in pixels are used to determine When sensors are mounted on the side of the road, it usually when vehicles are entering or leaving the zone. (Zones in means that they must be placed high enough to sense over video images can be considered "virtual inductance loops.") vehicles in nearby lanes in order to count and classify vehi- Activations of virtual zones can be used to determine volume cles in lanes that are farther away. Sensor height and angle of and lane occupancy. Two or more consecutive zones (set at view are also important for overhead-mounted sensors that a known distance apart) can be used, just as dual-inductance collect data on more than one lane of travel. The specific sen- loops are used, to measure vehicle speed and consequently sor mounting locations required by each device will vary overall vehicle length. This allows for vehicle classification with the device and are not discussed in this report. Specific based on total vehicle length. guidance on these details should be obtained from the ven- Image tracking relies on pattern recognition algorithms to dor of each device. (However, it will be noted that overhead- detect, recognize, and track specific kinds of vehicles. These mounted sensors tend to perform somewhat more accurately systems allow for more detailed data collection. (For exam- than the same sensor mounted at the roadside, all other things ple, they examine pixel images to detect axles, not just the being equal. This is most likely a result of the overhead posi- tion generally having a better "field of view" than the road- presence of a vehicle, in order to provide axle-based classifi- side position.) cations.) However, the complexity of the algorithms and short- The specific technologies presented in this section include comings of video image quality place additional constraints on their operation. Video, Video detectors of both types are sold by a number of dif- Radar, ferent vendors, and these systems can have very different Doppler microwave radar, capabilities. These differences are caused primarily by the Passive infrared, use of a variety of different data processing algorithms, each Active infrared, of which has different strengths and weaknesses. While con- Passive acoustic, and siderable experience has been gained as a result of the cur- Ultrasonic. rent use of these devices, the differences in specific vendor implementations make it difficult to identify the differences of those experiences. 8 This style of system has been used or tested in Ohio and New York among other states. 9 10 This style of system has been used or tested in Virginia and Minnesota among other The Minnesota Guidestar Non-Intrusive Traffic Detection Tests http://www.dot. states. state.mn.us/guidestar/projects/nitd.html.

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32 Factors that have been shown to affect video system per- one lane at a time with a single sensor. Radar technology has formance adversely include been in use in the United States for a number of years. Radar sensors can be either side-fired (mounted beside the Shadows (both stationary and moving shadows cast by roadway) or overhead mounted. A single, side-fired radar unit vehicles); can collect data on more than one lane, but a unit is required Direct sunlight; for each lane if overhead mounting is selected. (Overhead Reflections caused by wet pavement and headlights; mounting is more accurate, according to the manufacturer.) Transition from light to dark or dark to light; Because radar technology is relatively immune to weather Wind-induced pole movement; conditions (snow, fog, etc.), it is used in a number of loca- Environmental degradation of the video image caused tions where poor visibility conditions make video impracti- by (1) water on the camera lens, (2) icicles hanging in cal. Radar is easy to place, because side-fired systems can be front of the camera lens, (3) salt grime on the camera pole mounted at a height of only 5 meters (15 feet), which is lens, or (4) cobwebs on the camera lens; and considerably lower than for video systems that must often be Limited visibility caused by such phenomena as heavy mounted as high as 10.7 meters (35 feet). snow, heavy mist, or dust storms. Finally, conventional radar has the ability to detect slow- moving and non-moving vehicles. This means that system Each of these factors creates artificial changes in pixels count accuracy does not degrade significantly in stop-and-go traffic conditions. within the camera image (i.e., a change not caused by a vehi- In some system tests, radar has slightly undercounted cle passing through the image). Some of these causes are vehicles relative to counts made using conventional loop transient environmental conditions, while others are more per- detectors.11 manent and require corrective maintenance action. (Note: camera-based systems may require more frequent mainte- nance activity than conventional loop-based systems.) The accuracy of counts obtained from these systems is largely Doppler Microwave Radar dependent upon how effectively each system can deal with Doppler microwave radar is a variation on conventional these situations. radar systems. Doppler technology employs a continuous It is also apparent that the design and construction of sen- wave signal and measures the wave's Doppler shift as it is sor installations must take into account performance limita- reflected by passing vehicles. These detectors provide vehicle tions. Camera lenses need to be protected as much as possi- counts and speeds, but are not capable of detecting stopped ble from the elements. Similarly, placement of the cameras to vehicles and may be less applicable for the classification of minimize the effects of changing lighting conditions is also vehicles other than non-intrusive detectors. important for maximizing the performance of video-based systems. The two primary strengths of video image detection are (1) the ability to easily move "virtual sensors" to adapt to Passive Infrared changing lane configurations or to the need for new sensor Passive infrared devices detect the presence of vehicles by locations and (2) the ability of field staff to use a video mon- comparing the infrared energy naturally emanating from the itor to observe what the sensor is actually observing and to road surface with the change in energy caused by the pres- consequently (and easily) make adjustments to the operation ence of the vehicle. Because the roadway may generate either of the sensor. more or less radiation than a vehicle depending on the sea- Video detection has also the advantage of ability to collect, son, the contrast in heat energy is detected. from a single video image, data on more than one lane of traf- As with radar detectors, passive infrared detectors can be fic at a time. The keys to collecting multiple lanes of data mounted either on the side or overhead for data collection. from a single camera are (1) the ability to obtain a clear video These sensors provide the same detector output as conven- image of the lanes with sufficient pixel resolution to accu- tional loops: vehicle volumes and presence. Monitoring these rately monitor vehicle presence and (2) sufficient computing from two consecutive sensor locations allows the computa- power to monitor all "virtual detectors" in the image. tion of vehicle speed and consequently overall vehicle length. Sensor output from passive infrared appear to be unaf- fected by changes in weather conditions. While several ven- Radar dors sell these devices on the U.S. market, there are a rela- tively small number of current installations. Conventional radar-based detection uses pulsed, frequency- modulated, or phase-modulated signals to detect vehicles. 11 Lawrence Klein, Michael Kelley, and Milton Mills, "Traffic Detection Technolo- gies for a Modern Transportation Infrastructure," SPIE Conference 2592, Collision This technology is currently the only other non-intrusive Avoidance and Automated Traffic Management Sensors, October 2526, 1995, Philadel- technology that is designed to collect data from more than phia, Pennsylvania.