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"State-of-the-Art" Report on Non-Traditional Traffic Counting Methods
Final Report 503
Prepared by: Sherry L. Skszek 505 N. Tanque Verde Loop Rd. Tucson, AZ 85748
October 2001
Prepared for: Arizona Department of Transportation 206 South 17th Avenue Phoenix, Arizona 85007 in cooperation with U.S. Department of Transportation Federal Highway Administration
The contents of the report reflect the views of the authors
Technical Report Documentation Page
TABLE OF CONTENTS
APPENDIX A: SURVEY INSTRUMENT APPENDIX B: TRAFFIC COUNTING SURVEY RESULTS APPENDIX C: BIBLIOGRAPHY APPENDIX D: WEBSITE BIBLIOGRAPHY APPENDIX E: MANUFACTURER LIST APPENDIX F: MNDOT REPORT CONCLUSIONS
LIST OF FIGURES Figure 1. Product Classification
LIST OF TABLES Table 1. Manufacturer List Table 2. Product List Table 3. Freeway Detector Annualized Per-Lane Cost Comparison Table 4. Limitations of the Technology Table 5. Application Guide for Detector Selection on Freeways Table 6. State Departments of Transportation Table 7. Level of Satisfaction by State Table 8. Usage and Average Level of Satisfaction Table 9. Disadvantages Reported by Technology Table 10. Method of Data Collection Table 11. Frequency of Method Use Table 12. Device Manufacturers Table 13. Qualitative Assessment of Best Performing Technologies for Gathering Specific Data Table 14. Devices Evaluated in MnDOT Study Table B1. Level of Satisfaction Table B2. Disadvantages Reported Using Manual Observation Table B3. Disadvantages Reported Using Bending Plates Table B4. Disadvantages Reported Using Pneumatic Road Tubes Table B5. Disadvantages Reported Using Piezo-Electric Sensors Table B6. Disadvantages Reported Using Inductive Loops Table B7. Disadvantages Reported Using Passive Magnetic Devices Table B8. Disadvantages Reported Using Radar Table B9. Disadvantages Reported Using Passive Acoustic Devices Table B10. Disadvantages Reported Using Video Image Detection Table B11. Frequency of Method Use to Collect Count Data Table B12. Frequency of Method Use to Collect Speed Data Table B13. Frequency of Method Use to Collect Weight Data Table B14. Frequency of Method Use to Collect Classification Data Table B15. Traffic Data Reported Using Manual Observation Table B16. Traffic Data Reported Using Bending Plates Table B17. Traffic Data Reported Using Pneumatic Road Tubes Table B18. Traffic Data Reported Using Piezo-Electric Sensors Table B19. Traffic Data Reported Using Inductive Loops Table B20. Traffic Data Reported Using Passive Magnetic Devices Table B21. Traffic Data Reported Using Radar Table B22. Traffic Data Reported Using Passive Acoustic Devices Table B23. Traffic Data Reported Using Video Image Detection Table B24. Manufacturers Utilized by Each State
Any State using traffic data for the apportionment or allocation of Federal funds must have a traffic monitoring system that meets Federal Highway Administration requirements. As part of a traffic monitoring program, States are required to gather vehicle count, classification, and weight data. Since participation in federally funded programs is essential to the integrity of a State’s highway systems, the accurate, efficient collection of traffic data becomes a critical component of transportation infrastructure management. This report looks at the state-of-the-art of non-traditional traffic counting methods to facilitate informed decision making regarding changes to existing practices. The report is comprised of three sections—an evaluation of current technology, a survey of State Departments of Transportation (DOT) traffic counting practices, and a literature review. The evaluation of current technology was conducted through interviews with over fifty manufacturers of traffic counting devices as well as a review of the literature on existing technology. The survey of State DOTs involved sending a two-page survey to each of the fifty agencies requesting information on level of satisfaction with devices currently in use, disadvantages of the technology, manufacturer information, and data gathered using each device. Lastly, a literature review of new technology was conducted to uncover new trends in traffic counting practices. Two main categories were identified—intrusive and non-intrusive data collection devices. Intrusive devices are those that involve placement of the sensor technology on top of or into the lane of traffic being monitored. Conversely, non-intrusive devices do not interfere with traffic flow either during installation or operation. The information gathered was differentiated into one of these two categories. The type of traffic data collection devices available on the market has changed little in the past decade. The same thirteen technologies are still being utilized by State, county, city, and metropolitan organizations responsible for traffic monitoring operations. However, the devices have evolved as their use has come under greater scrutiny with the recent focus on "intelligent transportation systems." Such non-traditional technology as video image detection, Doppler microwave, passive magnetic, passive acoustic, active and passive ultrasonic, and active and passive infrared technology now are being used with increased frequency for data collection and traffic management. The second section of the report deals with the Arizona Department of Transportation Traffic Counting Survey. All fifty States submitted responses to the survey. The results showed that less than half of all States are using non-intrusive (non-traditional) methods for gathering traffic data. Although the level of satisfaction with intrusive devices is relatively high, there is pressure to find methods of data collection that will keep traffic counting professionals out of the lanes of traffic. A few manufacturers were identified as leaders in the industry with current technology. It is yet to be seen if they will continue to lead as the move toward non-intrusive technologies begins to dominate the marketplace. The last section of the report contains a review of the literature on emerging technology. There is little in the way of new devices; however, the information uncovered relates to improvements in existing technology. Manufacturers are looking toward "signatures" to improve on the accuracy of vehicle classification. This pattern matching technology is being used with inductive loops and passive acoustic devices to improve on current technology. Neural network software is able to use the unique characteristics of a vehicle designated as a "signature" to more accurately classify a vehicle even beyond the Federal Highway Administration’s thirteen classes. Piezo-electric sensors also have evolved with advances in the material used as the force transducer. Quartz materials, being highly insulated, are being employed to improve on the collection of weight-in-motion data. New technology is followed by a review of recent research on evaluations of non-intrusive traffic data collection devices. Studies have been conducted by organizations involved the transportation industry including the Federal Highway Administration, State DOTs, universities, and private industry in an effort to determine if the newer non-intrusive technologies are capable of more cost-effectively collecting reliable traffic data. The studies show promising results from the non-intrusive technologies but continued research and development is needed to provide appropriate documentation to convince traffic counting professional that a transition to new technology is in their best interest. In summary, the collection of accurate traffic data in a cost-effective manner is essential to the allocation of scarce resources needed to support an aging infrastructure. The pressure to move the industry forward will provide the impetus for manufacturers to continue to develop the newer non-intrusive technologies and show they can meet the stringent requirements set forth by today’s traffic counting professionals.
The purpose of this research project is to examine current state-of-the-art non-traditional traffic counting practices throughout the transportation industry. This information was gathered through interviews with manufacturers of existing technology and review of the literature. In addition, traffic counting professionals from state departments of transportation were surveyed to obtain information on their current practices and level of satisfaction with the systems they have in place. This report summarizes the information gathered and will be used during the decision making process involving the feasibility and cost effectiveness of improvements in Arizona Department of Transportation’s current traffic counting practices. The Federal-Aid Policy Guide established by the Federal Highway Administration mandates "requirements for development, establishment, implementation, and continued operation of a traffic monitoring system for highways and public transportation facilities and equipment in each State." Subchapter F of the Federal-Aid Policy Guide outlines general requirements for compliance with this policy. States must comply with these requirements when traffic data generated by the state are used for the following purposes:
A State’s traffic monitoring procedures also apply to the "activities of local governments and other public or private non-State government entities collecting highway traffic data within the State" if the data are used for any of the purposes described above. Since participation in federally-funded programs is essential to the integrity of a State’s highway systems, the accurate, efficient collection of traffic data becomes a critical component of transportation infrastructure management. As part of a traffic monitoring system, States are required to record traffic volumes, vehicle classification, and vehicle weight data. This information is collected at short-term counting stations and at long-term, continuous counting stations. Short-term counts are then adjusted for seasonal, day-of-the-week, and other factors as assessed at continuous count stations to provide estimates of traffic patterns throughout the State’s highway infrastructure. This information provides documentation to ensure the State receives appropriate levels of federal funding to maintain or expand its highway system. It also aids in the design of highway improvement projects. Decisions made regarding upgrades to traffic counting practices should be based on accurate, up-to-date information. This report summarizes the current state-of-the-art in traffic enumeration devices to facilitate this decision making process. This report is comprised of three components—an evaluation of current technology, a literature review, and a survey of State Department of Transportation (DOT) practices. The first section summarizes information supplied by manufacturers of devices used to collect count, speed, classification, and/or weight-in-motion data. Each manufacturer was asked to provide information regarding sensor technology, applications, classification algorithm, lane-monitoring capability, price, installation requirements, telemetry, calibration, power requirements, temperature requirements, and limitations of the system for each product. The second section contains the results of the Traffic Counting Survey circulated to the fifty State DOTs. Results were compiled in an Access database and summarized into tables for display in this report. The survey is included as Appendix A. Individual results from each state are included in Appendix B. The last section contains information gathered through a review of books, journals, Internet websites, and interviews with traffic counting professionals. Due to rapid advances in the area of traffic management, the review was limited to information from the past five years. A bibliography of relevant journal articles and websites dealing with traffic counting devices and transportation technology is included as Appendices C and D. 2.0 CURRENT TRAFFIC DATA COLLECTION TECHNOLOGY There are two main categories into which equipment for collecting traffic data can be placed—intrusive and non-intrusive devices. Intrusive (traditional) counting devices are those that involve placement of the sensor technology on top of or into the lane of traffic being monitored. They represent the most common devices used today including inductive loops, piezo-electric sensors and pneumatic rubber road tubes. Conversely, non-intrusive (non-traditional) counting devices such as passive acoustic and video image detection do not interfere with traffic flow either during installation or operation. Within these two broad categories, thirteen different technologies were identified for classifying devices used for recording traffic data. The collection of count, speed, class, and weight-in-motion (WIM) data are the focus for this report.
Figure 1. Product Classification A definition of each category, as used for purposes of this report, is listed below INTRUSIVE DEVICES Bending plate technology is most frequently used for collecting weight-in-motion data. The device typically consists of a weigh pad attached to a metal frame installed into the travel lane. A vehicle passes over the metal frame causing it to slightly "bend." Strain gauge weighing elements measure the strain on the metal plate induced by the vehicle passing over it. This yields a weight based on wheel/axle loads on each of two scales installed in a lane. The devices also is used to obtain classification and speed data. A pneumatic road tube is a hollow rubber tube placed across the roadway that is used to detect vehicles by the change in air pressure generated when a vehicle tire passes over the tube. A device attached to the road tubes is placed at the roadside to record the change in pressure as a vehicle axle. Axle counts can be converted to count, speed, and/or classification depending on how the road tube configuration is structured. Piezo-electric sensors are mounted in a groove that is cut into the roadway surface within the traffic lane. The sensors gather data by converting mechanical energy into electrical energy. Mechanical deformation of the piezo-electric material causes a change in the surface charge density of the material so that a change in voltage appears between the electrodes. The amplitude and frequency of the signal is directly proportional to the degree of deformation. When the force of the vehicle axle is removed, the output voltage is of opposite polarity. The change in polarity results in an alternating output voltage. This change in voltage can be used to detect and record vehicle count and classification, weight-in-motion and speed. [1] An inductive loop is a wire embedded into or under the roadway in roughly a square configuration. The loop utilizes the principle that a magnetic field introduced near an electrical conductor causes an electrical current to be induced. In the case of traffic monitoring, a large metal vehicle acts as the magnetic field and the inductive loop as the electrical conductor. A device at the roadside records the signals generated. [2] NON-INTRUSIVE DEVICES Manual observation involves detection of vehicles with the human eye and hand recording count and/or classification information. Hand-held devices are available for on-site recording of information gathered by one or more individuals observing traffic. 2.1.6 Passive and Active Infrared Passive infrared devices detect the presence of vehicles by measuring the infrared energy radiating from the detection zone. A vehicle will always have a temperature contrast to the background environment. The infrared energy naturally emanating from the road surface is compared to the energy radiating when a vehicle is present. Since the roadway may generate either more or less radiation than a vehicle, the contrast in heat energy is detected. The possibility of interference with other devices is minimized because the technology is completely passive. Passive infrared detectors are typically mounted directly over the lane of traffic on a gantry, overpass or bridge or alternatively on a pole at the roadside. Active infrared devices emit a laser beam at the road surface and measure the time for the reflected signal to return to the device. When a vehicle moves into the path of the laser beam the time it takes for the signal to return is reduced. The reduction in time indicates the presence of a vehicle. The mounting position for active infrared detectors is more variable. The Autosense devices from Schwartz Electro-Optics, Inc. are mounted over the lane(s) of traffic to be monitored or in a side-fire mount perpendicular to the lane of traffic. There also are portable, devices that are placed roadside so the laser beams are directed parallel to the road surface across the lane of traffic. Both active and passive infrared devices can be used to record count, speed, and classification data. Passive magnetic devices detect the disruption in the earth’s natural magnetic field caused by the movement of a vehicle through the detection area. In order to detect this change the device must be relatively close to the vehicles. This limits most applications to installation under or on top of the pavement, although some testing has been done with side fire devices in locations where they can be mounted within a few feet of the roadway. Magnetic sensors can be used to collect count, speed, and classification data. 2.1.8 Microwave - Doppler/Radar Doppler microwave detection devices transmit a continuous signal of low-energy microwave radiation at a target area on the pavement and then analyze the signal reflected back. The detector registers a change in the frequency of waves occurring when the microwave source and the vehicle are in motion relative to one another. According to the Doppler principle, when a moving object reflects the radar beam emitted from the detector, the frequency of the reflected wave is changed proportionally to the speed of the reflecting object. This allows the device to detect moving vehicles and determine their speed. The only sensors identified using Doppler microwave are produced by Microwave Sensors, Inc. and are used primarily as a detection device designed to trigger operation of a traffic controller. In this capacity, they are placed in an overhead mounting position. Radar (radio detecting and ranging) is capable of detecting distant objects and determining their position and speed of movement. With vehicle detection, a device directs high frequency radio waves, either a pulsed, frequency-modulated or phase-modulated signal, at the roadway to determine the time delay of the return signal, thereby calculating the distance to the detected vehicle. Radar devices are capable of sensing the presence of stationary vehicles. They are insensitive to weather and provide day and night operation. The device is placed in a side-fire mount off the shoulder of the roadway. This technology is capable of recording count, speed, and classification data. 2.1.9 Ultrasonic and Passive Acoustic Ultrasonic devices emit pulses of ultrasonic sound energy and measure the time for the signal to return to the device. The sound energy hits a passing vehicle and is reflected back to the detection device. The return of the sound energy in less time than the normal road surface background is used to indicate the presence of a vehicle. Ultrasonic sensors are generally placed over the lane of traffic to be monitored. Passive acoustic devices utilize sound waves in a somewhat different manner. These systems consist of a series of microphones aimed at the traffic stream. The device detects the sound from a vehicle passing through the detection zone. It then compares the sound to a set of sonic signatures preprogrammed to identify various classes of vehicles. The primary source of sound is the noise generated by the contact between the tire and road surface. These devices are best used in a side fire position, pointed at the tire track in a lane of traffic to collect count, speed, and classification data. Video image detection devices use a microprocessor to analyze the video image input from a camera. Two techniques, trip line and tracking, are used to record traffic data. Trip line techniques monitor specific zones on the roadway to detect the presence of a vehicle. Video tracking techniques employ algorithms to identify and track vehicles as they pass through the field of view. Different manufacturers technology may employ one or both of these techniques. Optimal mounting position for video image detectors is directly over the lane(s) to be monitored with an unobstructed view of traffic. Side mounting is feasible but large vehicles may obstruct detection zones. The mounting height is related to the desired lane coverage, usually 35 to 60 feet above the roadway. Video detection devices are capable of recording count, speed, and classification data. 2.2 MANUFACTURERS OF TRAFFIC COUNTING DEVICES A list of manufacturers was compiled through an Internet search and by conversations with traffic counting professionals. Any manufacturer producing one or more devices for collection of count, speed, classification, and/or WIM data was considered. Many systems are "open systems" in that the sensors and data collection devices may be from different manufacturers. This is the case with most pneumatic rubber tube and inductive loop systems. In addition, the data collection devices employed by these systems may utilize more than one sensor type. For the most part, the more sophisticated the technology, the more likely the system will be a "closed" system. Table 1 contains manufacturers identified by this researcher who were cooperative in supplying detailed product information and were responsive to questions regarding their products. The devices are categorized by their sensor technology; however, the product listing is limited to the devices used to interpret data output from the sensors. Manufacturers producing only sensors and not data recording devices were excluded from Table 1. A more detailed listing that includes contact name, address, telephone number, e-mail address, and website information for each manufacturer is included as Appendix E.
Each manufacturer was contacted for product information for any device they distribute that is used to collect traffic count, speed, classification, and/or WIM data. The focus was on devices designed specifically for use in high speed, freeway applications. Devices used primarily for presence detection at intersections for traffic signal applications or on freeway entrance ramps for traffic management were not considered. Table 2 summarizes devices currently on the market including manufacturer name, sensor type, and data collected. Although the devices are listed by sensor type, the emphasis was on acquiring information about the data recording and interpretation equipment that is attached to the various sensors. The sensor type used with a particular piece of equipment may or may not be made by the manufacturer listed. As previously stated, there is a wide range of open and closed systems available. Devices used for recording information obtained by manual observation were not included. Some issues that should be considered when selecting a particular product include traffic conditions at the site to be monitored, type of data to be collected, installation requirements, weather conditions, lane coverage, cost, and maintenance requirements. These requirements can determine whether a particular traffic counting device can or will work acceptably. It also is highly desirable for a new system to be field tested at the site in question prior to purchase of the device. Detailed information including technical specifications and installation requirements for each product in Table 2 are summarized in a Microsoft Access database.
Key to Sensor Types:
General considerations addressed in the product database are reviewed below. The installation requirements for each device are based on the type of sensor technology with a few exceptions. Looking first at traditional "intrusive devices," all pneumatic road tube products identified require the sensor to be placed across the roadway and attached to a counting device that is placed along the roadside. Installation generally takes less than an hour but requires some intrusion into the flow of traffic. Placement of road tubes is easy, quick and requires minimal technical expertise. Bending plates are much more labor-intensive to install. They require fixing the device to the roadway so intrusion in the flow of traffic is necessary. Piezo-electric sensors can be placed across the road surface or imbedded in the roadway. Imbedding the sensor requires cutting into the asphalt or concrete surface. The counting device is placed at the roadside. Installation can take less than an hour if the sensors are on top of the road surface or can take up to two days if placed into the roadway. Similar to some piezo-applications, inductive loop devices require the sensor to be imbedded in the roadway with the counting device placed at the roadside or in a nearby traffic cabinet. Again, inductive loop installation can take up to two days and will require lane closures. The non-invasive, non-traditional technologies identified could be divided into three groups based on installation requirements. The video detection, passive infrared, and ultrasonic devices require mounting directly over the traffic lane(s) with an unobstructed view of the traffic being monitored. The optimal height is typically 35-45 feet. Two manufacturers indicated roadside mounting is permissible in the absence of an overhead structure; however, accuracy diminishes the further the device is from the most distant lane being monitored. Installation time was consistently given as two hours for system set-up with additional time dependent on the availability of a suitable mounting structure. In addition, the presence of a bucket truck and flag support maybe required dependent on the installation site. Two manufacturers were identified who produced passive magnetic devices for freeway data collection—3M and Nu-Metrics. This technology requires that it be installed close to the road surface. The 3M Canoga micro-loop system is placed under the lane of traffic in PVC tubing without disrupting the road surface. A conduit is installed using horizontal directional drilling, without digging a trench. Nu-Metrics offers three passive magnetic devices that are installed by placing the small portable devices on or in the roadway. This technology is typically categorized as non-intrusive; however, placement of the sensors in the line of traffic seems to contradict this premise. Another manufacturer of passive magnetic devices, Safetran Traffic Systems, produces the IVHS sensor. However, the manufacturer recommends the device for detecting vehicle presence rather than highway traffic counting and classification. The last group—radar, passive acoustic, and active infrared devices—are typically mounted roadside on an existing structure such as a street light or sign post. Sensor placement will impact how many lanes of traffic can be successfully monitored. The time required for installation is similar to the video detection devices. Set-up of the device takes about two hours if there is an existing roadside structure for mounting the sensor. The only exceptions identified were the Multi-Lane Monitoring System (MLMS) by Spectra-Research and the SafeCount by Peek Traffic. These devices are portable, active infrared systems placed on the ground 10 to 15 feet from the lanes of traffic to be monitored. Installation time is less than one hour. 2.4.2 Power and Temperature Requirements Power and temperature requirements for each of these devices did not seem to present limiting factors with respect to product selection. The majority of the devices that were placed free standing along the roadside were battery operated and offered several options related to battery size, solar power, and rechargeable varieties. It is likely that power requirements would be of most concern in remote areas where power sources are unavailable. In this case, short-term portable counting devices could be utilized. Most single channel permanent installations offered battery options but multi-channel devices require 120 VAC. The operating temperature ranges for all devices were on the average from –30° to +65° C (-22° to +149°F). The Nestor Traffic Vision was a rare exception with an operating range of only +10° to +35° C (+50° to +95° F). Temperatures would be problematic only in regions of the country where weather extremes are frequent occurrences. However, it is important to keep in mind that the manufacturer's reported operating ranges may not take into account "real world" factors. Although the device may perform well in a test environment, there are "real world" conditions that can cause a device to fail. For example, the high summer temperatures in Arizona can cause the asphalt to shove leading to failure of inductive loops. Manufacturers may be unaware of these issues or reluctant to share them with potential buyers. Consequently, it is prudent to contact actual users for their experience prior to purchasing a new device. Table B24 in Appendix B lists the manufacturers of traffic counting devices used by each state to assist in this process. Data retrieval techniques ranged in complexity from reading traffic counts from a visual display on the recording device to having the ability to remotely configure, perform diagnostics and extract data via modem, landlines or wireless connection. Most systems offer more than one option for data retrieval with the degree of flexibility dependent more on the data collection device rather than the sensor type. The number of data retrieval options available increases with the level of sophistication and complexity of the equipment. The pneumatic road tube, inductive loop, and piezo-electric sensor systems consistently offer roadside data retrieval using data cards or a laptop. A few low cost models that record strictly traffic counts offer visual displays so that a computer is not necessary. With non-intrusive technology, remote data retrieval is more typically available. The minimum requirement is a receiving computer, either laptop or PC, with an RS-232 serial communication port being the most common standard for data retrieval. The purchase of additional software or data modules will increase the available options but also increases the price of the system. In general, most manufacturers are willing to work with the end user to configure a system that fits their data collection and retrieval needs as well as budget. Price information was requested from all manufacturers. The prices quoted were very dependent on site parameters that would be unique to a particular installation. There also were many issues that varied between manufacturers as to what was or was not included with the product. Some variables included data analysis software, types of sensors, rack or shelf mount format, data storage capacity and optional modules for data retrieval or WIM. Consequently, it was difficult to obtain information that was comparable across product lines. In considering equipment cost, on the surface the prices for non-intrusive devices appear to be higher. But, this may not actually be the case. The Texas Transportation Institute (TTI) study addressed the issue of life-cycle cost in its report Evaluation of Some Existing Technologies for Vehicle Detection. In this study, inductive loops were compared to other non-intrusive detection systems in several districts throughout Texas representing different sized urban applications. The elements that were considered in the life-cycle cost of each device were installation cost, maintenance costs, traffic control, motorist delay and related excess fuel consumption, additional pavement maintenance costs, and costs related to increased crash rates during installation and maintenance of some detectors. Table 3, reproduced from the TTI study, shows the per-lane cost comparison. It must be kept in mind that the TTI project summary covers the period from September 1996 to August 1999 so the price information is not current. However, it is possible to garner a relative cost comparison between the four different technologies. Readers should refer to the study for more information on specific details of how the figures were obtained. Table 3. Freeway Detector Annualized Per-Lane Cost Comparison
[Source: 5] One issue not addressed in the cost comparison is the level of expertise required for installation and operation. This may be a concern for some agencies. Although the non-intrusive technologies are more sophisticated, they are actually quite user-friendly. Set-up of most devices is with the use of intuitive, Windows-based software programs. Many vendors include in the price installation costs or the onsite presence of an individual during installation. There also are various end user training options available. As would be expected, each of the products listed in Table 2 has its limitations. Most manufacturers were reluctant to discuss limitations of their particular traffic data collection equipment but rather focused on general limitations of the technology. Familiarity with the limitations of each sensor type will help facilitate successful equipment selection. This information is listed in Table 4 on the following page.
Table 4. Limitations of the Technology
Comparatively assessing the performance of traffic counting devices is difficult. The differences in the technology necessitate very different installations. Selecting one particular section of highway to test all devices would seem to be optimal for comparison purposes but may not be the best assessment of a particular device’s capabilities. As has been stated previously, selection of a counting/classifying device should be based on several considerations, one of which is where the device will be installed. A site that may work well for video detection may not be optimal for passive infrared. The Texas Transportation Institute took a comparative look at the use of detectors in a freeway application in its study Evaluation of Some Existing Technologies for Vehicle Detection. The selection guide that was developed is reproduced as Table 5. TTI points out in its report that the reader should keep in mind the subjective nature of the evaluations when reviewing the data. In addition, one should remember this assessment is "only a snapshot, and it will surely change" as the technology continues to evolve. [5] Table 5. Application Guide for Detector Selection on Freeways
[Source: 5] Code: A = Excellent; B = Fair; C = Poor; D = Nonexistent; U = Unknown * A: Overhead mounting; B: Side-fire mounting
The type of traffic data collection devices available on the market has changed little in the past decade. The same thirteen technologies are still being utilized by State, county, city, and metropolitan organizations responsible for traffic monitoring operations. Some products have come and gone off the market and companies have been bought and sold, but the science remains pretty much the same. This is not to say the industry has been at a stand still. The devices have evolved as their use has come under greater scrutiny with increased usage. But, the increased usage has been more likely due to the recent focus on "intelligent transportation systems" (ITS) and the use of these devices in support of this movement. This is particularly true in the area of advanced traffic management systems (ATMS) where video image detection, Doppler microwave, passive magnetic, and passive acoustic technology are being used for signalized intersection control, incident detection and management, speed traps, and freeway metering control. As the need for collection of accurate, reliable traffic data is realized as essential for allocating scarce resources to support an aging infrastructure, greater pressure will be placed on manufacturers to make the existing technology used for traffic data collection more efficient and cost-effective.
The AZDOT Traffic Counting Survey was conducted to ascertain the current practices of State Departments of Transportation. In addition, each agency was asked their level of satisfaction with the technology in use, disadvantages identified, frequency of use and manufacturer name. The information will be used to assist in decision-making regarding changes to AZDOT’s current traffic counting practices. A two-page survey was sent to the fifty state DOTs on January 29, 2001. Prior to distribution of the survey each agency was contacted to obtain the name and address of an individual capable of providing the required information. Participants were given four weeks to respond to the survey. A list of each agency and the individual(s) completing the survey follows: Table 6. State Departments of Transportation
All fifty States returned survey results. The data were entered in a Microsoft Access database and summarized for this report. Following review of the results, individuals responsible for completing the survey were contacted for clarification of responses and to obtain additional or missing information.
The AZDOT Traffic Counting Survey included three questions. The questions are shown below with an example of the response format accompanying each. Only a small sample of each question type is included below. The complete survey is included as Appendix C.
Count Speed Weight Class Manufacturer
All fifty states returning results responded to question 1. The question asked agencies to rate their level of satisfaction (LOS) with each method for collecting traffic data. Responses were only to be given if the agency was actually using the equipment listed. The rating scale was from 1 to 5 with 1 being "very dissatisfied" and 5 being "very satisfied." Of the thirteen sensor technologies listed, no state reported using passive infrared, active infrared, Doppler microwave or pulse ultrasonic. Answers to the first question are shown in Table 7 on the following page. Table 7. Level of Satisfaction by State
The number and percent of states using each technology and the average level of satisfaction with each device is listed in Table 8. The methods have been arranged from left to right in decreasing frequency of usage among States. Table 8. Usage and Average Level of Satisfaction
According to the survey results, pneumatic rubber tubes, piezo-electric sensors and inductive loops are the most prevalent sensor technologies in use for collecting traffic data. Each is used by greater than 90% of the states reporting results with inductive loops the highest at 100%. The popularity of inductive loops is not surprising as it "continues to be the best all-weather, all-light condition sensor for many applications." [5] Participants were asked to rate their level of satisfaction with the thirteen technologies listed. Inductive loops achieved the highest score of all sensor types with consistent ratings of 3, 4, or 5 by all states and an average LOS of 4.4. Manual observation ranked second highest in LOS with an average of 4.0. This seems surprising due to the inherent inaccuracy and lack of consistency between observers that occur with any manual process. It definitely outperformed the more sophisticated technologies. The newer non-intrusive technologies—radar, video image detection, passive acoustic, and passive magnetic—rated consistently lower with the average LOS ranging from 2.8 to 3.4. This may be due to a number of factors such as complexity of the installation process, maintenance requirements, expense, or lack of experience and familiarity with the newer technology. However, with so few states reporting use of the later three technologies it is difficult to draw many conclusions from the results. The second survey question requested each state to indicate any disadvantages with their use of the different data collection devices. A summary of the results is shown in Table 9. The sensor types are listed on the left in order of decreasing frequency of usage. The devices with the greatest number of disadvantages per category as a percentage of the number of users have been shaded.Table 9. Disadvantages Reported by Technology
In looking at individual results by state, there does not appear to be any correlation between the manufacturer of the equipment used by the state and the disadvantages reported. For example, data accuracy was reported as a disadvantage of pneumatic road tube use. This was reported across the most prominent manufacturers and throughout geographic locations. It seems likely the disadvantages reported are a function of the type of technology rather than any other factor identified by this survey. There also are survey results for which there is no explanation such as system failure and installation cost reported as a disadvantage of manual observation. These results are likely information entry errors on the part of the participating DOTs. The third survey question requested three pieces of information—type of traffic data collected, frequency of method use, and device manufacturer. Participants were asked to indicate what type of data they gather using each of the thirteen sensor technologies and approximate percent of results reported using each method. Forty-nine states reported results for question 3. Individual results reported by each state are listed in Appendix B. A summary of all results is provided in the Table 10 below. Table 10. Method of Data Collection
According to these data, the most popular methods for vehicle counts reported by 47 out of 50 states are pneumatic rubber tubes and inductive loops. Inductive loops are the most popular for reporting speed data. As would be expected, piezo-electric sensors and bending plates were the most frequently used for reporting weight. Lastly, vehicle classification is most commonly reported using pneumatic rubber tubes and piezo-electric sensors. In reporting the type of data collected, participants also had to indicate the approximate frequency of use of each sensor type. Unfortunately, this section of the survey caused respondents some confusion. The intent of the question was to ascertain what percentage of vehicle counts are reported using each sensor type. For example, under vehicle count, a DOT may report < 25% using pneumatic rubber tubes, 51-75% using inductive loops, and < 25% using piezo-electric sensors. The total of the three sensor types approximates 100%. However, this was not the case for approximately 30% of the survey respondents. Instead of reporting totals across data type, the results appear to reflect percentage of results reported across sensor type. For example, under inductive loop, one participant reported that 25-50% of loop data are vehicle counts, < 25% are speed data, and 25-50% are classification data. The approximate total across sensor type totals 100%. It also appears that some States may have ignored the percentage descriptor and selected responses as though the quantities were absolute values. The respondents may have been trying to record the number of sensors in use rather than the percentage of the total results. Regardless, caution must be taken when attempting to draw conclusions from the complete set of results shown in Appendix B. In order to draw meaningful conclusions, results that were obviously incorrectly reported were eliminated from the summary in Table 11. The following numbers of States were included in each summary: count - 29, speed - 22, weight - 31, class - 28. The number of States reporting < 25%, 25-50%, 51-75%, or > 75% to indicate the percentage of data reported by a particular method are listed in the columns below. Table 11. Frequency of Method Use
Whereas Table 10 showed how States are using each sensor technology, Table 11 shows the frequency with which each device is used within a particular State for collecting a specific type of traffic data. The results show that the majority of States are using pneumatic rubber tubes to collect more than half of their vehicle count data, inductive loops for speed, piezo-electric sensors for weight, and pneumatic rubber tubes for the majority of classification data. The last portion of question 3 asked for information on equipment manufacturers. The intent was to gather information on the manufacturer of the data-recording device rather than the sensor. This was not clear to some respondents. Those reporting the manufacturer of their sensors were contacted for additional information. Some of the most commonly reported sensor manufacturers were Measurement Specialties, Vibracoax, Sperry Rubber, Trigg Industries and Hanna Rubber. Table 12 summarizes the manufacturers used by all States reporting results. The list is in alphabetic order by manufacturer name. Note that many States reported using more than one manufacturer’s equipment for collecting a particular type of data. A list of the equipment used by each state is included in Appendix B.
Table 12. Device Manufacturers
It is apparent when reviewing the survey results that a few manufacturers dominate the State DOT market. For bending plates, International Road Dynamics (42%) and ITC/Pat America (58%) were the only manufacturers reported. With pneumatic rubber tubes, the leaders are Peek Traffic and Diamond Traffic. Both companies manufacture cost-effective, easy-to-use devices for counting and classifying traffic. Unfortunately, the intrusive nature of road tubes makes them potentially dangerous for the road workers who install and maintain them. These two manufacturers showed similar market dominance with inductive loop sensors. Peek Traffic held 47% of the market with Diamond Traffic coming in second at 22%. The distribution of manufacturers among States using piezo-electric sensors is more wide spread. Peek Traffic remains the leader with 33% of the market and Diamond Traffic, Electronic Controle Mesure, and International Road Dynamics each having close to 19% each. The market for non-intrusive devices was quite different. In several cases, a particular technology may only be produced by one manufacturer. For instance, EIS Electronic Integrated Systems was the only company identified who distributed radar traffic data collection equipment. A similar situation exists with passive magnetic and passive acoustic technology. Nu-Metrics and 3M are the only two manufacturers producing passive magnetic. Electronic Integrated Systems is the only company producing passive acoustic equipment. There also have been some acquisitions over the past decade. Pat America purchased International Traffic Corporation in 2000 so these results have been combined throughout the report. In addition, StreeterAmet was purchase by Peek Traffic. These results have similarly been combined and listed under Peek Traffic. Lastly, GK, a British manufacturer, no longer produces counting devices for rubber tubes and loops. Less than half of all State DOTs (24 out of 50) are using non-intrusive methods for gathering traffic data. This may be due to the lack of comparative data showing the accuracy of these new technologies as compared to standard road tubes, inductive loops, and piezo-electric sensors. Other factors contributing to the reluctance to convert to non-intrusive technology may be cost and the level of technical expertise required to operate the devices. Both issues were addressed in Section 2.0. Inductive loops are probably the most consistently accurate device for vehicle counting applications. However, the newer non-intrusive technologies show great promise. As they show increased usage, they will continue to evolve and improve. Unfortunately, manufacturers cannot afford to invest in the research and development needed to continue to improve these devices without the assurance that a tangible market for their product exists. Additional cooperative studies validating the accuracy, reliability, and cost-effectiveness of these devices need to occur so that both groups will benefit. [3]
The purpose of the literature review is to explore advances in traffic counting technology that go beyond the traditional inductive loops and pneumatic road tubes. This was done through an extensive search of books and journal articles as well as websites associated with the transportation industry. State and federal transportation agencies, professional associations, and manufacturers of traffic counting devices were included in the search. As the goal of the review is to focus on "new developments" in traffic counting, this reviewer concentrated on information published after 1995. Due to ongoing advances in technology, anything beyond five years would likely be technologically outdated. This report reviews some of the important articles on advances in technology; however, it is recommended that a complete copy of these reports be obtained for a more detailed discussion of these in-depth evaluation projects. Information is provided on the report content and where the reader can find each report. Also, bibliographies of informative articles and websites are included in Appendices C and D for those who wish to read further and continue watching for new developments. 4.2 NEW AND IMPROVED TECHNOLOGY Despite extensive research, little was uncovered in the way of "new technology." Rather, the information found relates to improvements in existing technology. As the need for non-intrusive traffic detection devices becomes increasingly important with the evolution of ITS, the pressure is on manufacturers to invest in research and development to improve existing technology. Improvements have been made to the traditional inductive loop and piezo-electric sensors. Non-intrusive devices that have failed to catch on due to their high upfront cost and undocumented field performance also are under scrutiny. nductive Loops Some inductive loop manufacturers are looking toward using "signatures" to improve vehicle classification accuracy. Each vehicle has a characteristic signature resulting from its unique features. Aside from length, trucks have axle and hitch combinations that are unique to the vehicle type. [6] Partners for Advanced Transit and Highways (PATH) headquartered at the University of California in Berkeley and the University of Florida are both heavily involved in research to improve the classification ability of inductive loops. Two examples of classification devices using forms of this technology are the Peek’s Idris® Smart Loops AVC System and U.S. Traffic Corporation’s IVS-2000. With the Smart Loop System, the classification scheme is based on a per vehicle record which is comprised of vehicle length, number and spacing of axles, presence of dual tires and vehicle profile. Smart Loops use a special loop array per lane, 6’6’’ (2m) square and 6’6" (2m) apart with two optional axles loops in-between to reliably separate, profile and track each vehicle as it passes through the station. The system can be set-up and operated by remote telemetry. The manufacturer claims separation accuracy at > 99.96% and axle class accuracy at > 99.4%. [4,7] The IVS-2000 system uses a complete "inductive signature" to classify vehicles using advanced neural network software. The system classifies vehicles into 23 different classes—13 FHWA plus ten additional classes. The accuracy rate is reported by the manufacturer to be 85 to 90 percent using one or two loops. With the IVS system, a per vehicle, time-stamped record is created that is used to process classification data. The system operates with one or two loops per lane and can be used with existing loops. [4] 4.2.2 Passive Acoustic Devices The concept of neural networks for data collection and interpretation has potential beyond inductive loops. This pattern matching technology also is being used with acoustic sensors to improve on vehicle classification accuracy. Similar to the inductive loops, an "acoustic signature" is developed with the use of microphones and digital audiotape records. The neural net then uses this information to classify vehicles based on their unique acoustic signature. [8] In the area of piezo-electric sensors, Kistler Instruments Corporation is using quartz-based material in its force transducer design. Since the piezo-electric force transducers are ideally suited for measuring dynamic events, they cannot perform truly static measurements. The charge from a static load can be registered; however, it cannot be stored for an indefinite period of time. In this situation, "highly insulated materials are required to ensure a maximal discharge time constant and optimal operation of the charge amplifier (i.e., minimal drift)." Quartz has an ultra high insulation resistance that makes it ideal for static measurements. The Kistler system can routinely measure large forces for minutes and perhaps even hours. The quartz sensor can be used for either direct or indirect force measurements. [9] The Maine Department of Transportation reports having considerable success with the use of quartz sensors for collection of weight data. Readers interested in pursing use of quartz sensors may wish to contact the agency for more information on their experience. Non-intrusive traffic data collection devices were first employed in the 1940s with the use of magnetic sensors. Twenty years later, ultrasonic and microwave sensors came into use. [10] However, none of these devices came close to competing with the inductive loop in terms of accuracy and reliability. More recently, as safety, cost, increased traffic flow, complex road geometrics and traffic disruption have become issues of concern, traffic counting professionals are looking more closely at alternatives. Several studies were identified that deal with these concerns and evaluate the feasibility of replacing traditional inductive loops and pneumatic road tubes with non-intrusive devices for traffic data collection. Comparative Evaluations One of the first projects to evaluate traffic detection technologies was the Hughes Aircraft project, Detection Technology for IVHS, sponsored by the Federal Highway Administration. The 1996 study involved evaluating various devices at freeway and surface street arterial sites in Minnesota, Florida, and Arizona. The technology evaluated in the study included ultrasonic, microwave radar, infrared laser radar, nonimaging passive infrared, video image processing with visible and infrared spectrum imagery, acoustic array, microloop, and magnetometer detector technologies. In addition to these non-intrusive devices, high sampling rate inductive loop and conventional inductive loop devices were included as representing the "most consistently accurate" technology at the time. [3] The specific objectives of the project were:
The study focused on evaluating current technology for its acceptability in replacing inductive loops at permanent data collection sites for Intelligent Vehicle Highway Systems (IVHS) applications now know as Intelligent Transportation Systems (ITS). The evaluation centered on assessing performance in various weather and traffic conditions. Recommendations are given for best performance for low and high volume count and speed determination and in inclement weather. A qualitative assessment of the results is shown in Table 13. Although the study provides valuable information on performance, it does not address practical considerations related to ease of installation, calibration, and cost. The document can be obtained at TRIS Online through the search function, http://199.79.179.82/sundev/search.cfm. Table 13. Qualitative Assessment of Best Performing Technologies for Gathering Specific Data
[Source: 3] X indicates the best performing technologies. -- Indicates performance not among the best, but may still be adequate for the application. No entry indicates not enough data reduced to make a judgment. * Does not detect stopped vehicles. In May 1997, the report Field Test of Monitoring of Urban Vehicle Operations Using Non-Intrusive Technologies was published by the Minnesota Department of Transportation, Minnesota Guidestar and SRF Consulting. This report documents the results of a two-year study of non-intrusive traffic data collection devices. Each of seventeen different devices representing eight non-intrusive technologies was evaluated under differing traffic conditions including both intersection and highway locations. The report does not include a product-to-product comparison or evaluate one technology against another. Rather, it provides information on ease of system set-up and use, general system reliability, and system flexibility for the devices evaluated. [10] Even though the study was completed as recently as 1997, several of the devices are either no longer sold or have been revamped. The devices evaluated in the study include the following:
Table 14. Devices Evaluated in MnDOT Study
The study was comprised of two separate field tests—an Initial Field Test of selected devices from the list above and an Extended Field Test that included all devices on the list. The Initial Field Test was conducted on an interstate highway using an overpass bridge and installed poles for mounting locations. The traffic conditions included low-volume free flow and high-volume congestion. The test periods included both 24-hour and continuous counting intervals. Six inductive loops, originally installed as part of the previously mentioned Hughes’ project, were used to provide baseline count and speed data. Manual counts and speed observations were used to validate the accuracy of the loops. [10] The Extended Field Test was conducted at the same interstate highway location as the Initial Field Test but in addition an adjacent intersection site was added to the project. The intent of the Extended Field Test was to test the technologies under a variety of traffic and environmental conditions over a one-year period. The longer test period allowed for testing the devices against the harsh winter weather conditions of Minnesota that include snow, rain, fog, sleet, and high winds. In addition, the impact of various lighting conditions associated with seasonal positioning of the sun could be assessed. [10] Some of the most important conclusions of this study involved recommendations on device selection. The study found that the performance of one device over another within a particular technology type was more significant a factor than differences between technology types. The emphasis should be on choosing a well-designed and reliable product rather than limiting the selection to a particular technology. [10] In addition, the compatibility of the device with its intended use and installation site will also dictate how well it meets performance criteria established by the user. The "CONCLUSIONS" section of the Executive Summary from the MnDOT Study is reproduced in Appendix F. A copy of the 288-page report can be ordered directly over the Internet from the Minnesota Department of Transportation at http://www.dot.state.mn.us/guidestar/nitfinal/order.htm. The need to identify technology that can safely be installed without interrupting the flow of traffic continues to be a priority. Consequently, a second project continuing the work of this first study has been planned. Phase Two will continue to focus on historic data used primarily for planning purposes as well as investigate real-time ITS data collection applications. The five goals identified in the October 24, 2000 Draft Evaluation Test Plan are:
Readers should watch closely for the results of the second project as it will likely provide additional valuable information for decision makers in traffic monitoring divisions of State, county and municipal agencies. [11] Two years after the initial MnDOT study, the Texas Transportation Institute (TTI) in cooperation with the Federal Highway Administration and the Texas Department of Transportation published An Evaluation of Some Existing Technologies for Vehicle Detection. This report takes the Minnesota Guidestar report a step further by determining strengths and weaknesses of competing technologies. In particular, the study addresses reliability in the form of failure rates, accuracy rates, and cost comparisons. There also are selection criteria for decision-makers faced with replacing or upgrading existing traffic detection devices. The information is included in Table 5 of this report. Texas Transportation Institute extensively tested inductive loops and selected non-intrusive devices including the Accuwave 150LX Presence Detector, Nestor Traffic Vision Video Detector, Eagle Traffic Passive Infrared Detector (PIR-1), Electronic Integrated Systems RTMS, and International Road Dynamics Smart Sonic. In addition, there is a secondary discussion on the Econolite Control Products Autoscope video detection device based on its use by the Road Commission of Oakland County (RCOC), Michigan as part of their FAST-TRAC. However, RCOC uses the device primarily for adaptive signal control applications. [5] There are two particularly interesting parts of this report aside from the evaluation of non-intrusive devices. The first is the specification document for video image detection devices provided in the appendices. The specification document outlines procurement, installation, and performance requirements. This information is essential for transportation professionals who are considering the purchase of video image detection devices. The second part is the extensive discussion on TTI’s inductive loops experience and comparative assessment of ILDs against non-intrusive devices. It is TTI’s contention that a better understanding of ILD operation "should result in improved performance and longevity." [5] In addition, the "reliability and useful life" are directly related to the quality of the installation process. [3] The TTI report is available through the National Technical Information Service (NTIS) at http://www.ntis.gov, publication number PB2000-106667INZ. 4.3.2 Single Product Evaluations There have also been studies focusing on individual products and/or technologies. One of these tests is documented in the report Field Evaluation of a Microloop Vehicle Detection System from the Florida Department of Transportation. This July 2000 report tested the 3M Canoga® Vehicle Detection System Model 702. The study showed the microloop system detects the presence of slow moving vehicles and those passing at normal speeds very well. However, the system was not able to provide true presence of stopped vehicles. The system is also able to record average speed at accuracy levels very close to those of inductive loops. The ability to assess vehicle length for purposes of vehicle classification was questionable. Those considering this device should review the findings of this report for more detail. [12] The report can be downloaded at http://www.dot.state.fl.us/trafficengineering/terl.htm. California Polytechnic State University conducted a series of studies on the use of video image processing for traffic detection. The study was initiated in 1991 and has been ongoing as Cal Poly’s contract with the California Department of Transportation (Caltrans) is extended. The project has gone through several phases and continues to evolve as the technology evolves. The video image detection devices involved in 1997 phase III of the study include Rockwell TrafficCam System, Transyst Peek System, Econolite Autoscope, and Odetics Vantage. Although some of the earlier results are dated, the reports provide good background information for anyone considering the use of this technology. There also is an installation guidelines document in progress that gives detailed information on the intricacies of properly installing video devices. The reports can be found at http://gridlock.calpoly.edu. 4.3.3 Additional Information Resource New Mexico State University maintains the Vehicle Detector Clearinghouse (VDC) that is dedicated to providing information to transportation agencies on the capabilities of commercially available vehicle detectors. The VDC is a state pooled-fund project whose mission is "to provide information to transportation agencies on the capabilities of commercially available vehicle detectors by gathering, organizing, and sharing information concerning tests and test procedures in a timely, efficient, and cost-effective manner. Equipment types included in the VDC are devices that detect vehicle presence, speed, axles, classification (AVC), and weight (WIM). The clearinghouse will be a catalyst for developing standard test protocols." Until very recently (June 2001), no modifications had been made to the information on the VDC website, http://www.nmsu.edu/~traffic, for the past year despite the fact that the December 1999 newsletter indicates that a new contract provides for funding through December 2002. This may be due to the fact that the information was being assembled into the report A Summary of Vehicle Detection and Surveillance Technologies Used in Intelligent Transportation Systems, produced by the Southwestern Development Technology Institute at New Mexico State University. The report can be obtained at http://www.fhwa.dot.gov/ohim//tvtw/vdstits.htm. The VDC website has the potential to be an invaluable resource for monitoring new trends in the area of traffic data collection devices. An additional resource that was identified for new technology is the book Advanced Traffic Detection: Emerging Technologies and Market Forecast published by Scientific American Newsletters. According to the publisher this document is for "a user of detection equipment seeking guidance through a complex range of product offerings." It contains sections on Technology & Market Analysis, Market Share Data, Installation Details, and Individual Vendor Profiles. The book can be ordered on line for $1,995 at http://www.sanewsletters.com/its/ATDsummary.html. [13] The state-of-the-art of traffic counting devices is changing rapidly. There is a new focus in the industry to develop reliable, non-intrusive devices that are easy to use and cost effective to operate. However, there is much to be learned through the experiences of those who have evaluated these devices. It is recommended that the reader obtain the reports listed in this section to learn from the experiences of those who have installed and operated these devices in the field. The reports provide valuable practical information that can only be gained from working directly with the equipment.
SURVEY INSTRUMENT Arizona Department of Transportation Traffic Counting Survey The Arizona Department of Transportation is gathering information the traffic counting practices employed by other states. We would appreciate your response to the following questions. This information will be used to assist AZDOT in improving future traffic counting practices. 1. How satisfied are you with the data collection device(s) currently employed by your agency to collect traffic data? (Mark only those used.)
2. Please check any specific disadvantages you have noted with your use of the equipment types listed below.
3. Please indicate the approximate percentage each method represents of results reported and the manufacturer(s) of the equipment currently used to interpret your traffic data. (Mark only those used.)
TRAFFIC COUNTING SURVEY RESULTS Question 1. How satisfied are you with the data collection device(s) currently employed by your agency to collect traffic data? (5 = very satisfied, 1 = very dissatisfied) Table B1. Level of Satisfaction
Question 2. Please check any specific disadvantages you have noted with the use of the equipment types listed. Table B2. Disadvantages Reported Using Manual Observation
Table B3. Disadvantages Reported Using Bending Plates
Table B4. Disadvantages Reported Using Pneumatic Road Tubes
Table B5. Disadvantages Reported Using Piezo-Electric Sensors
Table B6. Disadvantages Reported Using Inductive Loops
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