Equipment for Collecting Traffic Load Data (2004)

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41 CHAPTER 4 A PROCESS FOR SELECTING EQUIPMENT Each of the technologies discussed in Chapters 2 and 3 has strengths and weaknesses for collecting classification and weight data. Under the right conditions, most of the tech- nologies can collect data of the quality needed for estimating traffic loads for the pavement design software. However, each of these technologies can perform very poorly when placed in environments that are not conducive to the tech- nology being used or when used incorrectly. As a consequence, the state highway agencies most suc- cessful at data collection own and operate more than one type of vehicle classification and/or WIM equipment. Different types of equipment are used in different operating environ- ments. This helps ensure the quality of data that are collected, but also forces understanding of and accounting for minor dif- ferences in data supplied by different devices. (For example, some agencies use dual-inductance loops to collect length- based truck classification data on urban freeways but axle sensor-based counters to collect classification data on rural roads. Special studies are needed to correlate these two data collection schemes. But through the use of detectors placed to collect traffic operations data, these simple correlation stud- ies provide access to large amounts of important truck count information that could not be collected otherwise.) Selecting technologies (and vendors) requires careful analy- sis of three different types of information: • Data collection needs of users, • Data handling requirements and capabilities of the high- way agency, and • Characteristics of available makes or models of equip- ment (e.g., cost, reliability, and data provided). Within each of these general subject areas are a variety of important issues. It is each agency’s responsibility to explore these issues and to balance the advantages and disadvantages of each technology when selecting equipment both in general and for specific data collection implementations. The material presented below briefly describes the issues that need to be considered when agencies select equipment for vehicle classification and/or truck weight data collection. In addition, Chapter 5 presents a series of best practices recom- mendations that describe tasks needed to ensure the collection of reliable, accurate traffic data. Adoption of these practices, or of variations in these practices, is likely to improve the quality of the data collected and reduce the overall cost of the data collection effort. 4.1 DATA COLLECTION NEEDS The traffic data needs for the pavement design software are being addressed in detail under other tasks of NCHRP Project 1-39. These needs require capability to collect the following: • Short-duration (48-hour) classification counts on roads and road segments where traffic loads will be needed, • Long-term classification counts (i.e., data collected for more than 1 year) at a limited number of locations around the state, and • WIM data collection at a limited number of locations. These capabilities are consistent with the general agency counting needs identified in the FHWA’s Traffic Monitoring Guide as meeting the needs of a wide variety of users. The specifics of the required data collection efforts are divided into vehicle classification issues, location issues, and count duration issues. 4.1.1 Classification Issues A good starting point when examining data collection equipment is the type of classification scheme the equipment is capable of providing. Axle-based classifications are pre- ferred for pavement design purposes, but length classification is acceptable when axle classes cannot be reliably collected. Truck characteristics (overall length, axle spacing, etc.) tend to differ from state to state. Each state highway agency should have an algorithm they have tested and certified that can correctly convert axle count and spacing information into accurate vehicle classification. Can this algorithm be imple- mented with the proposed equipment? If not, what flexibility is offered by the vendor in the classifications supported, and how do those classes correlate with the FHWA’s 13 classes or the classification system used by the state highway agency? Finally, if a classification algorithm other than the one tested and approved by the highway agency must be used, the agency must thoroughly test the new algorithm. Acceptance of the new data collection equipment should be contingent on the

satisfactory performance of the equipment in that test, and the highway agency should use those results to understand how data collected with the new device correlate with the data collected using the agency’s WIM scales. 4.1.2 Location Issues Often the classifications collected are not driven by the pref- erences of the user, but by the constraints of the location at which data must be collected. A number of location-specific constraints can affect the choice of data collection technology. Key location-specific constraints and their effects include the following: • Is the data collection site urban or rural oriented? (That is, is congestion likely to be present? Are vehicles often formed into closely space platoons?) Devices that work effectively in uncongested rural areas often do not work effectively in more congested urban conditions. (Tests performed by Minnesota Guidestar in 2001/2002 should provide guidance on which non-intrusive devices can accurately collect data in urban conditions. See http://www.dot.state.mn.us/guidestar/projects/nitd.html.) • Are there traffic signals or other control devices nearby that may affect vehicle speeds and/or spacings? (Classi- fication and WIM equipment should not be located near signals because vehicles frequently accelerate and decel- erate near intersections, causing problems in the perfor- mance of most classification and weight data collection devices.) • At what speeds are vehicles traveling? (Some devices do not operate at very low traveling speeds, while others are not effective at very high traveling speeds.) • On how many lanes do data need to be collected, and what is the layout of those lanes? (Some devices can only collect data in one lane, and that lane must be on the out- side of the roadway, next to a shoulder or median.) • Can detectors be placed safely in (or above) the travel lanes? Is formal traffic control needed for this purpose? (Inability to place sensors in the lanes of travel would indicate the use of non-intrusive detectors, and inability to work above the lanes of travel would further restrict the technology choice to one that can function from beside the roadway.) • Are there specific site constraints that need to be accounted for in the selection of equipment? (Will the road’s channelization be changing in the near future, so that permanent intrusive sensors are not cost-effective and non-intrusive sensors should be selected?) • Are there other features to the site that constrain or enhance the use of certain data collection technologies? (What is the availability of power or communications, and does that availability indicate the need to select a low- power consumption data collection technology? Does the presence of an existing sign, bridge, or other overhead 42 structure reduce the cost of non-intrusive sensor place- ment to the point where they have a cost advantage over intrusive sensors?) • Are there site conditions that prevent accurate data col- lection from taking place? (Is the pavement in too bad of a condition for WIM or classification equipment to operate correctly? Does the poor pavement condition warrant the use of non-intrusive data collection tech- nologies either because intrusive sensors will not sur- vive long or because the poor pavement will cause axles to jump the intrusive sensors?) • Is the pavement depth deep enough to allow sensor instal- lation? (If not, choose a different location, use a non- intrusive sensor, or build a special pavement slab deep enough to hold the sensor.) • Are there environmental conditions that restrict the use of specific technologies? (Do temperatures drop below lev- els at which some technologies work? Are temperature variations sufficient to cause calibration errors in some sensor technologies? Are there visibility constraints that limit the accuracy of specific non-intrusive data collec- tion technologies?) • Are there environmental conditions that are likely to badly impact the duration of data collection? (For exam- ple, are freeze-thaw conditions likely to reduce the expected sensor life?) In some cases, shortcomings of the site should warrant selection of another data collection location. For example, poor pavement condition affects the performance of all WIM sensors. Rather than accepting poor WIM performance (and perhaps choosing an inexpensive, inaccurate sensor because “bad data will be collected no matter what sensor is chosen”), consideration should be given to either (1) moving the data collection site upstream or downstream to a site more con- ducive to WIM data collection but with essentially the same traffic stream as at the original site or (2) improving the pave- ment condition prior to the installation of data collection sen- sors. The goal is to meet two criteria: (1) that the traffic data being collected correlate very closely with the traffic at the site for which the data are being collected and (2) that the new site has conditions that allow the equipment to perform accurately. 4.1.3 Count Duration Issues The intended duration of a count has a considerable impact on the type of data collection technology selected. The longer the desired count duration, the more likely a permanently mounted sensor is needed. For long-duration data collection sites, it is also impor- tant to determine the expected life span of the site itself. For example, because vehicle weight data are scarce on most roads in most states, it seems prudent to collect no more than 2 years’ worth of weight data at a specific site and then move

the data collection electronics to a new road in a different part of the state. With this type of scenario, the highest quality (and most expensive) WIM systems are not good investments. Instead, a more modestly priced sensor should be installed, with the intent of abandoning the site after 2 years. However, if data on the proposed site will be gathered for many years in order to track trends on key routes, it is often cost-effective to spend additional resources up front in order to reduce the cost of future maintenance and increase the life span of sensors placed at a given location. A similar type of situation can also affect the selection of classification technology. If the proposed data collection site is due to be repaved in 2 years, it may not be appropriate to place sensors in the pavement. Or it may be less expensive to install non-intrusive sensors that can be used even after the pavement replacement takes place. 4.2 DATA HANDLING AND OTHER AGENCY CONSIDERATIONS The next series of basic considerations when selecting equipment pertain to how effectively new equipment can be integrated into the existing (or planned) data handling system of the state highway agency. Perhaps the most important issues are what vehicle classi- fication categories can be collected (see Section 4.1) and how those classifications relate to the classes currently collected. However, there are a number of equally important factors to examine that relate to what other data are collected and how those data are handled within the highway agency’s data col- lection, storage, and reporting system: • How are the data retrieved from the data collection site? (Can the equipment be polled automatically using telecommunications? Does a staff person need to visit the site? Are the data extracted directly to a computer or must they be transferred to a data storage unit and then downloaded to a computer in a second step?) • How large are the files being transferred? (How much communications bandwidth is needed for the site?) • If remote communications capabilities exist, what com- munications options are supported? (For example, what telephone baud rates can be used? Does the system sup- port direct Internet connections? Are digital, wireless modems supported? Are other communications mecha- nisms supported that are already used by the highway agency?) • What computer formats are used as part of this data transfer, and are they proprietary to the vendor or can the state highway agency communicate to these devices using an existing standard (e.g., the National Transporta- tion Communications for ITS [intelligent transportation systems] Protocol, or NTCIP)? (Are these formats com- patible with existing central database software used by the state?) 43 • Does central system software provided by the vendor allow for the conversion of formats? • What levels of data aggregation are available when col- lecting data from the field (individual vehicle records, 5-minute summaries, or hourly summaries)? • How much data processing takes place at the site, and how much takes place at the central office? Can this dis- tribution of data processing be changed when setting up the data collection system? • Are the data available by individual lane or for all lanes at a site? • What error detection and reporting mechanisms are built into the vendor’s data collection equipment and soft- ware? Are error flags included in the data stream sent back from the field equipment? Another area of concern for a highway agency is the staffing resources needed to install, operate, and maintain the data collection equipment. Staffing issues relate to the number of staff needed to place, operate, and maintain data collection equipment, as well as the skill sets those staff need. Do the equipment maintenance staff need specific tool sets (oscillo- scopes, video monitors, specialized circuit boards or tools, etc.) in order to maintain the equipment (i.e., diagnose and repair problems)? In many data collection locations, an even bigger issue is the inability to gain physical access to the roadway when desired. (Many high-volume roadways allow sensor installa- tion only for a limited time during the night when traffic vol- umes are low and/or when other construction or maintenance activities are being performed. Can the technology selected be installed under these time constraints?) How long does it take to install the sensors, and what impact does that installation process have on the use of the existing roadway? Are those timeframes politically acceptable? 4.3 UNDERSTANDING EQUIPMENT CHARACTERISTICS Chapter 2 presented an introduction to the technologies available for collecting the classification and weight data required to meet the user needs determined from the effort described in Section 4.1. Additional information on equip- ment accuracy, reliability, and cost is available through the following: • References provided as part of this report, as well as those published in a variety of technical sources; • Experience gained by the highway agency as it uses spe- cific equipment; • Communication with other highway agencies about their experiences with specific types and models of data col- lection equipment;

• Specific tests done to measure the performance of equip- ment; and • Vendor responses to requests for proposals or requests for information published by the highway agency. It is important to collect device-specific information from these sources. Because a specific technology appears to be a good fit for a specific application does not mean that all devices using that technology will work equally well. Specific implementations of a given technology from two different vendors can result in data of very different quality. Similarly, the cost for specific technologies can vary considerably from vendor to vendor, along with the features supplied with the proposed equipment. Only by looking at the specifics of ven- dors’ proposals can these details be determined and compared. 44 It is important to obtain information about the perfor- mance of specific models offered by vendors. Similarly, it is important to determine what warranties and/or guaranties vendors supply with the equipment as these provide both assurances that equipment will perform as claimed and reme- dies if the equipment does not. Lastly, it is important to test the equipment when it is first placed in order to determine if the equipment meets the standards warranted by the vendor. Table 4.1 presents a summary sheet that can be used to high- light the specific data collection issues important to selecting the appropriate equipment for a specific project or set of data collection efforts. (Additional factors can also be added to reflect needs not discussed in this document.) It is up to each highway agency to weigh the relative importance of each of these issues.

45 Subject Area Issues/Concerns Technology/Vendor Review Comments Equipm Technology/Vendor/Model: ent Capability Type of Data Collected • WIM • Classification Types of Vehicle Classes Measured • 13 FHWA axle-based classes • Vehicle lengths only • Other (total number allowed) Desired/Required Sensor Location Can sensor be placed? • In pavement • Condition of pavement, planned pavement maintenance and repair? • On pavement • Traffic volumes • Non-intrusive • Availability of overhead structures or poles Count Duration • Seasonal changes? (in traffic generators?) • Portable (several days) • Correlation with permanent sites, reliability of measurements? • Permanent Output from Device • Level of aggregation • Can be polled from central source, or only from the site? • Specific • Flexibility of output formats • Quality-control metrics available for analysis of device output • Availability of standardized formats (NTCIP? Other?) Site Conditions Operating Environment • Temperature range and daily variation • Visibility constraints (fog, mist, dust) • Snow (loss of lane lines) • Free-flow or congested traffic (including other acceleration/ deceleration conditions) Number of Lanes • Are all lanes next to a shoulder? • Number of sensors required • Number of sets of electronics required Is Power Available? Can device run off of solar panels? Are Communications Available? • Telephone, DSL, wireless Bandwidth required from device • Other • Frequency of communications General Technology Price Total Cost = Sensor Cost x Number of Sensors + Cost of Electronics Staff Training to Install, Operate, and Maintain the Devices Equipment Needed to Install, Operate, and Maintain the Device Published Accuracy Achieved with the Technology Has the technology been used previously? Previous Experience with this Technology/Vendor Vendor support offered/available TABLE 4.1 Sample equipment selection analysis summary sheet