use of high speed profile data to optimize wim equipment

Florianópolis - Santa Catarina - Brazil April 3rd to 7th - 2011

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Use of High Speed Profile Data to Optimize WIM Equipment Location

Mark P. Gardner, P.E.; [email protected] Fugro Consultants, Inc.; Austin, Texas; USA

Abstract There are many considerations affecting the potential location of Weigh-In-Motion (WIM) equipment along a roadway. In addition to presence of utilities and safety issues, one must also consider if the roadway characteristics are favorable to the accurate measurement of WIM data. Roadway roughness has been shown to be one of the most important considerations in the expectation of accurate WIM measurements. A rough roadway will create vehicle (truck) dynamics that make the variability of data much greater than desired.

The United States (US) Federal Highway Administration (FHWA) Long-Term Pavement Performance (LTPP) program has developed practices and procedures for the use of high speed inertial road profilers to assess the smoothness of a roadway and hence the impact on the accuracy of the WIM measurements, as well as the use of this information to identify optimum locations for installation of WIM equipment. This presentation will discuss the use of high speed inertial profilers and indices developed for this purpose. The presentation will also introduce a manual of practice developed by the FHWA. This manual is available to, and may be useful to the attendees. The presentation will discuss a useful software tool (Optimum WIM Locator – OWL) that filters the profile data to identify optimum equipment locations. Together, these tools may be used to find locations suitable to the installation of WIM equipment, where the impact of roadway roughness on the quality of data will be minimized.

Key-words: WIM Site Location, Profile, FHWA LTPP

Utilização dos perfis dos dados de alta velocidade em rodovias para otimização da localização de equipamentos de pesagem em movimento

– WIM

Resumo São várias as considerações que afetam o posicionamento potencial de um equipamento de pesagem em movimento ao longo de uma rodovia. Além da presença de utilidades e de questões de segurança, também deve ser levado em consideração se as características da rodovia são favoráveis à medição de forma precisa pelos sistemas de pesagem em movimento (WIM). Observou-se que a irregularidade da rodovia é uma das considerações mais importantes na expectativa de medições precisas pelos sistemas de pesagem em movimento. Uma rodovia com pavimento irregular (rugoso) criará na suspensão dos caminhões efeitos dinâmicos, que resultarão numa variação de dados muito maior do que os desejados.

Os Estados Unidos da América (EUA), por intermédio do Federal Highway Administration (FHWA) “Long-Term Pavement Performance Program” (LTPP), desenvolveram práticas e procedimentos para a utilização de dados inerciais dos perfis da rodovia em alta velocidade, para avaliar a planicidade do pavimento dela e, consequentemente, o impacto na precisão das medições em um sistema de pesagem em movimento (WIM), bem como a utilização dessas informações para identificar os melhores locais para instalação de equipamentos de pesagem em movimento. Esta palestra discutirá o uso de criadores de perfis inerciais de alta velocidade e os índices desenvolvidos para tal propósito. A palestra também apresentará um manual prático desenvolvido pela FHWA. Esse manual estará disponível aos presentes. A apresentação discutirá uma ferramenta de software (Optimum WIM Locator – OWL) que filtra os dados do perfil da rodovia para identificar os melhores posicionamentos para os equipamentos. Em conjunto, essas ferramentas poderão ser usadas para encontrar os posicionamentos


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adequados para a instalação de sistemas de pesagem em movimento (WIM), em que o impacto da irregularidade da rodovia na qualidade dos dados será minimizado.

Palavras-chave: Localização da área de WIM, Perfil, FHWA, LTPP.

1. Introduction When the FHWA started the LTPP program, and data collection began in 1989, the technology and practice of WIM data collection was relatively new. Piezo-electric technologies were just coming on the scene as a low-cost alternative to more expensive load cell and bending plate technology, offering what appeared to be an excellent opportunity for more wide-spread data collection. This technology offered an in-the-road sensor that was less dangerous to use than capacitive pad technologies of the time. The LTPP program sought site-specific WIM data on each of the 2400 LTPP test sections across the United States and Canada. Unfortunately this goal was never achieved. Where data was provided, it many cases it simply was not of the quality expected or desired for a research activity. This left the LTPP group with a challenging concern. How would they be able to quantify the traffic loading on highway test sections where performance data was being collected?

In order to address the need for both quality and quantity of traffic data desired, the LTPP program decided to conduct a Pooled Fund Study for Traffic monitoring at selected LTPP site locations. In this study many state agencies donated money into a “pool”, which was used to fund the combined research activity. In order to ensure their funds were put to good use, the LTPP program implemented strict quality requirements for the installations. Among other requirements, LTPP recommended that the installation include 500 ft of Portland Cement Concrete (PCC) pavement, to include 400 ft in advance of the WIM sensors, and 100 ft after. This PCC pavement was to be built to ASTM standards for smoothness to minimize vehicle dynamics as much as possible, and the adverse affects they create on the variability of WIM measurements.

While the industry recognized the importance of pavement smoothness on data WIM data collection, this pavement attribute was much more important when using small sensors like the piezo-electric, because the contact area was so small. The focus on the impact of site smoothness on WIM system results heightened the awareness of this requirement, and efforts were made to achieve specifications and guidelines that would meet this requirement.

ASTM E 1318-09 Standard Specification for Highway Weigh-In-Motion (WIM) Systems with User Requirements and Test Methods, Section 6 – User Requirements provides guideline specifications for site conditions that are necessary for any WIM system to be expected to perform properly (1). These include Horizontal Alignment, Longitudinal Alignment, Cross Slope, and among others, surface smoothness. ASTM E 1318-09 requires that the smoothness of the paved roadway for 60m in advance and 30m after be sufficiently smooth that a 150 mm diameter plate 3 mm thick could not pass under a 5 m long straightedge at any point along the pavement. Provisions for a sweeping survey method are recommended. This is a time consuming and difficult process to complete. It also requires a lane closure for safety considerations.

Citing concerns for safety, testing time and efficiency, the FHWA developed an alternative method to assess the smoothness of a site, and therefore the acceptability of the site for WIM data collection. There was also a desire to measure the smoothness of the pavement over time, to determine when the roadway may reach an unacceptable level of roughness for the expectation of quality data to be realized. This alternative method uses high speed inertial profilers for collection of longitudinal profile data, and processing of that data to develop both long and short wavelength values that are compared to indices for acceptability. Recognizing the value of this effort, further development has resulted in a move towards the development of a process that assists in the location of a suitable location for a WIM sensor, based on the calculation of values and comparison with acceptable index values. The following sections of this paper will provide some details on the LTPP Pooled Fund Study and efforts made to facilitate the collection of research quality WIM data, and the use of the inertial profiler to evaluate WIM site acceptability.


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2. The LTPP Traffic Pooled Fund Study As specified in (2), selected LTPP project locations are the subject of a concentrated effort to obtain research quality traffic loading data. This effort focuses on acquiring weight data which meets the performance standards for gross vehicle weight and axle information in ASTM E 1318 Type I WIM systems (Highway Weigh-In-Motion (WIM) Systems with User Requirements and Test Methods, 1994.). The effort to upgrade traffic data collection for selected SPS experiments began in 1999 with input from the TRB LTPP Committee Traffic Expert Task Group (Traffic ETG). They noted that scarcity of traffic data as well as lack of knowledge about the limited data available constituted a deficiency in the ability of the LTPP program to get the maximum value out of the investment in performance data collection. The FHWA directed that a change in the traffic data collection process for these experiments be considered. The recommendation adopted was for a centrally run program for equipment installation, maintenance and data collection with a recommended equipment standard. States had the opportunity to participate in a pooled fund study that implements the recommended program. States electing to participate in the LTPP Traffic Pooled Fund Study Program are shown in Figure 1. The fund is structured so that a state may select which, if any, of the services it wishes to use.

As with any other LTPP data collection effort managed by FHWA, the pooled fund study required the development of equipment and pavement guidelines and data collection protocols. Inputs to the guidelines and protocols included the professional experience of the practitioners who constitute the Traffic ETG, FHWA staff and contractors and ASTM E-1318. It was the consensus of the Traffic ETG that the most likely installation to meet the specified accuracy requirements for research needs over a range of operating conditions was bending plate in portland cement concrete. The guidelines reflect that consensus. The suggested equipment guidelines are, however, performance based rather than equipment specific. The pavement guidelines include a sample grinding specification and smoothness specifications that address both short wavelength and long wavelength conditions. All of the guidelines were made available to the traffic data collection community prior to writing the data collection protocols. The data collection protocols developed include practices for initial site evaluation, on-going site validation, and installation and construction acceptance conditions.

Figure 1. States Participating in the LTPP Pooled Fund Study

To test the various protocols and guidelines, five pilot studies were conducted in the summer and fall of 2001. The pilots concentrated on site validation. It was shown that the documents developed were essentially correct as written. The accuracy requirement for weights for Type I equipment in ASTM E-1318 can be met with equipment currently in operation. Pavement smoothness makes a difference. The sensitivity of piezo ceramic sensors to temperature variation was observed. Greater variability of piezo sensors when compared to bending plate sensors under side by side testing was observed. The need to


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have a consistent means for measuring and documenting the quality of WIM data being collected was a significant finding as the pilots progressed.

Since the beginning of the LTPP Pooled Fund Traffic Study, years of research quality traffic data have been collected at some sites. This study proved to the industry that it is possible to collect high quality WIM data over a prolonged period; something that was questioned prior to this program. The LTPP Traffic Pooled Fund Study showed that to collect high quality traffic data over a long period of time, certain essential items had to be included in the data collection plan. These included:

• The site had to possess geometric and pavement conditions suitable to the collection of quality data (control horizontal and vertical grade, smooth and sound pavement);

• Installation had to follow strict guidelines, including calibration and development of a benchmark set of comparison data for determination of possible calibration drift over time;

• Data has to be closely monitored to address any potential equipment issues and/or calibration drift. Data collection is not a set-and-forget process.

• Periodic site validations to maintain site conditions amenable to quality data collection.

3. The LTPP Manual for Profile Measurements and Processing In order to ensure consistent measurement of high speed profile at all of its test site locations, including the WIM installations, the FHWA LTPP program developed a manual for profile measurements and data processing (3). This manual specifies the proper operating procedures and setups for operation of the profiler to obtain consistent and repeatable results. Figure 2 shows the LTPP Profile Manual.

Figure 2. The LTPP Manual for Profile Measurements and Processing

This manual is a valuable tool for the operation and setup of profile activities. The high speed inertial profiler provides a safe and efficient tool for both project-level and network-level data collection. Its use is much safer than manual methods, such as with a profilograph shown in Figure 3.


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Figure 3. Measuring Roadway Smoothness with a Profilograph

Measurement of smoothness with a profilograph is slow, and therefore generally only used for limited data collection efforts. Figure 4 shows a high speed inertial profiler in use at an LTPP Profile Comparison Workshop. These vehicles work at highway speeds, and if properly configured are capable of both longitudinal and transverse profile data collection. Using laser sensors and accelerometers, this equipment can safely and efficiently meet both project and network level needs. The data may be processed in real time, allowing for immediate determination of results. Of course, as with all tools, care needs to be taken in the settings used to collect data, so that the proper filtering is completed. The discussion of that activity is beyond the scope of this paper. The LTPP program has been using these high speed inertial profilers for years, measuring smoothness of the highway test sections in the program, and also the approaches to WIM systems collecting data for the test sections.

Figure 4. Use of a High Speed Inertial Profiler

4. Impact of Smoothness on WIM data Collection The impact of smoothness on WIM data may be clearly seen in an evaluation of plots of WIM data. It should be noted that pavement smoothness is not the only possible impact. Wind conditions and traffic flow may also affect consistency of results, but the author contends that roadway roughness will provide for systematic and consistent variability in the data results, whereas many of these other concerns may be short term environmental, or equipment specific issues that can be addressed and corrected. Figure 5 shows a gross vehicle weight distribution for a relatively rough site.


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Figure 5. Gross Vehicle Weight Distribution from a Relatively Rough Site Location

One can clearly see the variability in measurement results, and roadway roughness has a significant impact on this result. This data represents about 4 months of data collection. The vehicle dynamics created by the roughness of the road cause wide variability in the measured values, as different vehicles respond to the roughness of the road in different ways.

In contrast, Figure 6 provides about 3 months of data for a relatively smooth site location. The data is repeatable and consistent, and exhibits limited variability over time. These are the type of results possible when profile is one of the major considerations in WIM site location. Providing conditions conducive to quality data collection will result in better results.

Figure 6. Gross Vehicle Weight Distribution from a Relatively Smooth Site Location


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5. The Optimum WIM Locator As part of ongoing work at the US FHWA, a tool is under development which will be extremely useful to those seeking to select the best possible location for a WIM installation as related to roadway smoothness. The Optimum WIM Locator (OWL) is being developed and will be distributed as part of the ProVAL tool kit, which is a set of software prepared for management and quality assurance of High Speed Profile data. The actual statement of purpose for the OWL is “to estimate probably WIM scale performance using measured profile.” However, this relates to an opportunity to identify suitable locations for WIM scale installation based on pavement smoothness. It may also guide the identification of pavement locations that need smoothness improvements through grinding, overlay, or other means to accomplish the same purpose. Figure 7 shows a screen shot from the beta version of the OWL software in ProVal 3.2. At this point the Beta release is scheduled for February 2011, and the public release is scheduled for March 2011. More information about these tools should be available via the internet. The OWL provides a graphical means to determine smooth areas of pavement for location of WIM sensors, if they exists. Remember that this smoothness should exist for about 150 m, to include about 120m in advance of the sensor and 30 m after the sensor.

Figure 7. The Optimum WIM Locator Software in ProVAL 3.2

6. Summary This paper has provided information about the importance of considering profile in the determination of suitable locations for WIM sensor installation, and discussed the ways that smoothness may affect the data. The FHWA LTPP Traffic Pooled Fund Study was shown as an example of where guidelines, practices, and specifications have been used to collect high quality WIM data over a long period of time. It is hoped that this information will be useful as others develop or improve programs in the future.

More information about the LTPP program, including access to the manual for profile data collection is readily available on the internet at:

http://www.fhwa.dot.gov/research/tfhrc/programs/infrastructure/pavements/ltpp/

More information about the ProVAL software is readily available on the internet at: http://www.roadprofile.com/


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7. Bibliographic references 1. Annual Book of ASTM Standards 2009, Section 4, Construction; Volume 04.03; Road and Paving Materials; Vehicle-Pavement Systems; ASTM International.

2. SPS Traffic Site Evaluation – Pilot Summary and Lessons Learned; US Department of Transportation; Federal Highway Administration; May 2002

3. Long-Term Pavement Performance Program Manual for Profile Measurements and Processing; Perera, R.W., Kohn, S.D., and Rada, G.R.; FHWA Report Number FHWA-HRT-08-056; November 2008.

use of high speed profile data to optimize wim equipment

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