Bridges and Structures

I-35W ‘Smart Bridge’ Test Site Uses Vibration Data to Detect Bridge Defects

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By analyzing vibration data from the I-35W St. Anthony Falls Bridge, MnDOT is working to develop monitoring systems that could detect structural defects early on and ultimately allow engineers to improve bridge designs.

“With data spanning several years, the I-35W St. Anthony Falls Bridge offers a unique opportunity for investigating the environmental effects on a new concrete bridge in a location with weather extremes,” said Lauren Linderman, Assistant Professor, University of Minnesota Department of Civil, Environmental and Geo-Engineering. Linderman served as the research project’s principal investigator.

“This project gets MnDOT closer to using bridge monitoring systems in combination with visual inspection to help detect structural problems before they affect safety or require expensive repairs,” said Benjamin Jilk, Principal Engineer, MnDOT Bridge Office. Jilk served as the research project’s technical liaison.

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Completed in 2008, the I-35W St. Anthony Falls Bridge has a smart bridge monitoring system that includes hundreds of sensors.

What Was the Need?

In September 2008, the I-35W St. Anthony Falls Bridge was constructed to include a “smart bridge” electronic monitoring system. This system includes more than 500 sensors that continuously provide data on how the concrete structure bends and deforms in response to traffic loads, wind and temperature changes. Transportation agencies are increasingly interested in such systems. As a complement to regular inspections, they can help detect problems early on, before the problems require expensive repairs or lead to catastrophic failure. Smart bridge systems can also help engineers improve future bridge designs.

The smart bridge system on the I-35W St. Anthony Falls Bridge includes accelerometers, which provide data on the way the bridge vibrates in response to various stimuli, including structural damage. Vibration-based monitoring has the advantage of allowing damage to be detected at any location within the bridge rather than only at the specific locations where measuring devices have been placed.

However, it can be difficult to use vibration monitoring to detect damage when vibration is masked by the bridge’s natural response to traffic loads, wind, temperature changes and other environmental conditions. A crack in a bridge girder, for example, can produce a vibration signature similar to one produced by a change in beam length due to variations in temperature or other causes. Consequently, since 2008 MnDOT has conducted a series of projects using data from the St. Anthony Falls Bridge to establish a way to distinguish anomalous data indicating a structural defect or damage from background “noise” associated with other causes.

What Was Our Goal?

This project sought to develop a method for analyzing accelerometer data from the I-35W St. Anthony Falls Bridge that would show how the bridge naturally vibrates due to traffic, wind and other environmental conditions. With this fingerprint of the bridge’s natural vibration, engineers would have a baseline against which to measure anomalies in the data that might indicate structural damage.

What Did We Do?

A large amount of data has been collected from the bridge since its construction. To establish the vibratory fingerprint for the bridge, researchers examined the frequencies and shapes (or modes) of bridge vibration waves. The method they used to identify the data segments needed for the fingerprint was to evaluate the peak amplitude of bridge vibration waves and their root mean square (RMS), a measure of the intensity of free vibration.

The researchers applied this method to the vibration data collected on the I-35W St. Anthony Falls Bridge between April 2010 and July 2015, calculating the average frequencies for four wave modes and determining how they varied with the bridge’s temperature. They also calculated the way frequencies changed with the bridge’s thermal gradients, or variations in temperature between parts of the structure.

What Did We Learn?

The methods developed in this project were successful in establishing a fingerprint for the way the I-35W St. Anthony Falls Bridge vibrates due to environmental conditions, and a way to evaluate changes in vibration over time indicative of structural damage or other factors.

Researchers found that the ratio of peak signal amplitude to RMS in bridge vibrations was a strong indicator of data that should be analyzed, and was evidence of a large excitation followed by free vibration. By themselves, peak amplitude and RMS cannot distinguish between ambient free vibration and forced vibration.

Researchers were able to use this method to successfully analyze 29,333 data segments from the I-35W St. Anthony Falls Bridge. This analysis revealed that as temperature increases, the natural frequency of vibration tends to decrease. The magnitude of this change, they concluded, must be related not just to the elasticity of the bridge but also to other factors such as humidity. However, temperature gradients within the bridge did not appear to have a significant effect on the natural frequencies of the structure.

What’s Next?

MnDOT will continue to collect data from the bridge as it ages to further understand its behavior. This will provide an opportunity to determine how anomalies in vibration data correspond to cracking and other forms of structural distress. Ultimately, MnDOT hopes to use this bridge monitoring system in combination with visual inspection both to detect problems in bridges earlier and to develop better bridge designs. Researchers are also currently working on a follow-up project, Displacement Monitoring of I-35W Bridge with Current Vibration-Based System, to determine the effects of temperature on the bridge’s dynamic and long-term vertical displacements, which can be used to monitor the bridge’s stiffness, connections and foundations.

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This post pertains to Report 2017-01, Feasibility of Vibration-Based Long-Term Bridge Monitoring Using the I-35W St. Anthony Falls Bridge, published January 2017. 

Research

Videos trace progress in traffic operations, pavement design

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Last month, CTS debuted two videos about the many contributions U of M researchers have made—and are still making—in traffic operations and pavement design.

The videos are one of the ways CTS is marking 30 years of transportation innovation. Our goal is to show how research progresses over time—from curiosity to discovery to innovation. The videos also show how U of M research meets the practical needs of Minnesotans in the Twin Cities metro and throughout the state.

The first video focuses on improving traffic operations, a research focus since our earliest days. Professor Emeritus Panos Michalopoulos invented Autoscope® technology to help transportation agencies capture video images of traffic and analyze the information, enabling better traffic management. Autoscope was commercialized in 1991, and the technology has been incorporated into products sold and used worldwide.

Current traffic operations research builds on this strong foundation. For example, the U’s Minnesota Traffic Observatory, directed by John Hourdos, develops data collection tools such as the Beholder camera system. The system is deployed on high-rise rooftops overlooking a stretch of I-94 in Minneapolis—an area with the highest crash frequency in Minnesota—to help the Minnesota Department of Transportation reduce congestion and improve safety.

The second video showcases U of M research on pavement design. Developing pavements that can stand up to Minnesota’s harsh climate is a continuing priority for researchers, whose work has led to new methods, tools, and specifications to extend pavement life. The video also looks at how research teams are pushing the envelope with use of materials such as taconite waste and graphene nano-platelets for pavement applications.

 

Freight and Railroads, Policy and Planning

Mobility, labor, and competitiveness drive discussion at annual freight symposium

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How does the ability to move freight affect the economic health of a state, region, and even a city? How are the supply chains of businesses impacted by freight flow? And what challenges and opportunities does Minnesota face when it comes to leveraging and strengthening its freight modes?

The 2016 Freight and Logistics Symposium offered a thoughtful examination of those questions and explored other topics related to improved mobility in Minnesota, including congestion, regulation, labor shortages, and the value of all freight modes to the state’s economy.

The event, held December 2, 2016, in Minneapolis, included:

  • A presentation on the power of freight flow data in attracting industry to a location and ways to use data in making a compelling case for businesses to invest
  • A panel Q&A featuring four industry experts from diverse organizations that depend on reliable freight movement
  • A discussion of how the 2016 election results may affect freight transportation

For a full summary of the event, download the 2016 Freight and Logistics Symposium proceedings (PDF).

The symposium was sponsored by CTS in cooperation with MnDOT, the Minnesota Freight Advisory Committee, the Council of Supply Chain Management Professionals, the Metropolitan Council, and the Transportation Club of Minneapolis and St. Paul.

Bicycling, Research

Bicycle commuting improves public health, reduces medical costs

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According to the results of a new study, bicycle commuting in the Twin Cities metropolitan area reduces chronic illness and preventable deaths, saving millions of dollars annually in medical costs.

The findings are one component of a multifaceted project funded by MnDOT. In the final report, researchers in several U of M departments provide a comprehensive understanding of the economic impact and health effects of bicycling in Minnesota.

“MnDOT has long identified bicycling as an important part of the state’s multimodal transportation system,” says Tim Henkel, modal planning and program management assistant commissioner. “This first-ever study generated new information that will inform policy and program strategies on bicycling as we determine levels of future investment.”

Xinyi Qian, an Assistant Extension Professor in the U’s Tourism Center, was the project’s principal investigator. Dr. Mark Pereira of the School of Public Health, one of the co-investigators, led the health component of the project.

Pereira’s team began by measuring the amount of bicycle commuting among Twin Cities adults using data from the 2014 Minnesota State Survey. (The counties included were Anoka, Carver, Dakota, Hennepin, Ramsey, Scott, and Washington.) The team found that 13.4 percent of working-age metro-area residents (244,000 adults) bicycle to work at least occasionally, and the average bicycle commuter rides 366 miles per year.

The researchers next estimated the number of deaths prevented from that amount of bicycling using the Health Economic Assessment Tool developed by the World Health Organization (WHO). Their analysis found that bicycle commuting in the metro area prevents 12 to 61 deaths per year, saving $100 million to $500 million annually. “At current levels, roughly 1 death per year is prevented for every 10,000 cyclists,” he says.

The WHO tool estimates savings from prevented deaths but not from prevented disease. To estimate the effect of bicycling commuting on illness, researchers conducted an online survey of Twin Cities cyclists; participants also included three commuter groups and a bicycle parts manufacturer.

“We learned that bicycling is linked to lower risk of metabolic syndrome, obesity, and hypertension,” Pereira says. “For example, taking three additional bicycle trips per week is associated with 46 percent lower odds of metabolic syndrome, 32 percent lower odds of obesity, and 28 percent lower odds of hypertension.”

The illness assessment provides relative risk estimates that planners can use in cost-benefit analyses. “Current methods only consider risk reductions related to death rates, so the benefit of infrastructure projects is underestimated,” Pereira says. “By providing an estimate of the risk reductions for diabetes and heart disease related to cycling, we provide an input that will help project planners more accurately represent the benefits of these projects.”

While the research was conducted in the Twin Cities, the methods can be used in other locations and to compare changes over time. “The findings also provide a foundation for transportation and health care officials to take action,” Pereira says, citing several options:

  • Promote active transportation through policies and intervention programs, e.g., employer incentives.
  • Develop consistent safety education and encouragement messages statewide to increase bicycle commuting.
  • Continue to encourage and implement safe bicycling to school and access to bicycles for youth across the state.
Policy and Planning, Research

Transportation spending: How does Minnesota compare with other states?

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Transportation funding continues to be a contentious issue in Minnesota: Are we spending enough, too little, too much? One way to help answer that question is to compare spending with other states.

“A simple comparison, however, may not accurately reflect the real level of transportation funding across the states,” says Jerry Zhao, an associate professor in the Humphrey School of Public Affairs. “States face different levels of demand and costs due to different geographic, demographic, or labor market conditions.”

To better understand the factors that influence the transportation funding level, Zhao and Professor Wen Wang at Rutgers University developed a cost-adjusted approach to systematically compare highway expenses among states. They found that while Minnesota spends more than average on highways, its spending level actually ranks low in cost-adjusted measures.

“We controlled for the effects of some major cost factors, such as demographics and natural weather conditions, which are outside of the control of state and local officials,” Zhao explains. “We found that natural weather conditions have a significant impact on highway spending—a lower winter temperature is associated with higher highway expenditures.”

The effect of population size isn’t as straightforward: “There is some impact of economy of scale, but only to a certain threshold,” he says. While urban areas have greater complexity, the higher population density is associated with less spending per capita, probably due to spreading the costs across a greater population.

The analysis also found that state and local governments tend to spend less on highways when they are under fiscal stress, and states with a higher gross domestic product (GDP) appeared to spend more on highways per capita. “Essentially, highway investment decisions may be greatly influenced by the economic fluctuations and fiscal stresses faced by a state,” he says.

According to unadjusted 2010 data, Minnesota ranks 8th on highway spending per capita and 18th on its share of statewide highway spending in GDP. “But after adjusting for those factors that are largely out of control by transportation policy, we found that Minnesota’s rankings drop to 37th on highway spending per capita and 41st on the share of highway spending in GDP,” Zhao says. “This suggests that the relatively high level of highway spending in Minnesota is largely driven by the cost factors of demographics and weather conditions.”

“This study confirms what MnDOT has experienced and that transportation financing is more complicated than one would expect,” says Tracy Hatch, MnDOT deputy commissioner. “Not only is Minnesota’s transportation system significantly undercapitalized—there are considerable financial impacts from factors outside of our control.”

The analysis was conducted as part of the U’s Transportation Policy and Economic Competitiveness Program (TPEC). In previous work, TPEC researchers created the Minnesota Transportation Finance Database, which compiles data about Minnesota’s transportation finance and shows the change of transportation spending in Minnesota over time.

Materials and Construction, Research

Thicker may not equal stronger when building concrete roadways

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Transportation agencies have long placed high importance on the thickness of their concrete roadways, making it a major focus of control and inspection during construction. While it is commonly believed thicker concrete pavements last longer, there is little data to support this claim.

“One big reason for the lack of data on the relationship between concrete pavement thickness and performance is the destructive nature of these measurements,” says Lev Khazanovich, a former professor in the University of Minnesota’s Department of Civil, Environmental, and Geo- Engineering. “Concrete thickness is typically assessed by coring—a destructive, expensive, and time-consuming test that only offers widely spaced measurements of thickness.”

In a MnDOT-funded study, U of M researchers set out to fill this knowledge void by leveraging recent advances in the nondestructive testing of pavements that allow for large-scale, rapid collection of reliable measurements for pavement thickness and strength. They conducted four evaluations on three roadways in Minnesota using ultrasonic technology to collect more than 8,000 measurements in a dense survey pattern along with a continuous survey of observable distress.

“We found that both pavement thickness and stress measurements are highly variable, with a half-inch of variation in thickness about every 10 feet,” Khazanovich says. “Interestingly, three of the four surveys averaged less than design thickness, which is contrary to typical accounts of contractors building slightly thicker slabs in order to avoid compensation deductions.”

Data analysis showed that exceeding design thickness did not seem to increase or decrease pavement performance. However, a measurement of pavement strength and quality known as “shear wave velocity” did produce valuable findings. “A drop in the shear wave velocity strength measurement corresponded to an increase in observable pavement distresses such as cracking and crumbling,” Khazanovich explains. “This was especially apparent when we were able to easily identify locations of construction changes, where significant changes in shear wave velocity matched up with observable distress.”

The results of this study illustrate the importance of material quality control and uniformity during construction, since alterations in pavement strength and quality may significantly influence pavement performance. In addition, researchers say that despite inconclusive thickness results, it is still important that pavement has significant thickness to carry its intended traffic load over its service life. Finally, the study demonstrates that new methods of ultrasonic shear wave velocity testing are useful for identifying changes in construction and design that could lead to higher rates of pavement distress.

Public Transit, Research

For millennials, car ownership and family life may not be obstacles to transit use

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As the millennial generation comes of age, indications of a significant generational change in travel behavior have raised hopes of robust growth in transit use. As a whole, this generation owns fewer cars, drives fewer miles, and uses transit more than previous generations. However, one key question remains: will millennials continue their high rates of transit use as the economy improves and they increasingly settle down and start families?

“In older generations we have seen significant declines in transit use that coincide with the transition to family life and child rearing,” says Andrew Guthrie, a research fellow and Ph.D. candidate at the Humphrey School of Public Affairs. To gain insight into the question of whether the millennial generation will be different, Guthrie looked for changes in the extent that two factors—young children in a household and access to a vehicle—affect transit use.

The study, conducted with Humphrey School associate professor Yingling Fan, looked for evidence of these bellwether changes in the Minneapolis–Saint Paul region between 2000 and 2010. This period saw the opening of the region’s first modern light-rail line as well as numerous bus system improvements, including a network of high-frequency local routes. In addition, the region has a strong, knowledge-based economy and has seen an in-migration of millennials.

The researchers used data from the detailed Travel Behavior Inventory conducted by the Twin Cities Metropolitan Council in 2000 and 2010 to compare travel behavior at both the trip and person levels.

Their analysis revealed that both young children in a household and access to an automobile have become “weakening obstacles” to transit use. “Specifically, research models show that participants with access to an automobile were more likely to use transit in 2010 than in 2000, and that participants with young children in their households were less likely than others to use transit in 2000 but not in 2010,” Guthrie says.

“Our models provide strong evidence that the basic relationship between transit use and the presence of young children in a household has changed, as has the relationship between transit use and access to an automobile,” Fan adds. “In fact, regardless of the specific modeling approach, these two traditional obstacles to transit use either weakened or disappeared entirely between 2000 and 2010 in the Twin Cities region.”

According to the researchers, the findings suggest that transit may now be better able to hold on to market share as its millennial users mature and start families, especially in urban areas where walk-and-ride trips are most common. In order to attract and accommodate these transit users, researchers believe ensuring an adequate supply of family housing and family-oriented community features such as high-quality schools and playgrounds in transit-served areas will be critical.

The research this paper was based on was part of a larger project funded by the Metropolitan Council and MnDOT. The paper was recently published in the Transportation Research Record.

Bridges and Structures, Policy and Planning

U of M provides freeway ‘lid’ expertise for Rethinking I-94 project

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MnDOT is exploring the development of freeway “lids” at key locations on I-94 in the Twin Cities. To analyze the potential for private-sector investment and determine what steps might be needed to make lid projects a reality, MnDOT invited the Urban Land Institute (ULI) MN to conduct a Technical Assistance Panel with real estate experts and other specialists. The U’s Metropolitan Design Center (MDC) provided background and research for the panel.

A lid, also known as a cap or land bridge, is a structure built over a freeway trench to connect areas on either side. Lids may also support green space and development above the roadway and along adjacent embankments. Although lidding is not a new concept, it is gaining national attention as a way to restore communities damaged when freeways were first built in the 1960s.

According to MnDOT, roughly half of the 145 bridges on I-94 between the east side of Saint Paul and the north side of Minneapolis need work within the next 15 years. A shorter window applies in the area around the capitol to as far west as MN-280. In anticipation of the effort to rebuild so much infrastructure, the department wanted a deeper understanding of how attractive freeway lids and their surrounding areas would be to private developers and whether the investment they would attract would generate sufficient revenue to pay for them.

The three-day panel session was designed to consider the I-94 corridor and study three specific areas: the I-35W/Minneapolis Central Business District, historic Rondo Avenue in Saint Paul, and Fairview Park in North Minneapolis. It also included a “lightning round” for high-level observations of five other sites.

Mic Johnson, senior fellow with MDC, provided background about lidding and shared successful examples from around the country at the panel kick-off dinner. MDC has analyzed a wide range of freeway lid structures and identified seven basic lid typologies. “These typologies provide broad thematic guidance for thinking about what features best serve a location,” Johnson says.

The briefing book provided to panelists included detailed research by MDC about the economic opportunities of the area’s freeway lids. MDC also created four appendices (projects, case studies, prototypical lid diagrams, and health and economic value) for the panel final report.

MDC has been involved in lid-related activities for several years. Students participating in an Urban Design Studio course in fall 2013 taught by Johnson conducted an extensive analysis of the I-35W/Minneapolis area and created an architectural model of a lid connecting the U of M’s West Bank to Downtown East. Their model was displayed at the IDS Center.

MnDOT Commissioner Charlie Zelle requested that ULI MN convene the panel as part of the larger “Rethinking I-94” project, which is developing a vision for the corridor through a comprehensive public involvement process. “Lid projects are one way being considered that could reconnect neighborhoods such as Rondo that were divided by freeways in the 1960s,” Zelle says. The Rondo neighborhood was also featured in the USDOT’s Every Place Counts Design Challenge in July.

As part of its report to MnDOT, the panel concluded that private-sector development would not pay for the lids directly, but lids would create development interest that could generate significant long-term revenue to pay for lid maintenance, programming, and other amenities.

To build momentum and create an identity for lid projects, the panel also recommended that the area’s lids be considered as a whole under a single banner, not as separate projects, as part of a rebranded vision called the Healthy Communities Initiative. The final report is available on the ULI MN website.

(Adapted from the ULI MN report: Healthy Communities Initiative, Nov. 2016.)

Research, Traffic and Safety

Project seeks to ease traffic congestion in a roundabout way

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Freeways and highways aren’t the only urban roads with traffic congestion, even though traffic management strategies have been largely directed toward improving traffic flows there. So, U of M researchers have taken to city streets to reduce congestion in an innovative—albeit roundabout—way.

“There’s been a lot of research focused on controlling congestion on major highways and freeways, but there’s relatively less when it comes to looking at controlling traffic on urban arterials,” says Ted Morris, a research engineer with the Department of Computer Science. “It’s a very different picture when you get into urban arterials and the traffic behaviors going on there, because of the dynamics of route choice, pedestrian interactions, and other factors.”Image of overhead view of roundabout

Morris is part of a research team that aims to create a framework for testing and evaluating new urban traffic sensing and control strategies for arterial networks. The goal is to balance safety and efficiency for all users—especially in places where new types of urban transportation facilities are planned in the next few years.

The team is using the 66th Street corridor in Richfield as a test bed for its research. The city, along with Hennepin County, is in the process of converting a series of signalized intersections along the route to roundabouts over the next few years. The roundabout designs also incorporate new facilities for pedestrians, bikes, and bus transit as part of a multimodal approach.

Initially, the researchers sought to create a larger network of interconnected sensors and a live test bed, Morris says. But funding limitations kept the project area to approximately 10 miles of arterial roads, a portion of which will be supported by a network of interconnected traffic sensors. The research team is instrumenting major intersections along 66th Street with a reliable, low-cost, high-resolution camera mounted on a center pole and supporting electronics as the intersections are being reconstructed.

“You can zoom in pretty closely to capture all the different movements and events that we need to use for measurement and detection,” Morris adds. “The key to this, to really make it reliable, is you need to very carefully quantify gap acceptance and how that varies in time and time of day. You also need to know how pedestrian activities interact with the traffic flow.”

The use of roundabouts has grown in the region because they cost less to build and maintain than signalized intersections, they meet the latest design standards, and they improve safety by reducing traffic conflicts. But predicting the capacity of roundabouts can be especially challenging when factoring in pedestrian traffic, uneven traffic origin-destination flow, heavy vehicle volumes, and approach vehicle gap-selection timing.

In addition to creating a sensor network to obtain real-time vehicle and pedestrian data to help control traffic and keep it flowing smoothly, the researchers also are developing a traffic simulation model that includes almost all of Richfield—more than 140 signalized intersections covering 21 square miles, including the arterials. The simulation model will be used to develop and test traffic control strategies under different scenarios. Minnesota Traffic Observatory director John Hourdos is leading that effort.

This research and the field deployment system are funded through a collaborative grant from the National Science Foundation Cyber Physical Systems program. SRF Consulting is the industrial partner to help design the sensor network and evaluate the system.

Maintenance Operations, Research

Winter Decision-Making Crosses State Lines

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Winter weather events have a regional and often national impact. “Storms never stop at the state line,” said Tom Peters, research and training engineer, MnDOT Maintenance Operations. “That’s why it’s so important for us to know about winter maintenance efforts around the country, and particularly at neighboring states with similar climates.”

MnDOT leads the Clear Roads Transportation Pooled Fund Project (clearroads.org), a national winter maintenance research consortium. In 2015, Clear Roads launched a national survey to collect and report the annual winter maintenance operations of state DOTs. The effort included nearly 50 data points related to equipment, materials and costs.

The results, which are available at clearroads.org/winter-maintenance-survey as a Microsoft Excel-based spreadsheet, are available at no cost for users to examine, analyze and parse as needed. Beyond the raw data, the spreadsheet includes calculated statistics and an interactive map for plotting key metrics.

The results quantified much of what was known only anecdotally and provided useful, actionable data. “Data trends by geographic region and over time let us make more informed operations decisions,” Peters said. “We can also draw on this information to communicate with management, elected officials and the public about how MnDOT’s winter operations fit in a national context.”

As the lead state, MnDOT commits significant administrative time and attention across the agency to Clear Roads. “It’s rewarding and satisfying to see such a useful product as one of the payoffs for all this effort,” Peters said.

Additional data collection for the 2015-2016 winter season is already complete. Look for an update to the online database later this year.

Research in Progress

Clear Roads has nearly a dozen research projects in progress, including:

See all of Clear Roads’ current research projects at clearroads.org/research-in-progress.

What’s Next?

At its September meeting in Omaha, Nebraska, the Clear Roads Technical Advisory Committee funded five new projects:

  • Utilization of GPS/AVL Technology: Case Studies
  • Standards and Guidance for Using Sensor Technology to Assess Winter Road Conditions
  • Emergency Operations Methodology for Extreme Winter Storm Events
  • Weather Event Reconstruction and Analysis Tool
  • Training Video for the Implementation of Liquid-Only Plow Routes

What is Clear Roads? 

Clear Roads is a 33-member pooled fund program dedicated to winter road maintenance research. Led by MnDOT, Clear Roads projects evaluate winter maintenance materials, equipment and methods; develop specifications and recommendations; study and promote innovative techniques and technologies; and develop field guides and training curricula. Learn more at clearroads.org.

Minnesota's transportation research blog