Category Archives: Traffic and Safety

Materials and Construction, Traffic and Safety

Nontraditional Fog Seals Offer Value, Limitations Compared to Traditional Seals

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MnDOT conducted field and lab analyses of nontraditional fog seals used by local agencies around the state. Results show that agriculture-based bioseals offer value that must be balanced against temporary reductions in retroreflectivity and pavement friction. Bioseals offer greater friction and visibility than traditional fog seals.

“There is some value to the bioseals. They seal the pavement, and they’re clear so they have a minimal effect on striping. These applications are appropriate in certain areas,” said Bruce Hasbargen, County Engineer, Beltrami County.

What Was the Need?

Maintenance crews often spray pavement surfaces with a “fog” of liquid sealant after pavement has been in service for a year or more. These fog seals extend the water resistance of asphalt and protect pavements from oxidation.

Fog seals wear off after a few years, but can be inexpensively reapplied. The seals lengthen maintenance cycles, protecting asphalt between activities such as crack repair and surface treatment. Traditional fog seals, however, are dark, asphaltic mixtures that obscure pavement striping and reduce the reflectivity of materials. Fog seals also reduce friction, and so typically suit pavements with low-speed service conditions.

In recent years, city and county road agencies in Minnesota turned to bioseals—agriculture-based, clear liquids that manufacturers claim seal pavement against oxidation and water damage without concealing pavement markings. Bioseals are currently not less expensive than petroleum industry products, and little independent work had been performed to identify performance benefits.

What Was Our Goal?

To provide local agencies with more information about bioseal performance, the MnDOT Office of Materials and Road Research studied selected bioseal products in the lab and in the field (MnROAD test site pictured above), comparing them to traditional seals to determine product performance, durability and impact on friction and pavement marking visibility.

What Did We Do?

Following a literature review of fog seal treatments, investigators selected four seals for analysis: a traditional asphalt-emulsion sealer; a nontraditional, polymerized maltene emulsion longitudinal joint sealant (Jointbond); and two soy-based bioseals (RePlay and Biorestor). These seals were applied in 2014 to 8-foot shoulder sections built in 2013 on County Highway 75 in Wright County, north of Monticello. Seals were sprayed on shoulders outside painted markings, in shoulder space where investigators applied geotextile patches and strips of highly reflective striping tape commonly used on some roads. Untreated shoulder areas of 500 feet and 1,320 feet served as control sections.

Reflective marking tape on road shoulder
Researchers placed a swatch of geotextile and reflective pavement marking tape on shoulders before the shoulders were sprayed with nontraditional fog seals. Investigators then moved the textile and tape to MnROAD to study application rates and stripe performance.

After spraying, investigators removed the geotextiles to evaluate the quality of application work by bioseal distributors. They also removed some striping tape and reapplied it as shoulder striping to Cell 33 at the MnROAD test facility, where they could reliably monitor traffic passes over the biosealed markings and evaluate retroreflectivity over time. At the Wright County site, researchers examined pavement distress, friction properties and permeability on the shoulders for three years.

Lab studies included testing seal residue and stiffness in field-aged cores taken from the sealed test sections in year three. Finally, in year three researchers surveyed local agencies in Minnesota about their use of nontraditional fog seals.

What Did We Learn?

Geotextile coating levels showed that vendor application of bioseals is consistent and well-executed. Nontraditional seals do not obscure striping, but bioseals leave residue that temporarily reduces the retroreflectivity of sealed markings to below MnDOT-required levels. Acceptable levels of retroreflectivity returned to the Jointbond samples after 800 truck passes at MnROAD, and to Biorestor and RePlay samples after 1,600 truck passes.

Every tested seal reduced pavement friction. Recovery of friction for the three nontraditional products, which reduced friction by 11 to 17 percent, took about 200 days with no traffic. The traditional, asphaltic fog seal reduced friction by 67 percent and took longer to recover, remaining slippery for turning in wet conditions for over two years.

“Bioseals affect pavement friction, so agencies need to use some caution when using them. City streets are probably going to be very good for nontraditional seals,” said Eddie Johnson, Research Project Engineer, MnDOT Office of Materials and Road Research.

Each seal reduced pavement permeability for about two years; after two years, only the traditional seal continued to provide water protection. The permeability benefit of fog seals lasts significantly longer than the retroreflectivity reduction; when reflectivity recovers, the seals still provide water resistance. Field surveys also found that Biorestor and RePlay may help resist cracks.

Laboratory studies showed that high-temperature stiffness for every treatment was greater than control samples in the top layer than in the middle of cores, suggesting that seals may improve rut resistance of treated pavements in hot weather. Low-temperature stiffness was higher in the top sections for every treatment except the traditional fog seal.

Of the 57 agencies that responded to the survey, 32 have used nontraditional fog seals, preferring Biorestor and RePlay to others. Over half of these users recommend the use of such seals; responses suggest that bioseals offer sealing benefit for two years and, in some cases, up to six years.

What’s Next?

Nontraditional fog seals protect pavements from water and may help prevent cracking. Traditional seals offer longer-lasting water resistance, but also longer-lasting and greater friction reduction. Agencies must consider temporary reductions in retroreflectivity and friction for any seal, and may wish to continue using fog seals only in lower-speed environments.

Continued monitoring of applications would be helpful in determining long-term performance. The study observed that overlaying biosealed asphalt with a traditional fog seal should be effective in extending permeability.

This post pertains to the LRRB-produced Report 2018-18, “Nontraditional Fog Seals for Asphalt Pavement: Performance on Shoulder Sections in Minnesota,” published May 2018.  

Materials and Construction, Traffic and Safety

Upgrading MnPave-Rigid Design Software

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MnDOT has upgraded its software for concrete pavement design. MnPAVE-Rigid 2.0 is now easier to use and allows designers to select from more options for aggregate base types and thicknesses, and incorporate specific traffic values.

“In the original software, we only allowed one aggregate base thickness and one aggregate type. MnPAVE-Rigid 2.0 allows two base thicknesses and three base types,” said Tim Andersen, Pavement Design Engineer, MnDOT Office of Materials and Road Research.

What Was the Need?

MnDOT developed its own pavement design software, MnPAVE-Rigid, in 2014 that incorporated the methodology of the American Association of State Highway and Transportation Officials (AASHTO) Mechanistic–Empirical Pavement Design Guide (MEPDG). Minnesota’s pavement designers use MnPAVE to apply AASHTO’s most sophisticated design principles for both rigid and flexible pavement, focusing on mechanical properties of the pavement and prevention of early cracking and other distress.

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MnDOT engineers use sophisticated and complex M–E pavement design methods from MnPAVE-Rigid 2.0 software to design durable, high-performing concrete pavements.

AASHTO’s mechanistic–empirical (M–E) design methods entail hundreds of inputs, each a mechanical parameter, a measure of site-specific characteristics or a design goal. To simplify the input selection process, AASHTO’s M–E design software offers various input levels to reduce the data gathering and input burden. The most basic level uses default values for most of the inputs based on national averages, but still requires dozens of inputs for the number of pavement layers, traffic expectations, climate and other features.

MnPAVE-Rigid for concrete pavement design reduced that number of inputs to nine, operating like a module of AASHTO’s M–E software. MnPAVE-Rigid inputs work with a set of default values for jointed plain concrete selected by the MnDOT Office of Materials and Road Research in 2014, as described in the MnPAVE-Rigid 1.0 report.

“Many states ignored the challenge of adopting AASHTO M–E or they bought an AASHTO
software license. MnDOT used its accumulated knowledge of AASHTO M–E and Minnesota conditions to build MnPAVE-Rigid, and so can account for its M–E design results firsthand,” said Derek Tompkins, Principal Civil Engineer, American Engineering Testing, Inc.

Since implementing MnPAVE-Rigid 1.0, MnDOT has gathered feedback from users about their experience with the software. In the current project, MnDOT wanted to address this feedback, and expand and improve the original software by exploring additional options with some of the default parameters for concrete pavements.

What Was Our Goal?

The goal of this project was to update MnPAVE-Rigid 1.0 by expanding the range of inputs for traffic, subgrade type, base type and thickness, and to make the user interface more accessible.

What Did We Implement?

MnPAVE-Rigid 2.0 allows users to enter 11 inputs, including inputs related to specific traffic levels and aggregate base types; calculate the new design thickness; and print a project report that summarizes the inputs and the recommended thickness. The upgraded software is more user-friendly, and MnDOT can maintain or make future upgrades to the source code.

How Did We Do It?

Researchers met with the Technical Advisory Panel and reviewed the list of software improvements requested by pavement designers and the MnDOT Office of Materials and Road Research.

Because every change to an input affects a large number of default input variables, investigators ran over 21,000 simulations to analyze the impact of changes made to inputs for base type, base thickness, subgrade type and traffic level. The research team also modified the traffic input calculator to allow designers to enter traffic values from MnDOT’s weigh-in-motion and traffic counting data. The calculator runs input traffic data in software simulations and assigns the input an appropriate axle value for design.

MnPAVE-Rigid 1.0 ran designs based on Class 5 aggregate base over a subgrade like clay loam. Other aggregate types were added to simulations to determine how the software responds to these changes. Investigation also explored the addition of subgrade material options in design simulations.

The code developer modified elements of the advanced inputs tab and PDF report generation features to improve performance for software users, and rebuilt the software in JavaScript 2.0 code, including an installer for use with Windows software.

What Was the Impact?

MnPAVE-Rigid 2.0 is more user-friendly. Its tabs better match designer needs, and the software offers a design report PDF file for export. Instead of selecting from limited options for traffic volumes (default, normal and heavy), users can now input traffic data that the software will categorize. Designers can input Class 5 aggregate, Class 5Q (a higher quality aggregate with fewer fines) and open graded aggregate (no fines). Users can also choose 4-inch or 12-inch aggregate base thicknesses. An additional subgrade option was not included, as simulations indicated a sand subgrade input did not discernibly impact structural thickness outputs.

The AASHTO M–E software is expensive, and agencies that use it have to work closely with consultants to receive training and to explore or modify the code. MnDOT owns and manages the source code for MnPAVE-Rigid 2.0, can keep it secure, and can continue to change and upgrade it internally for Windows and Linux platforms.

What’s Next?

MnDOT designers around the state are reviewing the software, but it is essentially already in use. After full review, the Office of Materials and Road Research will post a link to the program on its website. Presentations about the software upgrades will be made at meetings for materials and soils engineers through the fall of 2018.

Still underway is an effort to further incorporate recycled material properties into MnPAVE Flexible, the design software for asphalt pavement.

This Implementation Summary pertains to Report 2018-17, “MnPAVE-Rigid 2.0,” published May 2018.

Maintenance Operations, Traffic and Safety

Reporting Driver Intrusions in Work Zones

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Data from a new system for tracking work zone intrusions may be used to change work zone design and policies, reducing the risk of injury and death from intrusion crashes.

MnDOT and the Local Road Research Board engaged researchers to develop a user-friendly system that allows highway crews to quickly record instances of motorists’ intrusion into work zones, using a laptop, tablet or paper.

“This collaboration resulted in a fast, efficient and easy-to-use system because crews and supervisors let us know throughout the process exactly what they needed to consistently report work zone intrusions,” said Nichole Morris, Director, University of Minnesota HumanFIRST Laboratory.

What Was Our Goal?

The goal of this research project was to develop and test an efficient, comprehensive and user-friendly reporting system for intrusions into work zones. It was essential for the system to be accepted by highway workers. The information collected from the system, which was modeled after the existing MNCrash report, would then be used to examine risk factors to reduce intrusions and danger to workers. Safety data would be relayed back to workers and to MnDOT managers, providing an empirical basis for design changes to work zones, as well as future policy recommendations to the state government.

“To reduce work zone intrusions and make work zones safer, we need to track and analyze the intrusions. This reporting system will generate the data we need to make smart changes and possibly to influence legislative policy,” said Todd Haglin, Emergency Management and Safety Manager, MnDOT Office of Administration.

What Did We Do?

To design a usable system for reporting work zone intrusions, research designers had to:

  • Understand the characteristics of the typical system user (in this case, the work zone supervisors and crew).
  • Develop common or typical intrusion scenarios to realistically test the system.
  • Conduct iterative testing with typical users (supervisors and crew members) and incorporate revisions based on test results.

The research team interviewed work zone supervisors from rural and urban truck station locations across the state: in Baxter, St. Cloud and Duluth and at Cedar Avenue near Minneapolis-St. Paul. Researchers sought to learn what crews and supervisors considered an intrusion and what they thought should be reportable elements of the intrusion, such as the work zone layout, weather, location, time, visibility, road conditions and maneuvers of the intruding vehicle.

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In a common type of close call, the dark car shown here fails to merge until it is too close to the work zone, forcing the silver car out of its lane.

Researchers used information gathered from the interviews to develop four typical intrusion scenarios—which were reviewed and revised by MnDOT supervisors—and used these scenarios to test the prototype reporting interface. Then they conducted usability tests with these scenarios and with actual intrusions that crews had experienced. Users suggested changes to the report format throughout the process.

Crews and supervisors collaborated with researchers during three rounds of testing, revising the reporting interface after each round. An online beta version had been supplemented with a paper version. Both versions were revised through this iterative design process.

What Did We Learn?

This design approach allowed the research team to produce a report interface incorporating the very specific needs of the work zone crews and supervisors:

  • The third major revision split the report decision flow into two options—a shorter report and a comprehensive report—based on whether the intrusion presented a risk to the crew. Without this revision, intrusions that workers considered minor were not likely to be reported.
  • Researchers surveyed users of the system with each revision. Supervisors liked the drop-down menus, the comprehensiveness of the system and its ease of use. They rated the final revision as good in terms of usability, ease of use and time to completion (five to six minutes on average).
  • The final design version was tested using a laptop, tablet and paper. Multiple reporting options made it more likely that workers and supervisors would quickly report data about a work zone intrusion before details were forgotten.

What’s Next?

Supervisors and workers involved in the design process gave high marks to the final version of the reporting system. The design is considered complete. Researchers had created the interface as a free-standing program, using the University of Minnesota’s digital resources to build and evaluate their design. For this reporting system to be made  vailable for use by MnDOT and other agency workers, MnDOT must engage MNIT, the state’s information technology professionals, to determine where the system will reside and to integrate it into the state’s existing computer platform.

This post pertains to Report 2018-09, “Work Zone Intrusion Report Interface Design,” published in February 2018. 

MnDOT is developing other initiatives to improve work zone safety,  including a personal warning sensor for construction workers. Search for “work zone” research projects here.

Maintenance Operations, Traffic and Safety

New Project: Real-Time Winter Weather Alerts Planned for Highway Message Signs

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The longest winter in recent memory might have ended, but MnDOT’s traffic and maintenance staff are already planning how to make future winters easier on Minnesota drivers.

Recently, the Regional Transportation Management Center was awarded funding to deliver real-time winter weather warnings via its roadside and overhead highway message signs. The RTMC displayed blizzard warnings for the first time during six storms last winter, but the alerts had to be manually entered.

“This is similar information that you receive on your cell phone or the evening news,” said Brian Kary, RTMC Traffic Operations director. “But for somebody who’s traveling down I-90 and just passing through, they might not realize that they’re entering an area with a blizzard.”

Another initiative aims to expand the road condition data that’s available during winter storms by piloting the use of mobile sensors on maintenance supervisor trucks and above-ground sensors at select Road and Weather Information System sites.

Both projects are among eight research implementation projects recently selected for funding by the governing board for MnDOT’s transportation research program.

Multi-lane highway with real-time message boards

Weather Alerts

Minnesota has nearly 300 Dynamic Message Signs, which currently issue real-time warnings about traffic incidents, road work and congestion. Around 200 are in the Twin Cities metro; the rest are in Greater Minnesota.

Kary’s two-year project will develop a system that can automatically relay critical weather alerts, which change frequently, are labor-intensive and error-prone when physically entered. Only blizzard warnings from the National Weather Service are initially planned, but the system will be capable of broadcasting all types of weather alerts.

A number of other states already issue weather alerts via their Dynamic Message Signs, so MnDOT has case studies to look at.

It’s possible that the signs could also someday relay information from MnDOT’s Maintenance Decision Support System and roadside weather sensors. A current pilot project uses weather sensors and flashings on a rural stretch of highway near Dassel Cokato High School to warn motorists and notify maintenance staff of unexpected blow ice.

warning sign indicating ice on road when flashing

Improving Road Condition Information

Over the next two years, the Maintenance Office will test the use of mobile and above-ground sensors to expand the geographic coverage of RWIS sites, which feed valuable weather and road surface information to highway operations managers and advanced traveler information systems. This might lead to the elimination of in-road sensors, which require lane closures to maintain and must be replaced during road construction projects.

The mobile sensors will collect road condition information, such as temperature, humidity, due point, and friction, from five maintenance supervisor trucks. The other non-invasive sensors will be attached to an RWIS tower or a pole near the roadway and use laser technology to read road surface temperature and condition (water ice, slush and snow).

See this related news story from KSTP-TV.

Traffic and Safety

County GIS Maps Help Road Departments Anticipate Slope Failure

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In a recently completed pilot study, researchers developed maps for two Minnesota counties that rank the failure potential of every slope using a geographic information system (GIS)-based model.

“GIS mapping has been applied to very small watersheds. The two counties in this study are huge areas in comparison. We used a physics-based approach that shows engineers where slope failure is likely to occur,” said Omid Mohseni, Senior Water Resources Manager, Barr Engineering Company.

What Was Our Goal?

The goal of this study was to determine if slope failure models could be developed to help counties anticipate where failures may occur. Researchers used publicly available data, research findings and geotechnical theory to develop failure models that could then be mapped with GIS in two topographically dissimilar Minnesota counties. These maps would identify slopes susceptible to failure so that county highway departments could develop preventive strategies for protecting roadways from potential  lope failure or prepare appropriate failure response plans.

What Did We Do?

Researchers began with a literature review of studies about the causes of slope failure, predictive approaches and mapping. They were particularly interested in research related to potential failure mechanisms, algorithms used for predicting failures and slope-failure susceptibility mapping.

Then investigators collected data on known slope failures in Carlton County in eastern Minnesota and Sibley County in south central Minnesota to identify failure-risk factors not found in the literature. Researchers reviewed various statewide data sets, identifying topographic, hydrologic and soils information that could be used in GIS-based modeling. Next, they developed a GIS-based slope-failure model by incorporating the available data with geotechnical theory and probability factors from hydrologic data, and writing computer code to allow the data to be input into mapping software.

Researchers tested the software on known failure sites to refine soil parameter selection and failure models. The refined models and software were then used to identify and map slope failure risks in Carlton and Sibley counties.

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A detailed GIS map of a length of County Highway 210, color-coded to show slope failure susceptibility along the roadway.

What Did We Learn?

After analyzing the literature and the failure and geotechnical data, researchers identified the following key causal factors in slope failure: slope angle, soil type and geology, vegetation, land use and drainage characteristics, soil moisture, and rainfall intensity and duration.

Researchers then developed mapping models for the two counties using three key data sets. The first was data from 3-meter resolution, high-quality lidar, which measures distances with laser range finders and reflected light, available through Minnesota’s Department of Natural Resources website. The team augmented this data with U.S. Department of Agriculture soils survey data, and with National Oceanic and Atmospheric Administration and National Weather Service hydrologic data for precipitation and storm duration information.

Based on research in geotechnical theory, researchers developed algorithms for anticipating failure and built these into the lidar-based topographic mapping model. They also developed input parameters based on the failure factors and established output parameters representing five levels of failure susceptibility: very low, low, moderate, high and very high.

After testing the GIS-based model against a slope along County Highway 210 in Carlton County, researchers confirmed that failure potential correlated well with documented or observed slope failure. The team further validated the model by applying it to several small areas in the adjacent Carver and Sibley counties, finding similarly effective correlation with identifiable failure sites.

Independent geotechnical experts examined the modeling software and further refined geotechnical, soil and hydrologic elements. Finally, the team developed maps of Carlton and Sibley counties that assigned failure susceptibility levels to slopes in the two counties. Viewing maps through the software remains the most useful way to examine slopes, although large-format maps are available.

“If county engineers have higher slopes adjacent to roadways, they can use this basic tool to predict slope failures and then hire a geotechnical consultant to investigate the site.” – Tim Becker, Public Works Director, Sibley County

What’s Next?

With additional funding, mapping could be extended to every county in Minnesota to further refine failure modeling. Maps may also be useful in identifying structures such as roadways, ecological features, transmission lines and pipelines, bridges and culverts that may be threatened by slope failure susceptibility. Potential risks could be used to prioritize slope treatment plans.

This research effort is part of a slope failure risk mitigation strategy that includes the recently released Slope Stabilization Guide for Minnesota Local Government Engineers. Another project, underway at MnDOT, is identifying, mapping and ranking slopes vulnerable to slides that could affect the state highway network. The project

This post pertains to the Local Road Research Board-produced Report 2018-05, “Storm-Induced Slope Failure Susceptibility Mapping,” published January 2018. More information is available on the project page.

Maintenance Operations, Materials and Construction, Traffic and Safety

Taking on potholes with new prevention and repair strategies

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Potholes are one of the biggest and most costly ongoing maintenance challenges faced by highway agencies. Despite considerable progress in pavement materials and mechanics, pothole repair has remained an area in which little progress has been made.

To make headway in this area, Minnesota transportation researchers studied critical factors in pothole formation and repair in order to identify solutions that would reduce the occurrence of potholes and increase the durability of repairs. They also investigated the potential of newer materials, such as taconite and graphite nanoplatelets (GNP), in repair mixes. Researchers looked at how to make winter patches more durable and also different shapes of patches.

“Our goal was to provide a scientific assessment of pothole repair materials and practices,” said University of Minnesota professor Mihai Marasteanu, the lead researcher. Project sponsors were the Minnesota Department of Transportation (MnDOT) and the Minnesota Local Road Research Board.

What Did We Do?

Researchers began by reviewing national and international literature about pothole causes and repair activities. They also surveyed MnDOT maintenance superintendents and local engineers on current repair practices.

Next, the research team conducted simulations of square, diamond, and round pothole repair shapes to determine if some shapes were more conducive to reducing stress in repair materials. This stress analysis included the use of different common pothole filling mixes and their interface with existing pavement materials.

In the next stage of research, the team evaluated six asphalt mixes for relevant mechanical properties: four winter mixes, a polymer-modified hot-mastic asphalt mix suitable for winter and summer use, and a summer mix in two forms modified with GNP. Mixes were evaluated for compaction and bonding, tensile strength, and water penetration.

Laboratory test of pothole repair sample
Pothole repair samples performed poorly in water penetration tests, which suggested that most mixes will perform poorly under seasonal freeze-thaw stresses.

Finally, researchers studied national and international pavement preservation and pothole prevention practices and the cost-effectiveness of pothole repair.

What Did We Learn?

Through this work, researchers learned that pothole prevention requires repairing pavement cracks as they develop—and sometimes, even timely repairs only slow pothole development.

Laboratory analysis showed that cold mixes compact and bond poorly. To be more effective, these materials require significant curing not possible in the field unless heating is provided. The polymer-modified mastic patching material that was heated was stronger than the winter mixes even at very cold temperatures. Most mastics are used in warm weather, but this material may be effective for winter uses.

Durable winter repairs require expensive patching materials and on-site heating technologies such as truck-mounted microwaves. “To make winter repairs last longer, you need to provide an external source of heat to cure winter patching materials,” Marasteanu says.

Taconite-based materials activated chemically or by heating potholes before and after filling offer promise for more durable repairs. GNP modifiers improved compaction, tensile strength, fracture energy, and fracture resistance in the summer mix.

Pothole repair samples performed poorly in water penetration tests, which suggests that most mixes will perform poorly under seasonal freeze-thaw stresses.

Also of note, the study’s exploration of pothole repair shapes found that circular repairs offer the best filling and compacting performance; repair materials cannot fill corners, even with significant compaction.

 

“We had been squaring off potholes, making sure patches were all at right angles. But in this study, we found that square patches increase stresses at the boundaries. The ideal is a circular patch,” said Todd Howard, Assistant County Engineer, Dakota County.

What’s Next?

The most common pothole repair in Minnesota is throw-and-roll with HMA (using a truck’s tires to compact shoveled-in asphalt). Newer, more durable repairs include taconite-based materials activated chemically or by heating potholes with a truck-
mounted microwave unit before and after filling. While promising and, in the case of the microwave method, potentially effective in extreme cold, these approaches require further research before becoming widely used in winter and spring repairs.

GNP-modified mixes also warrant further study, especially in winter mixes. If MnDOT can encourage cost tracking, analysis of the cost-effectiveness of various pothole repair methods, including the mastic tested in this research, may become possible.

This research is part of a larger effort by MnDOT to improve pothole repair approaches and develop pothole repair guidance for crews throughout the state, including a recently released asphalt patching best practices guide with decision trees.

This post pertains to the Report 2018-14, “Pothole Prevention and Innovative Repair,” published April 2018. Part of this story was adapted from a June 2018 article by the Center for Transportation Studies. Further information is available on the project page and technical summary.

Education, Pedestrian, Traffic and Safety

More Saint Paul drivers stopping for pedestrians, thanks to pilot study

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A two-year research project underway in the City of St. Paul is already improving pedestrian safety and driver behavior by applying lessons learned from a national award-winning pedestrian traffic study. The city began using the practices last fall with the “Stop for Me” campaign, and driver yield rates have already gone up by 9 percent.

Background

Each year, dozens of Saint Paul pedestrians legally crossing the street are struck by vehicles driven by motorists who fail to stop. In 2015, 40 pedestrians died in Minnesota after being hit by a motor vehicle; 900 were injured. In 2017, there were 192 vehicle-pedestrian crashes in Saint Paul, three of which proved deadly.

Pedestrian fatalities and injuries represent a growing percentage of traffic fatalities and injuries nationwide. For example, pedestrian fatalities comprised 10.9% of all traffic deaths nationwide in 2004, but 14.5% in 2013.

A recent study supported by the National Highway Traffic Safety Administration demonstrated that driver behavior can be changed on a city-wide basis. The introduction of highly-visible pedestrian right-of-way enforcement in Gainesville, Florida increased driver yield rates for pedestrians by 22% to 30%.

Objective

University of Minnesota researchers are charged with reviewing the City of St. Paul’s efforts to improve pedestrian safety and investigate whether a program similar to the one in Gainesville can change driver yielding for pedestrians and speed compliance. The activities in St. Paul are being planned together with city traffic engineers and enforcement officers and will include various educational, engineering and enforcement countermeasures and media campaigns.

Last fall, St. Paul began the “Stop for Me” campaign, which enforces pedestrian laws, increases driver and pedestrian education and works towards enhanced signage and other changes to crosswalks around the city.

A group of people holding signs with traffic safety messages
Stop For Me is a campaign to improve safety for people who use St. Paul’s sidewalks and cross its  streets.

On June 25, the St. Paul Police Department began the second phase of the campaign by ticketing drivers who fail to stop for pedestrians at crosswalks.

Additionally, police officers are ticketing drivers for “endangerment” if they pass a vehicle that is stopped for a pedestrian at a crosswalk. This citation leads to a mandatory court appearance for the driver.

Weekly stopping percentages can be viewed at eight intersections across the city from now until the end of fall.

Watch for new developments on this project (expected end date of August 2019) here.  Another MnDOT study is looking at pedestrian traffic safety in rural and tribal communities. Other Minnesota research on pedestrian travel can be found at MnDOT.gov/research.

 

 

Traffic and Safety

Refined ROI Methodology Shows Added Benefits of MnPASS Lanes

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Researchers have developed a more comprehensive, standardized method for evaluating and selecting MnPASS managed lane projects. The new methodology uses a return on investment (ROI) and benefit–cost analysis framework that includes a more extensive set of factors, variables and perspectives than earlier methods. After applying the broader range of impact categories from the new methodology to an earlier study of a MnPASS corridor, researchers found that the MnPASS projects provide more benefits than previously reported.

In the past, MNDOT used a series of evaluation methods—cost estimation, performance measures and travel demand forecasting—to select new MnPASS corridors. While the recommendations and results from these assessments were adequate, each evaluation used a different set of objectives and assumptions. The range of benefit–cost factors in earlier evaluations was also limited to travel time savings, operating costs and crashes.

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An overhead sign above a MnPASS express lane explains that drivers must have MnPASS or two or more occupants in a vehicle to use the lane during peak travel time of 6 a.m. to 10 a.m., Monday through Friday.

“Our previous corridor studies each had different benefit–cost elements, making direct comparisons difficult. This project makes it possible to quantify the transit, environment and travel time reliability benefits that we knew were there all along,” John Wilson, Economic Policy Analyst, MnDOT Office of Transportation System Management

MnDOT needed a more thorough and consistent benefit–cost analysis methodology to help decision-makers better assess MnPASS project alternatives; compare potential MnPASS corridors; and communicate why MnPASS is a financially effective, long-term strategy for addressing mobility and congestion issues.

What Was Our Goal?

The goal of this project was to develop a refined, standardized methodology to more accurately assess the ROI of MnPASS programs and projects. A refined assessment framework would include a broader range of financial and performance measures, allowing MnDOT to more thoroughly evaluate MnPASS investments.

What Did We Do?

Using ROI as the central framework, the research team set out to generate a more comprehensive method for estimating benefits and costs. To begin, team members identified limitations in the existing benefit–cost analysis methodology and developed a list of factors to include in the refinement process. Then they interviewed stakeholders from various agencies to better understand MnPASS planning and operations needs, as well as the data required to support the research and system benefits and costs.

Next, they began to develop the enhanced framework by defining economic, environmental and social ROI categories for MnPASS investments, and mapping the relationship between these categories and their associated benefits and costs. Benefit– cost analysis methods then were used to build the refined framework and to estimate benefit–cost ratios for projects. Finally, researchers applied the new framework to an earlier benefit–cost analysis of the Interstate 35 West (I-35W) North Managed Lanes project to compare the results of the new framework with the results from the earlier analysis.

What Did We Learn?

Researchers used additional benefit factors such as transit use, travel time reliability, emissions and noise to refine the ROI calculation methods. When they applied the new ROI framework to the I-35W project, they found that the MnPASS benefit–cost ratio significantly improves with the inclusion of transit and travel time reliability benefits.

“Benefit–cost analysis had not changed for a long time; we had looked at travel time, vehicle operating costs and safety. Now we have added travel time reliability, which is important because we are moving people, not just vehicles.”—Paul Morris, Senior Associate, SRF Consulting Group, Inc.

What’s Next?

The results from the comparative analysis yielded a notably higher benefit–cost ratio of 3.40 for the test corridor compared to a benefit–cost ratio of 2.11 in the original study, indicating that MnPASS projects have more positive effects than previously identified. Based on these findings, MnDOT will revise its benefit–cost guidance for evaluating MnPASS investments.

The research team was also able to measure the impacts of specific categories on the overall outcome of the calculations. Team members found that while the measures for reliability and transit impacts produced a meaningful change in the overall benefits, those for emergency response, emissions and noise impacts were very small relative to overall project costs. MnDOT will consider these findings in establishing updated procedures.

This post pertains to Report 2017-37, “Refining Return on Investment Methodology/Tool for MnPASS,” published October 2017. Other research initiatives to improve MnPASS operations can be found by searching “MnPASS” on MNDOT Research Services’ project pages.

 

Policy and Planning, Traffic and Safety

Traffic Tubes Still Provide More Accurate Counts Than GPS Smartphones

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Collecting traffic volume information from smartphone data, navigation systems and other GPS-based consumer and mobile technologies is not yet ready for use by MnDOT. However, the emerging technology offers useful information on driving origins and destinations for traffic monitors and planners.

“We’re on the cusp of using GPS technology to get traffic data from more facilities. We’re not there today, but we’ve spurred the industry to look at this opportunity,” said Gene Hicks, Director, MnDOT Traffic Forecasting and Analysis, who helped lead the research study on this topic.

MnDOT conducts traffic counts on its roadway network at regular intervals: every other year on state trunk highways, approximately every four years on city and county roadways, and every 12 years on low-volume roads. To make these traffic assessments, MnDOT currently stretches pneumatic tubes across traveled lanes and counts passing axles for up to 48 hours.

2017-49-p1-image
Using pneumatic road tubes to collect traffic data is an old and reliable practice, but installing them is time-consuming and puts workers in harm’s way.

“Using road tubes to collect traffic volume data is a proven method, but it’s an old practice and puts people in harm’s way. Smartphones may offer a useful alternative,” said Shawn Turner, Division Head, Texas Transportation Institute, who helped evaluate a beta version of traffic volume estimates derived from global positioning system (GPS)-based mobile devices.

What Was Our Goal?

With this project, researchers aimed to explore using smartphones and other GPS-based systems instead of pneumatic tubes to collect traffic volume data. The information collected was compared with actual volume counts from MnDOT traffic monitoring sites.

What Did We Do?

In May 2016 researchers began identifying data collection firms interested in participating in this research effort. These firms were developing products that gather, aggregate and analyze sufficient location data from GPS mobile devices to estimate traffic volumes. Researchers assessed and sorted packages from these firms to identify the best match for MnDOT’s needs.

Two firms that were initially interested withdrew from the project because their products were not ready for rigorous testing. The research team then developed an agreement to work with a third firm, StreetLight, to develop and evaluate traffic volume estimates from GPS-based devices.

Researchers and StreetLight worked together to develop and evaluate traffic volume data. Investigators provided MnDOT traffic count data to the vendor for calibration of its approach, and investigators suggested several ways to enhance StreetLight’s analytics.

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Smartphones, navigation devices and other GPS-based consumer and commercial personal devices collect data that can be used to develop traffic volume estimators.

 

The vendor developed its proprietary approach, combining GPS-based navigation data with location-based service data. The firm normalized these two data sets with U.S. Census population projections, then calibrated and scaled samples with data from 69 MnDOT permanent ATR sites. StreetLight then estimated traffic volumes for MnDOT based on 7,837 short-duration count sites.

What Did We Learn?

On multiple-tube, high-volume roadways, MnDOT expects an accuracy of over 95 percent. The correlation between AADT tube-based data and StreetLight’s data was 79 percent without calibration and scaling, and 85 percent when scaled and calibrated. GPS-linked traffic volume estimations are approaching acceptable accuracy for MnDOT, but are not yet sufficiently accurate to replace tube counting for assessing AADT.

Estimation accuracy varies heavily with traffic volume levels. At high levels of traffic, larger sample sizes of mobile devices seem to drive more accurate assessments. At over 50,000 AADT, StreetLight estimates reached mean absolute percent error levels of 34 percent. At AADT levels below 20,000, the percent error rates ballooned. At all traffic levels, GPS-based data was measured at 61 percent mean absolute percent error.

Low-volume roads and frontage roads where multiple roadways converge had to be removed from count sites for estimating AADT. Overall, some of the GPS-based data fell within 10 to 20 percent absolute percent errors, which is acceptable, but other estimates fell well outside an acceptable range, and the highest errors occurred in low-volume roadway assessments.

What’s Next?

GPS-based data offers granular information that tube counts cannot, like average annual hourly volume estimates, and origin and destination data. With improvements to analytical processes for all data, GPS-based data may provide value outside of AADT estimates.

Currently, MnDOT is evaluating origin-destination data that StreetLight is providing for use in traffic studies and planning analyses. Current research by the University of Maryland and the National Renewable Energy Laboratory is gathering data with better error rates and will be extended in Colorado, Florida and Rhode Island. MnDOT expects volume estimation from GPS-based data will continue to improve and will likely be an acceptable alternative to tube counting in a few years.

This post pertains to Report 2017-49, “Using Mobile Device Samples to Estimate Traffic Volumes,” published December 2017.

Traffic and Safety

New Software Models MnPASS HOT Lane Changes

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Until recently, Minnesota drivers could only enter or exit high-occupancy toll (HOT) lanes via select ramps and access points. But MnDOT has changed most of its MnPASS express lanes to an open-access system to allow more movement between general purpose and high-occupancy toll (HOT) lanes.

Although MnDOT prefers this open access system, there could safety concerns for allowing lane changes in certain locations, especially if traffic patterns change. A new design tool developed by MnDOT-sponsored research enables traffic engineers to evaluate existing or prospective HOT corridors and determine the impact of open or closed access. When data shows changes in traffic patterns and congestion, the software also allows technicians to model and design changes in MnPASS access to improve drive mobility and safety.

“Nothing like this has been developed anywhere else. There is a lot of debate around the country about high-occupancy designs. This tool helps us develop designs and monitor existing corridors,” said Brian Kary, Director of Traffic Operations, MnDOT Regional Transportation Management Center.

Background

In Minnesota, high-occupancy toll (HOT) lanes are used by buses, carpools and motorists with transponders that trigger payments for rush hour use. During rush hour, fees increase as congestion increases; at other times, the lanes are free for all users.

MnDOT research in 2014 concluded that both closed and open HOT lane configurations effectively and safely improve traffic capacity. But researchers anticipated that roadway sections on an HOT lane corridor may eventually experience safety and mobility problems if toll prices are lowered or traffic volume increases. During heavy traffic, driving speeds often vary dramatically between HOT and general purpose lanes. Drivers moving into MnPASS lanes may force other drivers to brake suddenly to avoid collisions and trigger shockwaves of slowed or stopped traffic behind them. MnDOT prefers open-access designs for the increased options they offer road users, but it was not clear how best to manage access to reduce the incidence of shockwaves and the safety and mobility problems they create.

A section of a MnPASS express lane.
MnDOT has changed most of its MnPASS lane markings from double solid lines to skipped double lines to allow more open movement between general purpose and HOT lanes. A new software tool allows RTMC engineers to reassess MnPASS access to respond to changing traffic patterns.

The goal of this research implementation effort was to develop a software tool that the Regional Transportation Management Center (RTMC) could use to assess corridor operations and design. Based on design recommendations from earlier research, the tool would allow RTMC users to predict the safety and mobility impacts of a change from open to closed HOT lanes, and estimate where in the corridor such changes could be implemented safely and effectively.

What Did We Implement?

This effort leveraged findings from three previous MnDOT studies. Models and methods from “Evaluation of the Effect of MnPASS Lane Design on Mobility and Safety” (Report 2014-23) were used for the architecture of this new system. Investigators drew on “Expanding and Streamlining of RTMC Freeway Network Performance Reporting Methodologies and Tools” (Report 2014-05) to implement methods for retrieving historical data from RTMC’s system, cleaning the data and integrating it into the new software package. The project team then incorporated data from “Safety Impacts of the I- 5W Improvements Done Under Minnesota’s Urban Partnership Agreement (UPA) Project” (Report 2017-22) to develop and calibrate the new tool for estimating traffic impacts and shockwaves. A car-following and lane-changing model developed in a 2013 University of Minnesota study provided effective methods for ensuring realistic vehicle modeling and shockwave generation.

How Did We Do It?

Investigators developed this system with links to MnDOT’s database to draw on historical data to identify patterns of traffic demand over time and generate predictions of points in the MnPASS lanes at which shifts from open- to closed-access HOT lanes will offer the most benefit. The program’s code integrates the various models, data sets and tools mentioned above into software that integrates smoothly with the RTMC’s current software and capabilities for collecting speed and volume data, developing interfaces to model impacts of changing open designs at certain points in the freeway corridor to closed designs. The tool includes a module for access design, a module for generating data and a web application.

“This tool is calibrated for the Twin Cities. It takes real-time data and diagrams each location separately for lane changes and reaction time. It took theoretical ideas and made them usable,” said John Hourdos, Director, Minnesota Traffic Observatory, University of Minnesota.

What Was the Impact?

After receiving training in using the new software, RTMC engineers have embraced the MnPASS design tool to regularly report MnPASS performance to the Federal Highway Administration and to generate quarterly and annual analyses and recommendations for changing specific locations from open access to closed access. Closing requires restriping and changing signage—operations that allow MnDOT to respond quickly and easily to shifts in traffic patterns and potential mobility and safety impacts.

The project also offers data for the broader transportation community in which experts debate the relative merits of open- and closed-access HOT designs. With MnPASS, MnDOT has emerged as a leader in open design; this software allows sophisticated modeling of design impacts for the national traffic operations community.

What’s Next?

The MnPASS design tool effectively monitors corridor behavior and design changes. Improvements may yet be made to allow data calibration and validation for slower traffic speeds that represent mobility breakdowns, and to refine aspects of car-following and shockwave models. Increased density scenarios can also be further improved to better accommodate traffic density increases in regular lanes alongside HOT lanes.

In its current form, the tool will be used to analyze open- and closed-access designs on the Interstate 394 corridor to determine the best locations for changes to MnPASS lanes, and positions MnDOT and the RTMC to respond to traffic demand changes in the future.

This post pertains to Report 2018-11, “A Tool for Designing MnPASS
Access Spacing,” published March 2018. The full report can be accessed at
mndot.gov/research/reports/2018/201811.pdf.