April 2017 Vol. 5, No. 3

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Geogrids improve pavement base stiffness

Image of aggregate road

Geogrid matting stabilizes and stiffens aggregate base courses. Photo: MnDOT

Soft subgrades put road designs to the test. When the ground below the structure is soft, pavements may experience cracking, rutting, and other types of failure. A recent MnDOT-sponsored project quantified the performance benefits of geogrids in pavement structures over soft soils.

In recent years, the Minnesota Department of Transportation’s MnPAVE flexible pavement design software has incorporated a mechanistic-empirical pavement design methodology, which allows for more accurate prediction of pavement performance based on design characteristics. The mechanistic properties of geogrids, however, had not been determined for use in MnPAVE.

In this project, researchers combined field and lab testing with sophisticated modeling to identify the extent to which a geogrid improves structural stiffness, so that its benefit can be assessed by designers constructing or reconstructing roads over soft subgrades.

Findings included:

  • Geogrids improve aggregate base resilient modulus, a mechanistic measure of stiffness.
  • Geogrids provide lateral restraint to aggregate base through interlocking of aggregate and friction between aggregate and a geogrid.
  • Aggregate base gradation, roughness, moisture content, and porosity affect the performance of aggregate bases enhanced with geogrids.
  • The improved analytical process provides mechanistic behavior models for aggregate-geogrid interaction that support reducing aggregate thickness for base courses enhanced with geogrids.

“This modeling work shows that geogrids give more stiffness to the pavement structure,” says Jim Bittmann, materials engineer with MnDOT District 2. “It validates what we’re already doing. In some cases, we may be able to use less asphalt once we have implemented these results into our pavement tools.”

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Weather management solutions improve roadway operations

EDC logo

Weather affects roadway operations year-round. Through round four of Every Day Counts (EDC-4), two distinct road weather management solutions were deployed that allow state and local agencies to manage road systems ahead of and during adverse weather events.

One of the solutions is Pathfinder, a collaborative process involving the National Weather Service (NWS), departments of transportation, and private-sector weather service providers to disseminate clear, consistent road weather information to the traveling public. The goal of this low-cost solution is to help drivers make safe and efficient travel decisions during weather events.

Utah is one of several states using the Pathfinder process to deliver messages via variable message signs about weather events such as snow, freezing rain, and high winds. A Utah study of snow events found a 40 percent reduction in vehicle-miles traveled, indicating that concise, consistent messaging works.

The second solution—integrating mobile observations—involves collecting weather and road condition data from government fleet vehicles, such as snowplows. The focus is on supplemental data from ancillary sensors installed on the vehicles, such as pavement temperature sensors, and it also includes native vehicle data such as windshield wiper status and anti-lock brake or traction control system activation.

The data provides maintenance managers with a detailed view of the weather and road conditions along the road network. This information supports a number of road weather management strategies, such as a winter maintenance decision support system that enables agencies to use only the necessary amounts of labor and equipment to pre-treat roads with salt and other materials. It also supports traveler advisories and warnings, ultimately resulting in improvements in safety and mobility.

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Manual helps agencies count bike, pedestrian traffic

Image of Bike Manual cover

As part of an ongoing effort to institutionalize bicycle and pedestrian counting in Minnesota, MnDOT has published a new manual designed to help city, county, state, and other transportation practitioners in their counting efforts. Other resources available thanks to the initiative include portable counting equipment, a website, and training programs.

The Bicycle and Pedestrian Data Collection Manual, developed by University of Minnesota researchers and SRF Consulting Group, provides guidance and methods for collecting bicycle and pedestrian traffic data in Minnesota. The manual is an introductory guide to nonmotorized traffic monitoring designed to help local jurisdictions, nonprofit organizations, and consultants design their own programs.

Topics covered in the manual include general traffic-monitoring principles, bicycle and pedestrian data collection sensors, how to perform counts using several types of technologies, data management and analysis, and next steps for nonmotorized traffic monitoring in Minnesota. Several case studies illustrate how bicycle and pedestrian traffic data can be used to support transportation planning and engineering.

The manual was completed as part of the Minnesota Bicycle and Pedestrian Counting Initiative, a collaborative effort launched by MnDOT in 2011. Other accomplishments include:

  • A district-based portable counting equipment loan program to support MnDOT districts and local jurisdictions interested in nonmotorized traffic monitoring
  • A MnDOT website for reporting annual and short-duration counts that allows local planners and engineers to download data for analysis
  • Provisions added to MnDOT equipment vendor agreements that enable local governments to purchase bicycle and monitoring equipment
  • Annual training programs for bicycle and pedestrian monitoring

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Guide helps small towns design multimodal networks

Image of biker

Photo: David Gonzalez, MnDOT

In many small towns and rural communities, active transportation is even more common than it is in urban areas. However, infrastructure to support active transportation is often limited or absent. The Small Town and Rural Multimodal Networks guide is a design resource and idea book to help small towns and rural communities support safe, accessible, comfortable, and active travel for people of all ages and abilities.

The new guide is intended to be a resource for transportation practitioners in small towns and rural communities. It applies existing national design guidelines in a rural setting and highlights small town and rural case studies. It addresses challenges specific to rural areas, recognizes how many rural roadways are operating today, and focuses on opportunities to make incremental improvements despite the geographic, fiscal, and other challenges that many rural communities face.

The document presents compelling photographs, visual illustrations, and technical diagrams to communicate design issues and solutions in a smaller scale setting. The guide development was funded in part by the Federal Highway Administration and Blue Cross Blue Shield of Minnesota.

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Zipper merge reduces crashes, congestion in work zones

Screenshot of Zipper Merge video

MnDOT Zipper Merge page

When most drivers see the first “lane closed ahead” sign in a work zone, they slow too quickly and move to the lane that will continue through the construction area. This behavior can lead to unexpected and dangerous lane switching, serious crashes, and road rage. “Zipper” merging, however, benefits individual drivers as well as the public at large.

Research shows that the dangers of early merging decrease when motorists use both lanes until reaching the defined merge area and then alternate in “zipper” fashion into the open lane. MnDOT recommends that drivers stay in their current lane up to the point of merge, then take turns with other drivers to safely and smoothly ease into the remaining lane.

However, when traffic is moving at highway speeds and there are no backups, it makes sense to move sooner to the lane that will remain open through construction. The bottom line is to merge when it is safe to do so.

Two helpful videos are on MnDOT’s zipper merge resources page: one is a “how to” instructional video, and the other is a public service announcement to share with your constituents and others.

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