Study Suggests 70 Percent RAP for Minnesota Gravel Road Surfaces

Researchers examined mixtures of recycled asphalt pavement (RAP) and aggregate for new gravel road surface layers in the lab and in the field. Although test results did not align perfectly, and field results were somewhat uneven, findings suggest that mixtures with 70 percent RAP content can reduce dust generation. After a year of service these roadways can match all-aggregate gravel road performance in terms of strength, but with a smoother ride.

What Was the Need?

Gravel roads offer a cost-effective option for road departments that wish to avoid the expense of asphalt and concrete roads in rural or low traffic areas. However, about an inch of gravel is lost from these roadways each year. Aggregate resources are diminishing, and gravel and crushed rock aggregate is growing increasingly expensive.

Gravel also generates dust that can reduce visibility, affect road performance and result in complaints from nearby homeowners. 

Recycled asphalt pavement (RAP) can be an effective component of new asphalt pavement mixtures. Many aggregate producers stockpile RAP that has been broken into the size of aggregate. But not all RAP works well mixed in asphalt, and some aggregate yards are too far from pavement projects to economically use RAP in pavement. 

Road agencies frequently use RAP in gravel roads. The asphalt content in RAP can bind with dust from crushed rock or gravel, helping manage fugitive dust. A recent study in Wyoming found that using RAP in new gravel surface applications at less than 50 percent of the aggregate resulted in good road performance and kept dust to a minimum. 

What Was Our Goal?

In light of the findings from the Wyoming study, researchers sought to determine the optimal level of RAP in an aggregate mixture for Minnesota gravel road surfaces. These new applications would offer good driving stability while also controlling fugitive dust. 

What Did We Do?

Research began with a review of the literature on RAP as an aggregate component of surface, base and subbase layers, as well as a survey of Minnesota counties on their experience with these mixtures. 

In the lab, the research team tested three RAP materials and virgin aggregate from two Minnesota locations in various RAP content levels for strength and compression. Investigators then compared the economic feasibility of 100 percent virgin aggregate use to 50 percent virgin and 50 percent RAP aggregate mixtures on a 1-mile aggregate road, including annual grading and eventual regraveling in the estimations.  

Research in the field focused primarily on six 1,000-foot gravel road test sections: four sections in Goodhue County using 15, 30, 45 and 60 percent RAP content, and two sections in Carlton County using 30 and 50 percent RAP. The studies entailed all-virgin aggregate control sections, and installations were made over roads with various subgrade soils that presented a variety of properties. Sites were tested for elasticity, bearing strength and fugitive dust generation. 

A secondary field study focused on RAP contents of 50, 70 and 80 percent in 3-inch surface courses for three test sections and one control section in Goodhue County. Sites were tested for elasticity, strength, dust generation, ride quality and surface aggregate looseness over time, and some lab tests were conducted.

“The 70 percent RAP mixture seemed to be about the best combination. We put RAP down in fall 2017, and by the next summer, it was working much like a regular gravel road.” —Charles Jahren, Professor, Iowa State University Department of Civil, Construction and Environmental Engineering 

Mounds of RAP at a gravel pit in Carlton County offer road agencies an alternative to
natural gravel and crushed aggregate for gravel roads. But RAP has to be used in the
right proportion with gravel.
Mounds of RAP at a gravel pit in Carlton County offer road agencies an alternative to natural gravel and crushed aggregate for gravel roads. But RAP has to be used in the right proportion with gravel.

What Did We Learn?

Previous research indicated that RAP can help reduce fugitive dust, offers value as surface courses, and can reduce moisture susceptibility of gravel roads in cold or wet locations. 

Lab mixtures with 30 percent RAP consistently produced high compressive strength values, and higher RAP levels generally correlated inversely with bearing strength. Improvements in dust reduction were limited until RAP levels exceeded 50 percent. 

Economic analysis determined that a 50/50 percent mix of RAP and aggregate would cost 1.5 percent more than an all-virgin aggregate surface course in terms of construction and maintenance, but potential reductions in dust generation, surface aggregate loss and regraveling after three years of service may produce savings from RAP use. 

Results from field testing defied clear recommendations on optimal RAP content. Generally, higher RAP content offered greater elasticity and lower levels of loose aggregate initially, but these benefits fell to equal or below non-RAP levels after a year. Higher RAP correlated with reduced dust generation, but again fell over the first year of service. In secondary testing, initial dust generation was lower with the 50 percent mixture than the others, but after a year was lowest with the 70 percent mixture. 

Ultimately, researchers found that after a year, during which fugitive dust production was reduced, the performance of a 70 percent RAP content aggregate surface course was most like a virgin aggregate surface course and offered a smoother driving surface. 

What’s Next?

“These findings provide another tool in the toolbox. They will be most useful to engineers who haven’t used RAP in gravel roads and to county engineers who have a RAP resource.” —Joel Ulring, Pavement Engineer, MnDOT State Aid for Local Transportation

While this research did not develop a definitive recommendation for an optimal RAP content in surface courses for aggregate roads, it did produce useful data on performance. The study did encourage a general sense that 70 percent RAP content for surface courses of approximately 2 inches may be effective and warrants systematic study for a three-year period. 

A researcher scrapes a gravel road surface with a modified garden hoe to measure loose aggregate levels.
A researcher scrapes a gravel road surface with a modified garden hoe to measure loose aggregate levels.

This post pertains to Report 2019-11, “Optimal RAP Content for Minnesota Gravel Roads,” published March 2019. For more information, visit MnDOT’s Office of Research & Innovation project page.

Study Underway to Harness Renewable Energy from Minnesota’s Highways

Sound barriers and snow fences along highways have the potential to provide clean energy in Minnesota.

A newly funded MnDOT study, Harnessing Solar Energy through Noise Barriers and Structural Snow Fencing, is investigating how to utilize existing noise barriers and snow fences on Minnesota highways to harvest clean, cost-effective energy.

“Snow fences and noise walls are structural barriers with a singular purpose. Snow fences are intended to limit snow from drifting onto our highways and noise walls are intended to reduce noise to a comfortable level for communities living near our highways. Finding a way to integrate solar that maintains their structural integrity could transform the use of these barriers from single purpose to multi-purpose,” says Dan Gullickson, MnDOT’s Blowing Snow Control Shared Services Program Supervisor, who is overseeing the research project.

Solar energy is energy captured from the sun and converted into thermal or electrical energy. It is a clean and abundant renewable energy source and generally requires very little maintenance after installation. Solar energy has a variety of uses, including providing electricity to power street lamps and homes, heating and cooling spaces, and heating water.

“We’ve seen some applications of solar panels on noise walls—primarily in European countries—but the addition of solar panels to snow fences is an entirely new concept,” says Gullickson.

The innovative nature of this project brings many unknowns that MnDOT hopes to answer, such as: Is it possible to engineer these structures without disrupting their functionality? What safety measures need to be taken to ensure the public and MnDOT workers stay safe if they come into contact with the panels? What are the lifecycle costs of installing and maintaining solar? How much energy could they generate and how does that connect with existing power grids?

One estimate shows that a thousand miles of solar panels could power all the street lights along Minnesota highways or 43,333 residential homes. (Assuming each solar panel is 330W and 1,000 panels could generate up to 330kW per mile.)

Harnessing solar energy on Minnesota highways: Solar energy can be used for heating, cooling, lighting, and warming water. 1,000 miles of solar panels on Minnesota highways could power: All of minnesota highway lights o 43,333 residential homes.

“We know Minnesota and North Dakota winters bring a lot of snow—which is disruptive to our travelers and farmers. We hope to create a sustainable solution that aids drivers and farmers, but also harnessing energy which would be able to offset the cost of construction and installation,” says Mijia Yang from North Dakota State University, the lead researcher.

Gullickson and a diverse team of MnDOT experts – from the field of environmental stewardship to traffic engineering – will guide the research and review findings.

The study will include surveys, lab testing or modeling of possible design options and a cost-benefit analysis—planned to be completed by of June 2021.

Currently, the research team is developing surveys to better understand public opinion on solar energy (including energy prices and solar panel infrastructure), power companies’ interest in purchasing solar energy generated through the right-of-way and legal considerations for harvesting solar energy through the rights-of-way.

 “Surveying the public and utility providers may uncover questions that we hadn’t previously anticipated. We hope to address those hurdles throughout the study,” says Gullickson. Follow along for project updates on MnDOT’s Office of Research & Innovation website.

Using Chemical Adhesives to Post-Install Epoxy-Coated Rebar in Concrete

The Minnesota Department of Transportation (MnDOT) had suspended the use of post-installed epoxy-coated rebar for concrete barrier repairs as a precautionary measure because chemical adhesives used in the process are not designed for use with coated bars. But laboratory testing (conducted in a recent MnDOT-sponsored research study) has now shown that using these adhesives with coated rebar for post-installation works well and provides a safety level 200 to 300 times that predicted by manufacturer specifications. MnDOT is considering research recommendations to modify the installation process in order to resume using coated rebar in post-installed concrete crash barriers.

What Was the Need?

As bridges age, local and state agencies are called on to repair and upgrade elements. Local agencies may install new light posts and replace chain-link rails, while state agencies may replace slabs on wing walls, deck and crash barriers, and other non-hanging bridge components. Work like this requires replacing concrete elements and installing new reinforcement bars. Until recently, MnDOT and local agencies used epoxy-coated steel rebar for these post-installed concrete panels and barriers.

Epoxy-coated rebar resists corrosion from salt and water that penetrate concrete members, especially at seams and cracks. Post-installed rebar requires chemical adhesives to secure the bar in place to effectively transfer loads from one concrete slab to another. Manufacturers test these adhesives with standard uncoated steel rebar to provide users with application guidance and to give engineers data on the tensile pullout strength of the rebar in concrete, a key property in the design of concrete bridge components.

As with many bridge materials, specifications for adhesives are conservative; the tensile strength of chemically adhered rebar can be assumed to be much higher than specified. Despite the almost certain safety of the practice, the MnDOT Bridge Office suspended the use of epoxy-coated rebar in post-installation applications because manufacturer specifications are based only on results from testing with uncoated rebar.

What Was Our Goal?

The goal of this project was to determine the effect of the epoxy coating on the tensile strength of rebar that is post-installed with a chemical adhesive. To achieve this goal, researchers surveyed other state transportation agencies to determine how these agencies use epoxy-coated rebar in certain post-installation practices. In addition, the research team conducted laboratory evaluations of common adhesives and epoxy-coated rebar installed in hardened concrete.

“The point of epoxy-coated rebar is to gain a longer life cycle. While MnDOT practices were not unsafe, there are ways the agency could be more accurate.”—Ben Dymond, Assistant Professor, University of Minnesota Duluth Department of Civil Engineering

What Did We Do?

Two post-installation applications were studied: crash barriers between bridge support piers and crash barriers on the edges of bridge decks. Investigators distributed a survey to all 50 state transportation agencies to learn the state of the practice related to postinstalling epoxy-coated rebar with chemical adhesives in hardened concrete bridge elements. The research team also studied MnDOT procedures for using adhesives with epoxy-coated and uncoated rebar during post-installation of rebar in crash barriers.

Investigators then conducted a laboratory study of the four most common adhesives used in Minnesota. They installed six uncoated lengths of rebar for each adhesive in one concrete slab and six epoxy-coated lengths of rebar for each adhesive in another slab. Rebar was pulled from the concrete to determine the tensile strength. 

In tests of adhesives and
reinforcement steel,
epoxy-coated rebar and
uncoated rebar showed
high pullout strength
when extracted from a
concrete test slab.
Two lengths of uncoated rebar (black) and two lengths of epoxy-coated rebar (green).

What Did We Learn?

Thirty states responded to the survey. Twelve of these state agencies do not use epoxy-coated rebar post-installed with chemical adhesives in concrete. Eleven of the 18 agencies that do use epoxy-coated rebar in these applications employ manufacturers’ data on bond strength; another six use a standard, national calculation method. 

Tensile pullout strength of the bars varied by adhesive type. In all cases, the manufacturer specifications for strength were very conservative and safe, identifying strength values 200 to 300 times lower than shown in laboratory tests. MnDOT’s approach to adhering epoxy-coated and uncoated reinforcement steel in hardened concrete barriers results in safe, reliable bonding and load transfer. 

The difference in strength between chemically adhered epoxy-coated steel and uncoated steel bars was on the order of 10 percent; in some cases, coated rebar was slightly stronger with the tested adhesive. Below are the pullout strength ratios of epoxy-coated bars to uncoated bars (reduced by three standard deviations) for the four adhesives:

  • Powers AC100+ Gold: 0.89.
  • Red Head A7+:1.02.
  • ATC Ultrabond 365CC: 0.98.
  • Hilti HIT-RE 500 V3: 1.11. (In this case, however, bars ruptured, and pullout strength was not definitively established.) 

Researchers recommended a modification factor for calculating bond strength to reflect the findings. Investigators also recommended changing a MnDOT bond stress specification or adopting manufacturer values, which would better meet national specifications for bridge design. 

“After exploring the epoxy-coated rebar practices of other states, we identified a difference in the installation design with epoxy-coated bars and uncoated bars.”—Joe Black, Senior Engineer, MnDOT Bridge Office 

What’s Next?

Although MnDOT is not currently considering the use of epoxy-coated rebar in post-installed concrete, this study suggests what steps may be necessary to reauthorize its use in these applications. One solution would be to specify longer epoxy-coated rebar to mitigate the minor potential loss in pullout strength, allowing bridge managers to leverage the corrosion-resistance benefit of epoxy-coated rebar in these applications. 

A researcher uses a dust-collecting drill to ensure clean insertion of rebar in concrete.
A researcher uses a dust-collecting drill to ensure clean insertion of rebar in concrete. 

This post pertains to Report 2019-07, “Anchorage of Epoxy-Coated Rear Using Chemical Adhesives,”  published February 2019. For more information, visit MnDOT’s Office of Research & Innovation project page.

Epoxy-Coated Rebar Bridge Decks Outperform Mixed Rebar Decks

Bridge decks reinforced with one layer of epoxy-coated rebar and a bottom layer of uncoated steel rebar show corrosion damage sooner than decks constructed with all epoxy-coated rebar. Inspection methods should be enhanced to add a rating for cracking density on the underside of bridge decks. Repairs to mixed rebar decks should be conducted once a key deck surface inspection element has received a condition rating of two and held that rating for seven years, which is sooner than the average repair time of 8.5 years.

What Was the Need?

In concrete bridge decks, steel reinforcing bars are necessary to add tensile strength and transfer loads to beams. Additionally, steel reinforcement in concrete bridge decks is designed to control cracking, which will extend the service life of the bridge.  

Steel also corrodes in salt environments, even when embedded in concrete. Water and road deicing chemicals can reach the steel and damage its strength and integrity. Between 1973 and 1990, MnDOT built approximately 660 bridges with more expensive, epoxy-coated rebar in the top layer of reinforcing matting and standard black rebar in the bottom layer. The coated top layer, only 3 inches below the deck surface, was expected to resist corrosion, and damage from salt and water would not reach the next layer of rebar, another 3 inches down in a 9-inch deck. 

In recent years, MnDOT has used another reinforcing strategy: mixing noncorrosive fibers into concrete mixes to help prevent or minimize cracking and resist corrosion. The older, mixed reinforcement bridges remain in service and few have been redecked. Performance of mixed reinforcement and fiber reinforcement in Minnesota bridge decks has not been compared to the performance of bridge decks constructed with only epoxy-coated rebar.

What Was Our Goal?

MnDOT sought to compare the performance of mixed rebar decks with all epoxy-coated rebar decks, and the performance of fiber-reinforced decks with no-fiber concrete decks. MnDOT also wanted to learn how to plan preventive maintenance efforts for mixed rebar decks.

What Did We Do?

Researchers reviewed reports from inspections, conducted every two years, for bridges with mixed reinforcement decks and decks with 3-inch strips of fiber reinforcement mixed into the concrete. They narrowed their review to bridge inspection data from 506 bridges with epoxy-coated rebar (including 35 control decks with all-epoxy rebar) built between 1973 and 1990, and 22 bridges with fiber-reinforced concrete and epoxy-coated rebar (including four controls with no rebar) built between 2012 and 2017. All of the bridges were inspected through 2017.  

Investigators then conducted site evaluations of 75 mixed rebar decks and 25 all-epoxy rebar decks, as well as 11 fiber-reinforced concrete decks with epoxy rebar and four without rebar. Site surveys focused on confirming the accuracy of recent inspection reports and recording signs of cracking, spalling and other deterioration conditions. 

Spalling has occurred because deicing chemicals and water have seeped into the concrete deck through cracks in the deck surface, causing uncoated steel reinforcement to corrode and concrete to spall.

What Did We Learn?

All-epoxy rebar decks outperformed mixed rebar decks, showing less cracking on the top and underside of the decks. Mixed rebar decks deteriorated at a quicker rate on bridges with steel beams than on bridges with prestressed concrete beams. Traffic levels and surface cracking did not appear to affect deterioration of decks in any group. 

“There’s not really a good visual inspection standard for quantifying cracking under the bridge deck, and that’s especially important for these types of bridges with mixed rebar. Using only epoxy-coated rebar in decks was a good idea.” —Ben Dymond, Assistant Professor, University of Minnesota Duluth Department of Civil Engineering

Data sets were too small to draw any conclusions about possible differences in performance of fiber-reinforced decks compared to bridge decks that were not built with fibers. 

Individual bridge elements, such as bridge deck surfaces, have historically been rated from one (best condition) to five (worst condition). Mixed rebar decks earned element ratings of three to four more frequently than all-epoxy rebar decks, and visual surveys identified more deterioration on the underside of mixed rebar decks than all-epoxy rebar decks. 

The research team recommended amending bridge inspection procedures to add a new rating element for quantifying crack density on the underside of decks to anticipate and prevent spalling and delamination on the underside of mixed rebar decks. The team also recommended that once the deck condition element in mixed-bar decks holds a rating of two for seven years, more robust cracking sealing techniques should be considered to prevent it from reaching a rating of three. (For most bridges, that repair typically occurs after 8.5 years of service.)

Finally, the team recommended continued evaluation of fiber-reinforced decks as inspection data is collected over time. 

What’s Next?

“These findings may help us shift some priorities for repairing or replacing mixed rebar bridges. We will continue to advocate for the use of all epoxy-coated rebar wherever we anticipate high levels of chlorides.” —Nick Haltvick, North Region Bridge Construction Engineer, MnDOT Bridge Office

This research confirms that MnDOT’s current practice of using only epoxy-coated rebar in bridge decks remains a durable solution and offers the best long-term value in terms of repair needs. MnDOT will continue to evaluate fiber-reinforced concrete deck behavior and may adopt a rating method for identifying crack density on the underside of concrete decks. 

A crack underneath a bridge deck, revealing signs of leakage and corrosion of steel reinforcement bars.
Cracks on the undersides of a bridge deck can leak as chlorides corrode steel and seep through the crack. 

This Technical Summary pertains to Report 2019-09, “Deterioration of Mixed Rebar and Fiber-Reinforced Concrete Bridge Decks,”  published February 2019. Visit the MnDOT research project page for more information.

Speed Notification System Warns Drivers Approaching Urban Work Zones

Using an innovative method to calculate vehicle trajectories and gather large amounts of driver data, researchers tested and evaluated the new Smart Work Zone Speed Notification system and determined that its messages successfully influenced drivers to reduce their speed. 

What Was the Need?

Maintenance and construction on Minnesota’s roadways often create travel disruption for drivers through traffic slowdowns and queuing. MnDOT has previously tested systems to inform drivers of traffic backups in rural work zones, but slowdowns near complex urban work zones are less predictable. Drivers traveling at highway speed may come upon congestion suddenly, resulting in abrupt braking and the risk of rear-end collisions. 

Posting advisory speed limit messages near these work zones has not been effective. To address the problem, MnDOT developed a Smart Work Zone Speed Notification (SWZSN) system designed to inform drivers of the actual speed of slowed downstream traffic near large urban work zones. Researchers from the Minnesota Traffic Observatory then tested and evaluated this system over time in an actual urban highway construction work zone. 

What Was Our Goal?

The primary objectives of the project were to quantify the speed notification system’s effect on drivers’ behavior and determine its impact on the safety of the work zone. With an effective work zone speed notification system, MnDOT’s goal was to create safer work zones by giving highway drivers real-time information that would influence them to slow down adequately before a congestion hazard, avoiding dangerous braking and collisions. 

What Did We Do?

A variable message sign, part of the SWZSN, displays the warning “Slow Traffic Ahead” in a construction zone.

The SWZSN was designed to collect traffic speed data throughout a work zone and run it through an algorithm, generating the appropriate message for drivers on a variable message sign, such as “35 MPH 1 Mile Ahead” or “Stopped Traffic Ahead.” 

MnDOT wanted to deploy the system within a project replacing 4.4 miles of Interstate 94 (I-94) east of downtown St. Paul. The construction was to be completed in stages between spring 2016 and fall 2017. This large project would include many lane closures and expected traffic congestion; the new system could mitigate some of the traffic disruption. The deployment and evaluation of the SWZSN took place in three phases:

  • Pre-SWZSN deployment (mid-2016): Gathering data from the work zone before the system was deployed to obtain data demonstrating drivers’ behavior and crash frequency data without the new system.
  • Post-SWZSN Phase I deployment (2016 into 2017): Gathering the same data from the work zone after the SWZSN was deployed.
  • Post-SWZSN Phase II deployment (the entire 2017 work season): Gathering data from the work zone after initial deployment, with troubleshooting and improvements of algorithms and messages in place, showing drivers’ behavior and work zone crash frequency with the improved SWZSN and the analyses of these data. 
A variable message sign, part of the SWZSN, displays the warning “Slow Traffic Ahead” in a construction zone.
Speed detection sensors were installed on poles every half-mile through a new highway construction work zone.

To collect data for the system, MnDOT’s Regional Transportation Management Center mounted Wavetronix speed detection sensors on poles every half-mile in the work zone, replacing old loop detectors. Researchers also deployed nine solar-powered cameras on mobile trailers about every half-mile. This allowed researchers to capture traffic flow images in more strategic locations where traffic queues were forming since the construction zones were complex, crowded and often changing, with many visual obstructions. Data were transferred primarily via an arranged wireless radio link to the Minnesota Traffic Observatory. 

Researchers then applied their own innovative methodology—a Trajectory Extraction Tool (TET)—to the traffic images captured by the cameras using video alone to calculate a vehicle’s deceleration rate when approaching traffic congestion. The cameras were positioned to optimize TET performance. Researchers gathered tens of thousands of data points for analysis from traffic in the work zone over the course of the project. 

What Did We Learn?

During the first year of SWZSN implementation, the project team identified discrepancies in the speed notification algorithm, such as unreasonable or delayed messages. By the second construction season, those anomalies were significantly reduced. The most significant results of this project showed that in situations where messages communicated to drivers were consistent and accurate, reductions of more than 30 percent in the selected deceleration rates were observed.

Most importantly, the speed notification system is clearly noticed by drivers and results in a statistically significant influence on drivers’ behavior, suggesting that downstream speed notification is an effective traffic control tool.

What’s Next?

This evaluation project was considered a success: The system is more effective than previously used work zone advisory speed limits. The SWZSN has already been deployed in two highway construction sites and will be used for future projects.

This Technical Summary pertains to Report 2019-21, “Evaluation of the Smart Work Zone Speed Notification System,” published June 2019. Visit the MnDOT research project page for more information.

Evaluating the Use of Central Traffic Signal Control Systems

MnDOT sought to determine the full range of intersection control information (ICI) currently used in the state and how it could best be made accessible for state transportation system needs. Researchers created the Regional Database of Unified Intersection Control Information, a machine-readable, cloud-based unified ICI system. They determined steps MnDOT could take toward more effective use of its central traffic signal control system, such as mitigating traffic disruption around construction zones and participating more fully in emerging technologies such as vehicle information systems and vehicle automation.

What Was the Need?

Traffic signal control has evolved since the 1950s from simple time-based signal protocols to current dynamic systems that allow adjustment of signals to traffic conditions. Intersection control information (ICI) is increasingly important to transportation agencies, researchers and private companies involved in developing traffic models and technologies. 

Historically, the availability of traffic signal control information in Minnesota and traffic data formats have varied across jurisdictions. Nationally, increased use of central traffic signal control systems (CTSCS) has supported recent trends toward more dynamic traffic models and control, as well as toward advances in automated intelligent vehicles. This quickly evolving environment makes the creation of a unified, standardized system of ICI in Minnesota both feasible and necessary. 

MnDOT sought to examine the use of CTSCS to manage all aspects of traffic near construction zones more strategically and effectively in order to mitigate the frequent and often severe disruption of traffic these zones can cause. This project was initiated to determine the state of Minnesota’s ICI systems and to develop guidance for reaching MnDOT’s goal of a unified ICI and better statewide traffic management through CTSCS. 

A bird’s-eye view of a diverging diamond interchange in Bloomington, Minnesota. Two diamond-shaped formations of many converging and diverging lanes of traffic are seen on either side of a multilane highway.
Unified ICI can identify all the parameters traffic signal controllers need to effectively manage challenging highway configurations like this diverging diamond interchange in Bloomington.

What Was Our Goal?

This project had three objectives:

  • Deliver guidance and tools to collect ICI from all Twin Cities metro area jurisdictions and automate the importation of this information into each jurisdiction’s CTSCS and signal performance measure (SPM) applications. A stated priority was the ability to import this data into MnDOT’s digital products used in construction design.
  • With the help of all stakeholders, define the most inclusive format to represent all required information.
  • Design a Regional Database of Unified Intersection Control Information (RDUICI), and propose methods and tools for importing and exporting data between the RDUICI and all CTSCS and SPM applications by local jurisdictions. 

“A unified set of intersection control information is valuable for developing a regional signal timing database to model construction project impacts and provide standardized information for use with connected vehicle technologies.”

—Kevin Schwartz, Signal Optimization Engineer, MnDOT Metro Traffic Engineering

What Did We Do?

Researchers distributed surveys to signal operators and transportation model builders to identify the contents and develop the format of the unified ICI. 

A survey sent to 153 signal professionals sought to learn how operators in diverse jurisdictions store and distribute ICI. Responses from 42 participants helped researchers assess the availability of ICI and the degree of effort a regional unified ICI would require.

A second survey was sent to 58 designers, modelers and planners who have experience working with MnDOT signal information to learn about ICI’s various uses; 25 people responded. Researchers also interviewed a selected group of signal operators and modelers to gain more detailed information. 

A four-sided traffic signal hangs from a pole over an intersection. The traffic light illuminated on the visible side is red.
Most of Minnesota’s traffic signals use complex controllers to manage traffic, responding to information gathered from multiple sources, such as loop detectors and other sensors.

These surveys and in-depth interviews allowed researchers to create intersection models of varying complexity to drive the identification and categorization parameters of the proposed unified ICI. Researchers developed a complete unified ICI for a diverging diamond interchange, a complex interchange that is difficult to represent with traditional intersection models. Researchers also developed a relational database schema for containing the data set in a machine-readable format. This schema is a starting point for developing a system for standardizing the management and availability of ICI across jurisdictions. 

What Did We Learn?

Researchers documented all intersection signal control codes in use. They showed the feasibility of a unified ICI and demonstrated it through the example of a fully coded diverging diamond interchange. They learned that some data in older formats would need to be digitized to be included.

“Identifying the needs of different stakeholder groups allowed us to produce an organized, comprehensive format for intersection control information.”  

—John Hourdos, Research Associate Professor, Minnesota Traffic Observatory, University of Minnesota

Further investigation and communications with the software developers of MaxView, MnDOT’s CTSCS, showed researchers that current systems could not be used to store the entire unified ICI. While the systems contain much of the unified ICI data set, some detailed geometric information is missing that is critical to understanding the intersection control. MaxView also contains information that is not readable by other systems. 

Because of these challenges, researchers suggested managing unified ICI through a custom-built, centralized cloud repository. This solution would only require that vendors develop tools for exporting the information they have in a unified ICI format. The cloud repository would then be accessible to signal control vendors and to MnDOT, and security would remain intact. 

What’s Next?

MnDOT now has the full range of intersection signal control data used across the state. Researchers have determined it can be imported, stored and delivered through a cloud-based method. With these findings, the agency can begin to consider projects that use CTSCS for construction zone disruption mitigation and intelligent vehicle technologies.

This Technical Summary pertains to Report 2019-14, “Evaluation of a Central Traffic Signal System and Best Practices for Implementation,” published March 2019.  Visit the MnDOT research project page for more information.

Sediment Control Log Guidance for Field Applications

Researchers tested sediment control logs in the lab and in the field to determine the relative filtration capabilities of these devices. They also developed design guidelines for correct selection and contributed to ongoing educational efforts. 

What Was the Need?

Whenever MnDOT or its contractors engage in construction, maintenance or other projects that substantially disturb the soil at a project site, they are required to use practices that reduce sediment discharge from the site when it rains. Sediment control methods are used as perimeter barriers around stockpiles, for inlet protection, as check dams in small drainage ditches and also along natural waterways such as streams, ponds or wetlands. 

A commonly used method is the sediment control log (SCL)—a linear roll constructed with an outer sleeve of varying permeability that is filled with natural biodegradable infiltration materials such as straw, coconut fiber (also known as coir), compost or rocks. MnDOT’s SCLs range from 6 to 9 inches in diameter and up to 30 feet in length.

While MnDOT has used SCLs extensively for many years, these devices often fail because their performance is not well-defined or understood. SCLs are also frequently installed incorrectly or in inappropriate locations. Because SCL use represents a substantial cost to the agency, MnDOT sought to learn actual performance parameters as well as optimum locations and installation methods. 

What Was Our Goal?

The goal of this project was to improve practitioners’ ability to select the appropriate SCL for a specific purpose and location. To achieve this goal, researchers sought to:

  • Determine the hydraulic characteristics of SCLs—how SCLs constructed from different encasement fabrics and internal media allow the passage of water. 
  • Evaluate the sediment removal efficiency of these SCLs and the effect of trapped sediment on their hydraulic characteristics.
  • Develop design guidelines for selecting SCLs based on log materials and the characteristics of the watershed where they will be installed. 
  • Organize the selection guidelines into a format that can be used by field practitioners for amending or upgrading the device. 

“This study compared the sediment filtration capabilities and effective life cycles of a range of sediment control logs. This new knowledge will allow us to reduce costs in all areas of sediment control log use and more effectively protect the environment.”

—Dwayne Stenlund, Erosion Control Specialist, MnDOT Office of Erosion Control and Stormwater Management 

What Did We Do?

First, researchers conducted a literature review of studies published from 1995 to 2013 that examined a variety of sediment control methods. 

Next, they determined the physical characteristics of 12 SCLs filled with diverse biodegradable media, ranging from straw; coconut fiber; wood fiber; wood chips; light, medium and heavy compost; and rock. Then they investigated the hydraulic characteristics of the SCLs, most importantly the volumetric flow rate through logs of various media, using the flume at the University of Minnesota’s Biosystems and Agricultural Engineering Laboratory. 

A sediment flume was constructed at this laboratory that researchers used to evaluate the sediment removal efficiencies and failure rates of a subset of five logs. The subset was selected to capture the range of hydraulic response representing a variety of log materials.

Researchers also examined field installations of SCLs in locations across the state to learn how SCLs were installed and, if failing, how they had failed.

A long, black sediment control log. Dried sediment is on top of a section of the log that traverses a shallow, eroded ditch. The log is held in place with two wooden stakes on the downslope side.
Overtopping occurred at this failed SCL installation, indicated by the dried sediment on the log. 

Finally, they produced two SCL selection tools and developed training materials about SCL use. 

What Did We Learn?

From the literature review, researchers reviewed seven laboratory studies and nine field studies examining a wide range of sediment control methods. They found no studies similar to this project that compared different kinds of SCLs for their sediment removal efficiency, life cycles and appropriate siting. 

Researchers investigated the physical characteristics of 12 SCLs, including diameter, density and percent volumetric pore space. They conducted material size analysis and other tests to determine saturated moisture content, capillary moisture content, saturated conductivity and other relevant hydraulic measures. Using results from the laboratory flume, they documented the flow rates of water through the SCLs. 

The physical characteristics of the 12 SCLs varied substantially. For example, densities ranged from 2.18 pounds to 18.5 pounds per cubic foot. Hydraulic characteristics, such as the amount of water retained and the rate of fluid flow through the medium, also varied widely. 

The subset of five logs tested for sediment removal efficiency showed how much sediment each log could filter at three flow rates and how much sediment buildup would cause log failure. These results combined with earlier hydraulic data allowed researchers to extrapolate the relative comparative longevity of different SCL media and to develop two SCL selection tools: one for ditch checks and one for perimeter control. The tools will guide practitioners to select the correct SCLs using watershed area, basin and ditch slope. Researchers also adapted the results of the investigations into a set of training materials for erosion control and stormwater management.

What’s Next?

The two decision tools will guide the selection of correct SCLs for particular locations. SCL training materials have already been implemented in the erosion control and stormwater management certification workshops. 

“Sediment control log failure is a worldwide problem. This research takes a substantial step toward a better understanding of the parameters within which SCLs can be effective, clarifying with data their capabilities as well as their limitations.”

—Bruce Wilson, Professor, University of Minnesota College of Science and Engineering

This post pertains to Report 2019-23, “Sediment Control Log Performance, Design and Decision Matrix for Field Applications,” published May 2019. Visit the MnDOT research project page for more information.

Preparing Roads for Connected and Autonomous Vehicles

Proprietary technologies, industry competition and federal regulatory concerns are slowing the advent of defined standards for connected and autonomous vehicles (CAVs). Researchers investigated the state of CAV implementation to help local agencies begin preparing for the infrastructure needs of these vehicles. CAV-friendly options are considered for eight infrastructure categories. Since truck platooning is the likely first application of this technology, and optical cameras appear imminent as an early iteration of sensing technology, researchers suggest that wider pavement striping and well-maintained, uniform and visible signage may effectively serve the needs of CAVs in the near future while enhancing infrastructure for today’s drivers. 

What Was the Need?

For transportation agencies, which manage infrastructure in time frames of decades, the potential of connected and autonomous vehicle (CAV) technology influences infrastructure upgrade plans. 

New pavements and overlays, traffic signal systems and signs may serve for decades, while pavement markings face shorter life cycles. Optimizing spending today requires anticipating future infrastructure needs, and the infrastructure requirements of CAVs may differ from standards currently in place.

It remains unknown how imminent the CAV future is, and competing technologies and designs for guidance systems, sensor formats and other facets of the developing vehicle technology keep outcomes unsettled. Enthusiasm in the technology and automotive sectors for this new model of road user tools nevertheless suggests that short-term preparations warrant consideration within the current limited-budget environment for infrastructure improvements. 

How local agencies can best brace their roadway systems for a CAV-driven shift in road usage remains unclear, and public transportation officials cannot predict what the technology will look like if and when autonomous vehicles roll onto streets in significant numbers. 

What Was Our Goal?

Researchers sought to create a toolbox for local road agencies to use in preparing for CAVs in the next five to 10 years. Recommendations would help agencies leverage ongoing infrastructure plans and expenditures to prepare for CAVs and the potential technologies for roadway navigation and travel the vehicles will deploy. 

What Did We Do?

Researchers began by studying the literature, attending conferences and consulting with industry experts to describe likely CAV technologies and potential implementation timelines. Based on this research and discussions with the project’s Technical Advisory Panel, investigators developed recommendations in eight categories of infrastructure needs. The research team also prepared seven case studies showing how road agencies have addressed different aspects of preparing for CAV fleets. 

What Did We Learn?

Industry competition and proprietary technologies make CAV outcomes difficult to project, and federal standards and regulations have yet to develop to meet potential forms of the technology. 

“Connected and autonomous vehicles are further away than we think. Full integration of driver assistance technologies—which is where the real power in CAVs is at this time—may be a slow process.”

—Shauna Hallmark, Professor, Iowa State University Department of Civil, Construction and Environmental Engineering 

Some consensus within the CAV industry suggests truck platooning, in which two or more CAV trucks follow one another at distances of 30 to 50 feet, seems the most promising initial implementation of CAV technology within the next five to 10 years. In addition, optical cameras will be a likely early iteration of sensing technology. Accommodating these technologies will impact two infrastructure categories—pavement markings and signage. Recommendations for these infrastructure needs follow: 

  • Pavement Markings. Consider California’s plans to install 6-inch-wide lane lines (the current Minnesota standard is 4 inches) on highways and Interstates during regular maintenance and new construction within three years.
  • Signing. Ensure that signs are standardized, easily visible, and not blocked, damaged or faded. 

The other six infrastructure categories impacted by CAVs entail less-specific recommendations: 

  • Traffic Signals. Create space at signal control cabinets for additional hardware related to CAV technologies.
  • Consistency and Standardization. Install and maintain striping, signing and signals consistent with CAV algorithms and technologies.
  • Pavement Maintenance. Continue to keep road surfaces well-maintained.
  • Data Capture and Information Sharing. Begin or continue collecting and organizing data for bridge heights, speed limits, load restrictions, crosswalks, roadway curvatures and other infrastructure characteristics. 
  • Communication Infrastructure. In new construction and information technology infrastructure built for agency use, ensure adequate conduits for power and fiber optic cables. 
  • High-Resolution Mapping. Consider developing high-resolution mapping capabilities.
U.S. DOT image shows current work zone warning signals that may be adopted in connected and autonomous vehicles.
Intelligent transportation system features like work zone warnings may be incorporated in CAVs.

What’s Next?

Case studies about developments in Los Angeles and in Iowa, Michigan, Ohio, Virginia and Wyoming explain how agencies are preparing for the needs of a CAV-friendly infrastructure.  

“Making sure that signing and striping are visible will be essential for accommodating autonomous cars. It’s also going to be good for all drivers, especially with an aging population.”

—Douglas Fischer, Highway Engineer, Anoka County 

A pilot project in Anoka County, Minnesota, informed decisions about signage to ensure visibility and consistent placement. Pavement markings were also addressed; currently the county continues to place 4-inch edge lines, lane lines and centerlines after resurfacing projects, and painting lines to 10-foot lengths at 40-foot gaps. Conversion to 6-inch markings could be accommodated on existing pavements; however, if a new standard is required for skip stripe spacing, it may only be economically feasible to do so on new surfaces.

This Technical Summary pertains to Report 2019-18, “Preparing Local Agencies for the Future of Connected and Autonomous Vehicles,” published May 2019. Visit the MnDOT research project page for more information.

Bus–Highway Connections Make Transit More Competitive With Driving

Researchers developed a method for associating travel times and travel costs with transit mobility. In an evaluation of bus–highway system interactions, investigators found that park-and-ride lots and managed lanes put suburban and walk-up urban transit options on equal footing. Bus–highway system interactions improve access to job locations and have improved transit access to job sites by about 20 percent compared to automobile access. When wage-related costs are included, the benefit of automobile use over transit use diminishes significantly.

What Was the Need?

Bus service in the Twin Cities relies on MnDOT-built park-and-ride (PNR) lots and managed lanes—lanes for buses on streets and highways, including high-occupancy lanes—to help transit users travel from the suburbs and urban locations to job, retail, service and entertainment sites. 

One measure of how a transit system of PNR lots and bus service works for users is job accessibility—the number of jobs that can be reached by a mode of transportation within a certain travel time period.

The type of lanes a bus uses impacts travel times via bus, and the differences in these travel times in turn impact the transit user’s ability to reach locations using walk-up transit service. The transit alternative to walk-up service is drive-to-transit service via PNR lots. The Twin Cities transit system intersects with over 100 PNR lots where transit users park their vehicles and take express and limited-stop services to business districts and job locations. 

Understanding the impact of managed lanes and PNR lots on transit effectiveness in terms of job access requires diving into transit and travel data; developing ways to measure accessibility for walk-up, drive-to-transit and automobile-only travel modes; and adjusting methods so the cost of travel and the time of travel can be reasonably compared between modes. 

A MnPASS lane on Interstate 394 at the General Mills Boulevard exit. The express lane is closest to the highway median, indicated by a white diamond-shaped marker on the pavement and separated from three other traffic lanes by a solid white line. A highway sign above the lane indicates the fees for lane use.
MnPASS lanes are managed lanes that offer buses quicker access to downtown.

What Was Our Goal?

MnDOT sought to evaluate how the bus and highway systems interact in terms of job accessibility. The research would consider how managed lanes and PNR lots affect job accessibility for walk-up and drive-to-transit users, compare these findings to automobile-only usage, and profile how well the transit system of the Twin Cities serves users in terms of cost to use and travel time. 

What Did We Do?

In the first stage of work, the research team focused on the managed lane network to determine how it contributes to walk-up transit accessibility. Investigators developed a computer program to modify transit schedule data to reflect how buses operate in different managed lane configurations and calculate walk-up access to jobs systemwide. 

In the second stage, the team developed a method for calculating accessibility via PNR use, and PNR accessibility in terms comparable to access via walk-up transit and automobile use. 

In the third stage, researchers developed a mixed-mode accessibility profile of the system. 

“The researchers did more than just measure mobility; they quantified access to employment in terms of travel time and travel cost, as well. Results put park-and-rides and suburban transit on equal footing with walk-up transit in urban environments.”

—Jim Henricksen, Traffic Forecaster, MnDOT Metro Traffic Forecasting and Analysis 

The research team incorporated a monetary dimension to travel time accessibility measures, associating costs of automobile use, parking fees, transit fare and travel time with travel modes in a value of time unit to compare accessibility between automotive and transit usage. 

What Did We Learn?

Study results showed that PNR lots and managed lanes offer greater access to job sites. The longer the trip to a job site, the more competitive transit becomes with driving for commuting to work. Bus–highway interactions via managed lanes and PNR lots improve transit job accessibility relative to automobile use by 3.8 percent in a 30-minute commute and by 19.1 percent in a 60-minute commute. For the 60-minute scenarios, transit accessibility from the suburbs to the central business district improves by 319,322 jobs for the average worker. 

For managed lanes, the greatest benefit is for suburban regions near express routes. On the I-94 corridor, where the greatest improvement by transit to accessibility is felt, every mile of MnPASS lanes offers an increase of 98 jobs accessible to average riders. 

With express bus service, travel times from PNR lots to destinations decrease by an average of 10.7 minutes for the system. Compared to walk-up transit travel, drive-to-transit from suburban areas offers accessibility values roughly three times greater than travel by walk-up transit, in part because time spent driving in suburbs gets users to more transit facilities than the same time spent walking.  

“We developed tools and methodologies, and applied them metrowide to bring new insights to the role of highway operations and planning on access to jobs through transit.”

—Andrew Owen, Director, Accessibility Observatory, University of Minnesota

Researchers found pockets in the Twin Cities where transit and PNR are more competitive with automotive travel per dollar of travel. These areas highlight urban locations where the transit network is the most robust and suburban areas where automobile travel times are long compared to express transit. When researchers applied wage value to time spent traveling, the benefit of driving rather than using PNR lots and transit dropped 89.6 percent. The relative value of transit may increase further if measures account for productivity on transit. 

What’s Next?

This research helps MnDOT plan future PNR and managed lane facilities to maximize benefit to transit services. Value of time models and comparisons offer a way to measure the relative value of transit to automobile use in accessing jobs. 

Future analysis may include long-term fixed costs associated with vehicle ownership and show further improvement in the comparative value of transit services to automobile use. Methods from this study may also be applied to other mixed-mode transit options, like biking, scooters or ride-sharing to transit access points.

This Technical Summary pertains to Report 2019-17, “Accessibility and Behavior Impacts of Bus-Highway System Interactions,” published April 2019. Visit the MnDOT research project page for more information.

New project: Effectiveness of Teenage Driver Support System

The Minnesota Local Road Research Board (LRRB) has funded a follow-up study to determine whether a monitoring system it field tested for new drivers, called the Teen Driver Support System (TDSS), affected teenagers’ long-term driver behavior.

Background

Motor vehicle crashes are the leading cause of teen fatalities. Because of inexperience and risk-seeking propensity, new teenage drivers are more prone to behaviors such as speeding and harsh maneuvers, especially during their first few months of licensure.

In an effort to reduce risky driving among new teenage drivers, in 2011, the LRRB funded a one-year field operational test of a prototype system developed by the University of Minnesota’s ITS Institute, which enabled parents to monitor their child’s driving behavior.

The software ran on a teen’s smart phone, which was mounted to the dashboard and provided instant feedback about risky behavior to the teen and communicated to parents if the behavior continued.

The system didn’t allow incoming or outgoing phone calls (except 911) or texting while driving. It provided visual and auditory warnings about speeding, excessive maneuvers (e.g., hard braking, cornering), and stop sign violations. It also monitored seat belt usage and detected the presence of passengers, two known factors that increase the risk of fatalities among teen drivers. The system could also be programmed to monitor if the teen was driving after the curfew set by parents or required by Minnesota’s graduated license requirements.

In January 2013, the University of Minnesota launched a 300-vehicle, 12-month field operational test in Minnesota to determine the effectiveness of the TDSS in terms of its in-vehicle information and feedback to parents.

Research results indicated an overall safety benefit of TDSS, demonstrating that in-vehicle monitoring and driver alerts, coupled with parental notifications, is a meaningful intervention to reduce the frequency of risky driving behaviors that are correlated with novice teen driver crashes. In particular, the system was shown to be an effective strategy for reducing excessive speeds when used with parental feedback and potentially even without parental involvement.

Project Scope

The TDSS study was cutting-edge at the time. Today, there are many systems in the marketplace which families may seek out to provide added support for their novice teen drivers. However, the long-term effectiveness of these systems is largely unknown. Furthermore, the extent to which the TDSS reduced crashes, injuries, and citations among those who participated in the study is unknown.

This new study will collect information on study participants’ self-reported driving behaviors and driving attitudes, as well collect traffic violation and crash history records from the Minnesota Department of Public Safety.

This study proposes to not only provide a follow-up to the TDSS study to further explore the benefit it may have had on participants, but also determine to what extent families, schools, and other organizations should continue to invest in in-vehicle coaching systems similar to the TDSS. Ultimately, the TDSS is a low-cost system, which, if found to have long-term efficacy beyond what was demonstrated in the original study, could help guide cost-effective implementations to reduce crashes among teen or other driver groups.

Watch for new developments on this project.  Other Minnesota research can be found at MnDOT.gov/research.

Minnesota transportation research blog