All posts by mndotresearch

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.

Concrete Grinding Residue Doesn’t Appear to Negatively Affect Roadside Vegetation and Soil

A new MnDOT research study determined that depositing concrete grinding residue (CGR) slurry at specific rates on roadside vegetation and soil may not cause lasting harm to plant growth and soil quality; however, follow-up research is recommended.

Study results showed that CGR did not appear to hinder vegetation growth or soil quality, but did change soil chemistry. At some roadside areas, the increase in soil pH enhanced plant growth. Results cannot be generalized for all soil types, plant communities, concrete residues or water sources in Minnesota. Access to real-time slurry disposal activity is needed for a thorough investigation.

Study background

Construction crews use diamond grinders to level newly cured concrete with adjacent slabs of older pavement and to smooth new pavement surfaces for improved friction and tire traction. Diamond grinders are fitted with hoses for rinsing grinding burrs with water to keep the burrs clean and prevent overheating. Vacuum lines then collect the residual dust and rinsing fluids, generating a slurry of concrete grinding residue (CGR) that is frequently discarded on roadside slopes and vegetation. 

When slurry dries, it leaves pale gray patches on roadside vegetation and other features, lightening the soil surface for a season or more. The effect of this slurry on vegetation, soil and drainage was unknown. Engineers and researchers presumed that the concrete dust temporarily coats roadside turf and plants, raises the soil pH, clogs soil pores and inhibits water drainage, invites invasive species to take root, and may infiltrate storm drains and waterways. 

What Was Our Goal?

MnDOT needed to study the impact of CGR on roadside vegetation and soil. Research would evaluate sites where residue has been deposited and determine its impact on vegetation and soils common to state roadsides. 

What Did We Do?

A literature review indicated that related research has been limited and that vegetation samples of only one or two species have been examined. Researchers developed two approaches for investigating the impact of CGR on plant density, plant growth and soil properties. 

First, researchers collected CGR slurry from a slurry tank at a Minnesota construction site to replicate residue application at the Kelly Farm, an Iowa State University research site near Ames, Iowa, that features prairie vegetation similar to that found along Minnesota roadsides. They applied slurry at application rates of zero, 10, 20 and 40 dry tons per acre. Plant cover, soil chemistry and soil structure properties, such as plant biomass, density, hydraulic conductivity, infiltration and pH, were measured before the slurry was applied and again at one-, six- and 12-month intervals after application. 

Second, researchers visited two roadside locations along Interstate 90 near Austin, Minnesota, where CGR had been applied. The research team evaluated vegetation content and cover, took soil samples and compared survey results to neighboring roadside environments that had not received CGR slurry.

The infiltrometer system setup at the Kelly Farm site in Ames, Iowa.
This water infiltrometer measured infiltration of water at the roadside environment test site.

What Did We Learn?

Statistical analyses established that at the Kelly Farm, CGR did not significantly impact soil physical properties and plant biomass, but did alter soil chemistry. Levels of soil pH, electrical conductivity, metals content and other properties rose significantly after CGR application. These effects increased with increases in application rate and decreased at increased soil depths. These changes did not reduce soil quality, and higher pH levels did not persist after one month. For certain warm-season grasses and legumes, increased pH improved plant growth. Some nutrients such as calcium and magnesium leached from CGR could benefit plant growth as well.

“Concrete grinding residue or slurry can, under certain conditions, be a benefit. It can act as a liming agent, changing soil pH in a positive manner.” —David Hanson, Integrated Roadside Vegetation Manager, MnDOT Roadside Vegetation Management

The two roadside environments yielded differing results. Slurries had been deposited in 2009 at the first site and in 2013 at the second. At the first site, soil bulk density and hydraulic conductivity in the slurried areas did not differ significantly from measures at the nonslurried areas; at the second site, the levels differed significantly. At both sites, electrical conductivity, calcium content and base saturation values were higher at the areas with CGR than the areas without CGR. 

Researchers concluded that at the Kelly Farm and at the roadside locations, slurry applications at a rate of up to 40 tons per acre did not reduce soil quality and vegetation growth for longer than three years. 

What’s Next?

Efforts to access grinding operations and CGR deposits in real time were not embraced by Minnesota’s concrete industry, and researchers were unable to properly assess residue composition and rates, and volumes of slurry deposition on roadway environments. A thorough investigation of residue impact will require such access and follow-up on site conditions after established periods of time. 

Researchers noted that findings cannot be easily generalized since CGR compositions may vary depending on source and water quality, influencing soil and vegetation differently, and soil and plant communities may differ in response to comparable CGR applications. Investigators recommended that MnDOT develop quick field measures of slurry pH, electrical conductivity and alkalinity to use in adjusting slurry spreading rates at grinding sites.

“This study was a great start to this topic. Follow-up research is recommended to evaluate live projects, field demonstrations and data collection.” —Halil Ceylan, Professor, Iowa State University Department of Civil, Construction and Environmental Engineering

This technical summary pertains to Report 2019-06, “Concrete Grinding Residue: Its Effect on Roadside Vegetation and Soil Properties,” published January 2019. Visit the MnDOT research project page for more information.

Rout-and-Seal Offers Slight Cost–Benefit Over Clean-and-Seal Repairs

In a recently completed study, Minnesota researchers compare the performance and cost-benefit of the clean-and-seal versus rout-and-seal techniques for repairing asphalt pavement cracks.

Survey results, construction data and field evaluation of new repairs and their performance over two years gave rout-and-seal repairs a slight cost–benefit edge over clean-and-seal repairs. At an average performance index level, rout-and-seal offered about four years of service before failure; clean-and-seal offered about three years. The study also recommends rout-and-seal for use over clay and silt subgrades in most conditions. Decision trees were developed to help planners and repair crews select an appropriate repair method.

Background

Preserving asphalt pavements so they maintain performance for decades requires a variety of repairs, including sealing cracks. Cracks allow water to seep into pavement structures, leading to damage from freeze-thaw expansion, stripping of the asphalt’s bond from the underlying structure, potholes and crack expansion.  

For most crack repairs, road crews clean the crack and apply an asphaltic filler or sealant. MnDOT uses two approaches to repair cracks and create a smooth ride for passing vehicles: clean-and-seal and rout-and-seal. Both treatments force traffic closures. 

Clean-and-seal asphalt crack repair begins by using compressed air to clean the crack before sealing it.

With clean-and-seal, compressed air is used to remove debris from the crack before a sealant is applied. With rout-and-seal, a saw or router is used to grind a shallow trench or reservoir over the crack. The routed seam is then filled with an asphaltic sealant. 

After routing a shallow channel over the pavement crack, repair workers fill the crack with asphaltic sealant.

Rout-and-seal requires more time and, in many cases, slightly more sealant, making it more expensive than clean-and-seal. Some agencies favor clean-and-seal because it is less expensive, reduces the time crews are on the road and frees more time to maintain other cracks. 

What Was Our Goal?

Researchers sought to determine which of the two repair methods offers the better value over time. If rout-and-seal delivers a longer-lasting repair, it may be more cost-effective than clean-and-seal in terms of life-cycle cost. The research team also needed to develop guidelines for selecting the most suitable repair method for the damaged pavement. 

What Did We Do?

Researchers conducted a literature search to see how agencies around the country approach asphalt crack repair. The research team then surveyed Minnesota road agencies to see which repair method agencies prefer and how long repairs typically last. 

To review performance of crack sealing, researchers evaluated the MnDOT construction logs of old repair sites and visited 11 new repair sites. These locations were revisited two, six, eight, 12 and 18 months following the repair. To calculate a performance index rating, researchers recorded data about site conditions that included sealant age, traffic level, subgrade soil type and crack sealing performance. Two sites were removed from the analysis when local crews applied chip seals to the pavements.

Investigators calculated performance index levels for each repair method at each site. They gathered cost data where available from bid-letting paperwork and determined life-cycle costs. Finally, the research team created decision trees that planners and maintenance crews can use to help select crack repair methods. 

What Did We Learn?

“This study provided very useful information. The rout-and-seal has a better cost–benefit over the life of the pavement than the clean-and-seal, however, they are relatively close. Agencies will need to decide if they have the manpower or resources to perform one over the other.”

—Dan Knapek, Assistant County Engineer, Sherburne County Public Works

Limited research was identified that compared clean-and-seal and rout-and-seal treatments. Most studies of asphalt crack sealing compared unsealed and sealed pavement performance and have established that sealing does extend pavement life. None compared cost–benefits of the two methods.

Of 47 survey respondents, 68 percent use rout-and-seal and 32 percent use clean-and-seal. Responses identified no clear trends in life expectancies for the two methods, with predictions for service until failure falling predominantly in two to 10 years for clean-and-seal and two to 15 years for rout-and-seal. The most common criteria for choosing a method were crack or pavement condition (46 percent of respondents) and predetermined maintenance schedules (24 percent). 

Analysis of MnDOT construction data found no statistically significant difference in life expectancies for the two methods, with service lives of 6.4 years for rout-and-seal and 6 years for clean-and-seal. A similarly slight advantage for service lives of both treatments was identified for low-volume roads over higher-volume roads. 

After one year of service, the new seal sites delivered strong performance index figures. Short-term performance on rural roads was identical for the two methods. After the severe 2018-2019 winter, however, performance dropped significantly; spalling damage was frequently observed at rout-and-seal sites. 

Analysis of old and new seal projects showed that at an average performance index level, rout-and-seal repairs last about four years and clean-and-seal about three. Life-cycle cost analysis found rout-and-seal slightly more effective. Because the difference is slight, factors such as treatment cost, life expectancy, ease of operation, traffic level and crew manager opinion may guide selection of sealing strategies. 

What’s Next?

Researchers developed two decision trees for selecting a repair method: one for pavement management and another for maintenance crews. Rout-and-seal is recommended for pavements over clay and silt subgrades. 

Research that extends monitoring of the new crack seal sites for up to five years would provide useful data on performance and comparison of the effectiveness of the two methods.

“To help select an appropriate crack repair method, we developed two decision trees: a detailed one and a simple one with only three variables—crack size, traffic level and the number of times a crack has been sealed.” 

—Manik Barman, Assistant Professor, University of Minnesota Duluth Department of Civil Engineering

This Technical Summary pertains to Report 2019-26, “Cost/Benefit Analysis of the Effectiveness of Crack Sealing Techniques,” published June 2019. Visit the MnDOT research project page for more information.

MnDOT’s Smart Bridge Sensors Are Leveraged to Measure Vertical Displacement

A Minnesota Department of Transportation research study has developed a new method for estimating vertical displacements on bridges using accelerometers installed on the Interstate 35W St. Anthony Falls Bridge in Minneapolis. The dual-model approach shows potential for using these sensors to measure vertical displacement on steel, cable-stayed and other less-stiff bridges where traffic generates higher vibration frequencies. The method expands the industry’s knowledge of how to use smart sensors in new ways.

What Was the Need?

Since September 2008, the I-35W St. Anthony Falls Bridge has carried traffic over the Mississippi River in Minneapolis and funneled sensor data to researchers and MnDOT bridge engineers. This smart bridge features over 500 sensors that monitor strain, load distribution, temperature, bridge movement, and other forces and functions.

Sensors help designers and bridge managers learn more about how bridges shift and flex over time. Concrete expands and contracts, and bearings shift; sensor systems continuously gather data about these minute changes, offering an alternative to time-consuming inspection.

Sensors attached to a steel beam to study vibrations in a laboratory.
Sensors attached to a steel beam to study vibrations in a laboratory.

Researchers continue to identify potential uses for sensor data and new ways to use such information to analyze bridge properties and performance. In a 2017 study about monitoring bridge health, researchers learned to distinguish and associate specific vibration frequencies with structural damage, weather conditions and other factors. These frequencies were gathered by accelerometers, which measure structural vibrations triggered by traffic and environmental conditions.

Decks, piers and other structural elements displace vertically under loads and environmental conditions. Researchers and bridge managers wanted to know if accelerometers could be used to measure vertical displacements and help monitor bridge health.

What Was Our Goal?

MnDOT needed a procedure for measuring and monitoring vertical displacement on bridges under traffic and environmental forces. Investigators would use the sensor systems on the I-35W St. Anthony Falls Bridge to design and analyze this procedure.

“We need to learn more about sensors because we don’t have a lot of experience with them. This study gave us valuable information about accelerometers and the information they provide,” said Benjamin Jilk, Complex Analysis and Modeling Design Leader, MnDOT Bridge Office.

What Did We Do?

Indirect analysis and measurement of vertical displacements rely on estimations obtained through modeling. Investigators evaluated the most well-developed approach for measuring vibration frequencies like those tracked by accelerometers and refined the method. The team developed a dual-model approach: One model estimates loads and the other estimates displacements.

In a laboratory, investigators evaluated the impact of loading on displacement and vibration frequencies on a girder with contact sensors and accelerometers under moving and stationary loads. Researchers applied the dual-model analysis to laboratory displacement readings to compare the effectiveness of the model with contact sensor responses to loading.

Using laboratory data, investigators tuned the dual-model approach to accelerometer data available from the I-35W St. Anthony Falls Bridge. The research team then applied its identified tuning approach to the data from the bridge’s 26 accelerometers to determine the procedure’s suitability for estimating vertical displacement from vibration response on this bridge and its potential for other structures in the MnDOT bridge system.

New Project: Extreme Flood Risks to Minnesota Bridges and Culverts

Extreme flooding is a threat to Minnesota’s transportation infrastructure and the safety and economic vitality of its communities. A spate of recent flooding events around the state has demonstrated this and heightened the level of concern. Furthermore, climate change — a factor not traditionally accounted for in the design of the state’s infrastructure — is projected to enhance precipitation and the threat of flooding in coming decades.

Given this, MnDOT is undertaking an effort to better predict the threat flooding poses to its bridges, large culverts and pipes, which may be increasingly called upon to convey higher, more frequent flood flows than they were designed for.

The state transportation research program recently launched a two-year extreme flood vulnerability analysis study, which will develop a methodology for characterizing the vulnerability of the state’s bridges, large culverts, and pipes to flooding.

The effort builds upon the previously completed Flash Flood Vulnerability and Adaptation Assessment Pilot Project (2014), which scored bridges, large culverts, and pipes in MnDOT Districts 1 and 6 for flood vulnerability, allowing detailed assessments of adaptation options for each of their facilities to be prioritized.

This new study, which will be conducted by WSP, aims to develop and test ways to enhance the vulnerability scoring techniques used in the previous study and ensure their applicability throughout the state. Researchers will not actually undertake the statewide assessment, but specify an approach that could be used for it. They will also explore how the outputs of the analysis can be incorporated into MnDOT’s asset management systems. The results of this work will be a clear path forward for MnDOT to use for prioritizing adaptation actions — a key step towards enhancing agency resilience and maintaining good fiscal stewardship.

Project scope

The primary intent of this study is to develop a methodology for characterizing the flood vulnerability of bridges, large culverts, and pipes statewide. As part of the development process, the methodology will be tested on a limited, but diverse, set of assets across the state. Following a successful proof of concept, recommendations will be made on how the outputs (i.e., the vulnerability scores) can be incorporated into the state’s asset management systems.

By determining which facilities are most vulnerable to flooding through the techniques developed on this project, MnDOT can prioritize where adaptation measures will make the biggest impact, ultimately decreasing asset life-cycle and road user costs. Without the development of assessment techniques, adaptation measures run the risk of being implemented in a more reactive and/or ad-hoc fashion, with less regard to where the biggest “bang for the buck” can be realized.

This project will produce several technical memorandums, and is expected to be completed in early 2021.