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Research, Traffic and Safety

Preparing Roads for Connected and Autonomous Vehicles

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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.

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.

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.
Materials and Construction, Research

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

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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.

Maintenance Operations

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

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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.