MnDOT is working on ways to reduce crashes at intersections by making stop signs and stop lights more visible to motorists. The agency will apply reflective red metal strips on nearly 1,000 stop sign posts and fluorescent yellow tape around 100 traffic signal lights across the state this summer.
“We think these two low-cost safety countermeasures will help reduce crashes at these higher risk intersections,” said Derek Leuer, traffic safety engineer.
The stop sign project will be implemented on locally owned roads that intersect with two-lane, two-way state highways. The highways chosen are considered moderate- and high-risk crash corridors.
The reflective red strips will be installed on the stop sign post directly beneath the stop sign.
Rural intersection crashes are a serious issue in Minnesota, according to Leuer. From 2008 to 2012, there were 533 serious and fatal injury crashes at rural state highway intersections.
“This project aims to reduce those fatal and serious injury crashes in the state by making the stop signs easier to see,” he said. “Fatal right-angle crashes often are the result of one or more drivers failing to comply with a stop sign.”
The traffic signal project includes installing fluorescent yellow tape around the rectangular back plate that contains the green, red and yellow traffic signal bulbs. Leuer said this is a proven Federal Highway Administration safety countermeasure already used by other states.
“The reflective tape will make the signals look bigger and help motorists be more aware of them,” Leuer said. “This will be especially helpful at night and in low-visibility conditions.”
The florescent yellow tape will go on signals at intersections that are considered higher risk for crashes and may have a record of past crashes.
Cost of both projects is about $500,000.
MnDOT will evaluate both projects for effectiveness on an ongoing basis over the next three years.
“The installation of red reflective strips to stop sign posts and yellow fluorescent tape around signal lights may become another low-cost tool to help MnDOT improve roadway safety and move Minnesota toward zero deaths,” Leuer said.
This article by Sue Roe originally appeared in the June 13, 2018 MnDOT Newsline.
Lane-departure crashes on curves make up a significant portion of fatal crashes on rural Minnesota roads. To improve safety, solutions are needed to help drivers identify upcoming curves and inform them of a safe speed for navigating the curve.
“Traditionally there are two ways to do this: with either static signage or with dynamic warning signs,” says Brian Davis, a research fellow in the U of M’s Department of Mechanical Engineering. “However, while signing curves can help, static signage is often disregarded by drivers, and it is not required for roads with low average daily traffic. Dynamic speed signs are very costly, which can be difficult to justify, especially for rural roads with low traffic volumes.”
In a recent project led by Davis on behalf of MnDOT and the Minnesota Local Road Research Board, researchers developed a method of achieving dynamic curve warnings while avoiding costly infrastructure-based solutions. To do so, they used in-vehicle technology to display dynamic curve-speed warnings to the driver based on the driver’s real-time behavior and position relative to the curve. The system uses a smartphone app located in the vehicle to provide the driver with visual and auditory warnings when approaching a potentially hazardous curve at an unsafe speed.
“Highway curves [make up] 19 percent of the total mileage of the paved St. Louis County highway system, yet these curves account for 47 percent of all severe road departure crashes,” says Victor Lund, traffic engineer with St. Louis County. “In-vehicle warnings will be a critical strategy to reduce these crashes.”
To begin their study, researchers designed and tested prototype visual and auditory warning designs to ensure they were non-distracting and effective. This portion of the study included decisions about the best way to visually display the warnings and how and when audio messages should be used. “To create the optimal user experience, we looked at everything from how to order the audio information and when the message should play to the best length for the warning message,” says Nichole Morris, director of the U’s HumanFIRST Lab and co-investigator of the study.
Next, a controlled field test was conducted to determine whether the system helped reduce curve speeds, pinpoint the best timing for the warnings in relation to the curves, and gather user feedback about the system’s usefulness and trustworthiness. The study was conducted with 24 drivers using the test track at the Minnesota Highway Safety and Research Center in St. Cloud, Minnesota. The selected course allowed drivers to get up to highway speeds and then travel through curves of different radii, enabling researchers to learn how sensitive drivers are to the position of the warnings.
Based on the study results, the system shows both feasibility and promise. “Our in-vehicle dynamic curve warning system was well-liked and trusted by the participants,” Davis says. “We saw an 8 to 10 percent decrease in curve speed when participants were using the system.”
The project was funded by MnDOT and the Minnesota Local Road Research Board.
An update to BullConverter allows MnDOT’s statewide weigh-in-motion (WIM) system to adopt systems from more manufacturers. The BullReporter upgrade adds new reporting functions, including a View Vehicles function that provides an image of a vehicle along with a graphical representation of WIM data, such as weight and speed.
This upgrade, developed through a research study, expands the commercial traffic information that the Office of Traffic System Management can provide to the MnDOT Office of Bridges and Structures, local and state permitting agencies, the Minnesota State Patrol and other Minnesota authorities.
“With BullReporter, now we can produce daily, weekly and monthly reports of the overweight vehicles that cross over WIM sensors,” Benjamin Timerson, Transportation Data and Analysis Program Manager, MnDOT Office of Transportation System Management.
What Was the Need?
Weigh-in-motion (WIM) systems measure characteristics of individual vehicles on the road, generating records of data that include vehicle type, speed, axle weights and spacing. When a vehicle crosses WIM sensors in the pavement, it triggers electrical signals that are transmitted to a WIM controller, which converts the signals into usable WIM vehicle data. A number of manufacturers produce WIM sensors and controllers, and each vendor employs its own methods of processing signals and producing proprietary WIM data.
In 2009, MnDOT began using BullConverter/BullReporter (BC/BR) software with heterogeneous WIM systems. BC converts incompatible, proprietary data into a uniform comma-separated values (CSV) format. BR generates reports from the converted CSV data, allowing the analysis of WIM data over different systems.
Currently, MnDOT’s Office of Transportation System Management (OTSM) uses WIM systems from International Road Dynamics (IRD), but recently began evaluating systems from Kistler and Intercomp. In a current study, investigators are evaluating the use of Intercomp WIM controllers with Intercomp sensors, IRD controllers with Kistler sensors, and Kistler controllers with Kistler sensors. These new WIM system combinations require new conversion functions in BC.
What Was Our Goal?
The goal of this project was to upgrade the BC/BR software package by improving existing functions and incorporating new functions that will convert Intercomp and Kistler formats to the Bull-CSV format and refine export functions in BC. MnDOT also wanted to expand data reporting capabilities and analytical options in BR, including a View Vehicles capability for analyzing individual vehicles.
What Did We Implement?
MnDOT funded enhancements to the BC/BR software package to include Kistler and Intercomp formats and develop new data retrieval, statistical assessments and report generation applications, including View Vehicles.
How Did We Do It?
MnDOT provided the original BC/BR developer with a detailed list of enhancements and new conversion and reporting functions. The team developed a new WIM data downloading tool for Kistler controllers that would connect the controllers through the Internet and download and archive the raw data. Developers added two new conversion functions in BC to support conversion from Kistler and Intercomp formatted data to CSV-formatted data. The team also updated the export function in BC.
The software team then added View Vehicles report, a new reporting function, to BR. View Vehicles allows queries of vehicle records under any combination of parameters, including lane numbers, date and hour ranges, class numbers, gross vehicle weight (GVW), speed range, axle weight ranges and warning flags. Retrieved vehicle data are then displayed in web or PDF formats with a digital photo of the vehicle and graphics of selected WIM parameters.
The team added histogram functions for GVW and equivalent single-axle load (ESAL), which would retrieve a set of vehicle data based on user-selected parameters and then plot a graph or produce a spreadsheet. Developers enhanced a few other elements of BC/BR, wrote a manual for editing classification schemes and trained OTSM staff on the editing procedures.
What Was the Impact?
Deploying the updated BC/BR software package has significantly helped MnDOT and other state agencies. OTSM now can produce many different reports with a range of user-selectable data queries that can be customized to share with the MnDOT Office of Bridges and Structures, the Minnesota State Patrol and overweight permitting offices.
Expanded GVW and ESAL data generated with the updated software can be used in evaluating designs for new bridge construction. Permitting offices can draw upon BR reports to request changed axle configurations of overweight vehicles to prevent bridge damage. OTSM can also provide reports and vehicle images for compliance activities to the MnDOT Bridge Office, permitting offices and the State Patrol.
In addition, the updated BC/BR can provide data on traffic volume and vehicle class to the Office of Traffic Safety and Technology, can inform design decisions by the Office of Materials and Road Research, and can offer a wide range of useful information to the Office of Freight and Commercial Vehicle Operations.
“This software allows us to use different WIM systems and generate reports and analysis by integrating incompatible systems. We added more capabilities in BullConverter and increased BullReporter functions from 40 to more than 60,” Taek Kwon, Professor, University of Minnesota Duluth Department of Electrical Engineering.
BC and BR are now fully updated for current needs and are in use by OTSM. The upgraded software will be used until industry changes or new analytical needs arise at MnDOT.
Roadways in Minnesota’s Red River watershed are prone to flooding and overtopping, where wide flows of water wash across the surface of the roadway. Repairing the resulting damage to roadway embankments can be costly and time-consuming, requiring lengthy road closures. Protecting roads from destructive scour could significantly reduce the cost and time of repairs after a flood event. Researchers investigated three “soft” design methods using full-scale models and field monitoring, with flexible geogrid mat providing the best erosion protection. Regardless of protection technique, any physical separation from the soil beneath led to failure by creating a pathway for water to follow. Establishing root growth and vegetation would improve the performance of all techniques by anchoring the soil.
“This project developed a fairly complete matrix of useful erosion protection measures that our own staff can implement—techniques that are less elaborate and more cost-effective than hiring contractors,” said J.T. Anderson, Assistant District Engineer, MnDOT District 2.
“This project was a combination of basic and applied science, and is a great example of the university and MnDOT working together successfully to solve problems unique to our geography and climate,” said Jeff Marr, Associate Director, Engineering and Facilities, University of Minnesota St. Anthony Falls Laboratory.
What Was the Need?
Roadways in the Red River watershed are prone to flooding and overtopping, where wide flows of water wash across the surface of the roadway. Downstream scour and erosion of roadway embankments can result in breach or washout of the entire roadway. Repairing the damage caused by flooding and overtopping can be costly and time-consuming, requiring lengthy road closures. Frequent flood events in recent years reinforce the need to protect roadways where flooding is likely to occur.
Raising the roadway to prevent overtopping is not a feasible solution, as flood plain law does not allow moving the problem elsewhere by backing up the water. The most cost-effective option is to allow floodwaters to overtop roadways and to try to protect their embankments from scour. Protecting roads from destructive scour and erosion by developing cost-effective scour prevention measures could greatly reduce the cost of repairs, as well as the time required to reopen the roadway after a flood event.
What Was Our Goal?
The goal of this project was to investigate the effectiveness of slope protection techniques to shield overtopped roadways and their downstream embankments from scour and erosion. A further goal was to use cost-effective methods that could be installed by local agencies instead of contractors. The researchers evaluated several “soft” design methods using an integrated approach of full-scale models and field monitoring.
What Did We Do?
Using the findings from a literature review, the research team developed a field-based program to collect data on the hydraulics associated with full-scale overtopping events. Researchers recorded flood stage at several locations near the Red River during over-topping events and evaluated the failure modes under natural conditions. Annual field monitoring occurred from 2013 through 2016 during overtopping events.
Next, the research team conducted a series of experiments at a full-scale laboratory facility to study the hydraulic and erosional processes associated with overtopping. The facility simulated a transverse section of a roadway and included an upstream water supply, road crest, shoulder and downstream embankment slope.
Two slopes were examined in the lab: 4:1 (horizontal:vertical) and 6:1. With bare soil used as a control, three erosion protection techniques were investigated: armored sod hydraulic soil stabilization, turf reinforcement mat (Enkamat) and flexible concrete geogrid mat (Flexamat). All three are alternatives to riprap and other hardscapes, and encourage vegetation to grow through a mat, helping to stabilize the soil and protect the embankment from scour and erosion.
What Did We Learn?
The researchers were able to draw some definitive conclusions from the laboratory experiments:
Bare soil with no vegetative cover (the control) is highly susceptible to erosion and is the worst-case scenario. New installations should have established vegetation before the first overtopping event is expected.
All three mitigation techniques reduced erosion, but the flexible concrete geogrid mat provided the best protection. Researchers noted that these results describe overtopping that occurred immediately after the protection treatments were installed. Established vegetation and root growth would likely improve the performance of all techniques.
Initiation of erosion appears to be linked to small-scale inconsistencies in the soil, erosion control material and placement of the protection technique. Small failures can quickly develop into mass failure of the embankment.
Failure occurred in areas where the protection technique physically separated from the surface of the soil and exposed a direct pathway for the water to flow. Inflexible protection techniques were the poorest performers.
Common locations for failure were the toe of the slope and the upstream transition from the shoulder to the soil slope, with steeper slopes failing most often.
No mature vegetation existed on the embankment slope in the laboratory flume, which mimics the post-construction period in the field. Full vegetation is more typical for much of an embankment’s life cycle. Since one of the most important functions of vegetation on a slope is the ability of its roots to anchor soil, further study of these techniques with mature vegetation could provide a better understanding of their effects.
Future studies should include other stabilization techniques as well as the effects of overtopping on frozen and thawing soils, through-embankment seepage or piping, and various soil types on performance of the stabilization technique. Future projects could also evaluate the use of multiple techniques along with the study of anchoring improvements and longevity of the erosion control products.
“This project will find out the behavior of the DMS and RICWS under AASHTO defined design loads and develop the retrofitting system to avoid the experienced problems that will improve the public safety, reduce the maintenance cost and minimize impact to the traffic,” Lin said.
RICWS have exhibited excessive swaying under wind loads, leading to safety concerns regarding failure of the support structure at the base. It is believed the heavy weight of these signs has brought the frequency range of these systems too close to that of the wind excitations. There is a need to investigate the wind-induced dynamic effects on these sign structures and to propose modifications to the systems to reduce the likelihood of failure. There is also interest in investigating the dynamic behavior of the DMS, particularly the loads on the friction connection.
This research project involves a field investigation to determine the structural performance of these two types of sign structures. Laboratory tests using a towing tank facility and a wind tunnel will be performed on scaled models and opportunely modified models to improve performance and minimize unsteady loads.
The outcome of this project is expected to develop an understanding of the RICWS and DMS sign structures and to provide modifications to improve the structural performance of the RICWS sign structures while maintaining the crashworthy requirements. The results will help to ensure the uninterrupted service of these sign structures, which are important to public safety.
Task 1A: Development of Field Instrumentation Plan and Instrumentation Purchase
Task 1B: Experimental Determination of Load Effects and Dynamic Characteristics of Post Mounted DMS in Field
Task 2A: Development of Numerical Models to Investigate Post Mounted DMS Sign Demands and Fatigue
Task 2B: Validation of Numerical Models to Investigate Post Mounted DMS Sign Demands and Fatigue
Task 3A: Investigation of Design Loads and Relevant Fatigue Considerations for DMS
Task 3B: Analysis of Design Loads and Anticipated Fatigue Life of DMS
Task 4: Experimental Determination of Dynamic Characteristics of RICWS in Field
Task 5: Development and Validation of Numerical Models to Investigate RICWS Signs
Task 6: Numerical and Experimental Investigation of Drag and Vortex Shedding Characteristics of RICWS Signs Using Scaled Models
Task 7: Numerical and Small-Scale Experimental Investigation of Modifications to RICWS Sign Panel to Reduce Effects of Vortex Shedding
Task 8: Numerical and Analytical Investigation of Noncommercial Means to Damp Motion of RICWS Blankout Sign Structure
Task 9A: Research Benefits and Implementation Steps Initial Memorandum
Task 9B: Research Benefits and Develop Implementation Steps
Task 10: Compile Report, Technical Advisory Panel Review and Revisions
Task 11: Editorial Review and Publication of Final Report
The project is scheduled to be completed in March 2019.
Researchers studied driving behavior at four multilane roundabouts to better understand the relationship between traffic control designs and driver errors. Data collected showed that certain traffic control changes decreased turn violations but failed to eliminate yield violations. Researchers were unable to identify long-term solutions for improving roundabout design and signage, and recommended further research to improve the overall safety and mobility of multilane roundabouts.
“Even though the study did not provide a silver bullet on how to prevent crashes at multilane roundabouts, it did create an efficient tool to analyze and quantify driving behavior data,” said Joe Gustafson, Traffic Engineer, Washington County Public Works.
“This study has advanced our knowledge in multilane roundabout safety and is one step closer to providing much needed improvements to roundabout design guidance,” said John Hourdos, Director, Minnesota Traffic Observatory, University of Minnesota.
What Was the Need?
Roundabouts have been shown to improve overall in-tersection safety compared to traditional traffic signals. However, noninjury crashes are sometimes more frequent on multilane roundabouts than on single-lane roundabouts due in part to driver confusion. Common driver errors such as failing to yield and turning violations on multilane roundabouts have contributed to an increase in noninjury crashes.
Given the benefits of improved mobility, traffic throughput and injury reduction of multilane roundabouts, reducing the noninjury crash rate at multilane roundabouts is important to facilitating their use by Minnesota cities and counties. Identifying solutions to reduce common driving violations would be more sustainable than the current practice of converting multilane roundabouts back to single-lane roundabouts.
In a previous study on a two-lane roundabout in Richfield, Minnesota, researchers demonstrated that traffic control changes could reduce some of these driver errors. However, more data was needed to support study results. Understanding driver behavior and improving traffic control devices are key factors in designing safer multilane roundabouts.
What Was Our Goal?
With limited research on modern multilane roundabouts, the Minnesota Traffic Observatory sought to collect more data to evaluate the correlation between traffic control design features and collisions. Instead of conducting manual observations, researchers used an innovative video analysis tool to collect and process recorded videos of driving behaviors at test sites. Based on the analysis, they attempted to identify driver behaviors and error rates to help reduce noninjury crashes at multilane roundabouts.
What Did We Do?
The research team selected four multilane roundabouts in Minnesota — two in Mankato, one in Lakeville and one in St. Cloud — to observe undesirable driving maneuvers. At each roundabout site, researchers mounted video cameras at key locations to record one to two weeks of driving behavior. Only one roundabout could be observed at a time because only one set of specialized video equipment was available.
The raw videos were processed to produce a data set for analysis. Researchers used TrafficIntelligence, an open-source computer vision program, to automate extraction of vehicle trajectories from the raw footages. They used the same software to correct any errors to improve data reliability. The resulting clean data from the recorded videos were supplemented with historical crash frequency data reports obtained from the Minnesota Department of Public Safety. Collectively, data from both sources allowed researchers to thoroughly investigate the frequency and crash types from the four roundabouts. A statistical analysis of the data revealed that turn violations and yield violations were among the most common driving errors.
Researchers also looked at how violation rates vary with the roundabout’s location and relevant design features. Based on these findings, researchers proposed signage and striping changes to reduce driver errors at the two Mankato test sites. After the changes were implemented, they collected additional video data.
What Did We Learn?
This study provided one of the most comprehensive analyses to date of driving behavior at multilane roundabouts. Researchers were successful in finding solutions for reducing turn violations, but they were unable to identify solutions for yield violations despite the vast amount of data.
Minor differences in the design at each roundabout presented specific challenges. The analysis focused on how each varying design feature impacted driving behavior. Proposed traffic control changes such as extending solid lines between entrance lanes, adjusting the position of yield signs and adding one-way signs successfully decreased turn violations. However, data from before and after traffic control changes showed an insignificant impact on decreasing yield violations. Importantly, the study produced a list of ineffective traffic control methods that can be eliminated from future studies, saving engineers time and money.
The TrafficIntelligence tool was crucial in efficiently processing and cleaning large amounts of raw video. With improvements made to the software program, the tool should be an asset to future research on roundabouts and to other studies requiring observations of driving behavior.
The traffic control changes that were successful at reducing crashes at two-lane roundabouts should be implemented by traffic engineers. In particular, large overhead directional signs or extended solid lines between entrance lanes should be installed to help reduce turning violations. The analysis method used in this study could also be used for a robust before-and-after evaluation of future modifications to traffic control devices.
Additional research could further scrutinize the data already collected, and researchers recommend that more data be collected to examine additional traffic control methods and other intersection design elements to improve the overall safety and mobility of two-lane roundabouts. This research could also serve as an impetus for the study of numerous roundabouts in a pooled fund effort involving several states.
Under simulated conditions, drivers were not distracted by controlled work zone-related messages delivered through smartphones. In fact, driving performance improved. Researchers also learned that the location of the smartphone did not affect the driver if the message included an auditory component.
“The main goal was to determine whether in-vehicle warnings conveyed through smartphones would be distracting to the driver. We found that wasn’t the case,” said Ken Johnson, Work Zone, Pavement Marking and Traffic Devices Engineer, MnDOT Office of Traffic, Safety and Technology.
“We learned that drivers had a lower mental workload when they experienced the in-vehicle messages. It really didn’t matter what modality we used. Half the messages were auditory only, and half were auditory paired with visual,” said Nichole Morris, Director, University of Minnesota HumanFIRST Laboratory.
What Was the Need?
Highway work zones require drivers to reduce speed and be aware of work crews, lane closures, traffic backups, construction equipment and other potential hazards on the roadway.
Transportation departments have long employed stationary warning signs, sometimes supplemented by portable changeable message signs (PCMSs), to alert drivers to upcoming construction projects. However, some previous studies have indicated that stationary warning signs are not always effective. In addition, PCMSs are costly and may be difficult to deploy in the field.
Smartphone technology offers an opportunity to deliver accurate and early in-vehicle warnings about road construction miles ahead. Digital messages could alert drivers about upcoming work zone conditions and improve safety for drivers and workers in the field.
But receiving in-vehicle messages about work zone conditions could distract drivers from safely operating their vehicles. MnDOT needed to study the advantages and disadvantages of using smart-phones to deliver in-vehicle work zone messages.
What Was Our Goal?
The primary goal of this project was to determine whether smartphones have the potential to safely deliver effective and accurate messages to drivers about upcoming road construction on Minnesota highways.
What Did We Do?
The research team developed and conducted an online survey that focused on Minnesota drivers’ perceptions of work zone safety and on their attitudes toward using smartphones and potentially receiving in-vehicle messages regarding work zone conditions.
Data from the surveys was used by the HumanFIRST Laboratory at the University of Minnesota to develop a driving simulation study designed to determine whether in-vehicle messages sent by smartphones could promote safe driving in work zones. The study analyzed 48 drivers operating a driving simulator within two work zones to test reactions to in-vehicle messages as compared to messages displayed on an external PCMS system. Researchers collected data about each participant’s visual attention, driving performance, mental workload and opinions on smartphone technology.
Researchers also reviewed previous national studies and published works to identify environmental and driver behavior risk factors related to work zones.
What Did We Learn?
An analysis of the simulation results showed drivers were very responsive to receiving in-vehicle messages regarding work zones and roadway hazards. Messages presented through smartphones did not cause driver distractions. In fact, some drivers’ performance actually improved following delivery of audiovisual messages.
Drivers preferred to receive audio messages, and researchers learned that a synthesized female voice (like Apple’s Siri) resulted in greater awareness and acceptance from the driver than a more natural or prerecorded voice.
Survey findings showed that only 5 percent of participants use a dashboard mount for their smartphones, while the vast majority keep their phone in the cup holder, on the console, in a backpack or purse, or on the passenger seat. A few participants said they hold their smartphone while driving. Investigating the safety impact of this behavior paired with an in-vehicle messaging system, researchers found that the location of the smartphone within the simulator (on the dash or passenger seat) did not negatively impact driver safety or performance, providing the work zone message contained the auditory component.
In-vehicle messages required less cognitive effort from drivers, and drivers had greater recall of the hazard warning message versus stationary PCMS signage.
A significant number of survey participants, nearly 20 percent, provided unprompted feedback that it was the state’s responsibility to provide factual work zone messaging information and to ensure in-vehicle technology employed does not pose a distraction.
MnDOT will need to continue research into the viability of smartphones as the way to deliver in-vehicle work zone messages. The simulation study provided the findings needed to advance the project to field testing, where drivers would respond to in-vehicle messages from smartphones on a test track or under real roadway conditions. Another potential topic to explore through further research is the viability of messages delivered through electronic interface or dashboard features offered on some newer vehicles.
MnDOT should identify the medium needed to deliver in-vehicle messages and use the prescribed syntax outlined by the study for communicating messages. Researchers noted the existing 511 service provided by MnDOT currently provides road, traffic, weather and other information. A study should be undertaken to determine whether the 511 or a third-party app would be most appropriate for a future statewide in-vehicle messaging program.
An analysis of crash data revealed that congestion-related improvements implemented on I-35W in the Twin Cities did not introduce additional safety risks. When installed strategically, improvements like priced dynamic shoulder lanes can alleviate congestion and improve safety for motorists.
“Rather than just conducting a before-and-after analysis of crashes, we also wanted to compare the expected crash rate based on changes in traffic conditions,” said Brian Kary, Freeway Operations Engineer, MnDOT Metro District.
“Probably the most significant finding was that rear-end crash risk shows an inverted U-shaped relation to lane occupancy,” Gary Davis, Professor, University of Minnesota Department of Civil, Environmental and Geo-Engineering.
Davis served as the study’s principal investigator, and Kary was the technical liaison.
What Was the Need?
The Urban Partnership Agreement (UPA) is a federally funded program managed by the Federal Highway Administration to explore ways to reduce congestion on urban freeways. The Twin Cities area was one of four urban areas selected to test several innovative technologies through the UPA. These included high-occupancy toll (HOT) lanes, engineered revisions to ramps and auxiliary lanes, and a priced dynamic shoulder lane (PDSL) system on segments of the Interstate 35 West (I-35W) corridor. Work on implementing these innovations in the Twin Cities ran from spring 2009 through fall 2010.
MnDOT may decide to incorporate selected innovations, including the conversion of bus-only shoulder lanes to PDSLs, in other corridors. Decision-makers needed to better understand the safety-related benefits associated with the UPA improvements.
What Was Our Goal?
The goal of this project was to compare the incidence of crashes occurring on I-35W before and after implementation of the UPA improvements. Researchers wanted to determine whether any increase in crashes was due to the installation of the PDSLs or to other changes in the transportation network.
What Did We Do?
Researchers started by compiling data files on variables such as traffic volume and lane occupancy, weather conditions, and the presence or absence of UPA improvements for the relevant portions of I-35W. A second set of data was prepared using the Minnesota Crash Mapping Analysis Tool (MnCMAT) to identify crashes that took place on I-35W from 2006 to 2008 and from 2011 to 2013, the three years before and after the UPA project.
Investigators established three regions — HOT, Crosstown and PDSL — and divided each region into sections so that traffic demand and lane geometry would be constant within a section.
The data files were analyzed to determine the likelihood of a rear-end crash based upon the time of day, traffic volume, weather and other conditions.
What Did We Learn?
The analysis indicated that the increase in crashes on the most analyzed sections of I-35W was not likely the result of installation of PDSLs and other UPA improvements. A noted increase in crash rates was instead tied to reconstruction work that removed a bottleneck in the Crosstown Commons area, where I-35W shared right of way with Trunk Highway 62 (TH 62). There were some exceptions, however. Fewer crashes occurred on a section of the freeway south of I-494 during both study periods. An increase in rear-end crash risk north of the Minnesota River was due to weather and traffic conditions. In addition, researchers identified an inverted U-shaped relationship between lane occupancy and crash risk along several sections of the I-35W study area.
The findings supported the contention that PDSLs, when installed strategically, are safe and can provide transportation departments with an additional resource for managing congestion and improving traffic conditions along the Twin Cities freeway network.
Installation of PDSLs in the corridor did decrease the bottleneck at TH 62, but the improvement literally moved the problem down the road by creating a new bottleneck close to downtown Minneapolis.
From the MnCMAT database, the research team found 5,545 records of various types of crashes that took place from the beginning of I-35W to the I-35W/I-94 junction during the two three-year study periods. Rear-end crashes were by far the most prevalent type of crash, with 1,513 recorded before the UPA improvements and 1,657 during the three subsequent years.
Researchers encountered some challenges in preparing the data files for analysis. Careful screening of loop detector data was needed to identify questionable statistics and required a review of individual crash reports to verify crash locations.
Through this research, MnDOT gained valuable insights into the impact of the UPA improvements on crash incidents along areas studied on the I-35W corridor. The methodology employed supports using PDSLs on other sections of the freeway network.
MnDOT is exploring different software options for developing a “mesoscopic dynamic traffic model” that can more accurately predict road construction impacts than current macroscopic models like the Twin Cities Regional Travel Demand Forecasting Model.
“Dynamic traffic assignment is an emerging model type, and there are a lot of software platforms with different methodologies. MnDOT was interested in reviewing their pros
and cons,” said Jim Henricksen, Traffic Forecaster, MnDOT Metro District, who helped lead a recent research project that analyzed different software packages.
“A team maintains the Twin Cities Regional Travel Demand Forecasting Model. Any mesoscopic model would require a similar maintenance effort to keep the model from becoming obsolete as construction adds new lanes,” said John Hourdos, Director, Minnesota Traffic Observatory, University of Minnesota, and principal investigator for the study.
What Was the Need?
Traffic modeling is a valuable tool used in transportation planning to predict the impacts of new construction or maintenance projects. MnDOT currently has modeling tools available in two scales: macroscopic and microscopic. Macroscopic-scale planning level tools such as the Twin Cities Regional Travel Demand Forecasting Model predict driver route choice and the number of drivers that will travel on a given road at a given time. Microscopic-scale traffic simulation, on the other hand, models driver behaviors such as gap acceptance or acceleration rates. MnDOT uses microscopic-scale simulation to plan capacity-increasing projects, but the tool is only feasible on the corridor level because generating the simulation requires a large amount of data and computing power.
To bridge these two scales, MnDOT is developing a mesoscopic-scale dynamic traffic assignment (DTA) model for the Twin Cities. This model falls between microscopic- and macroscopic-scale modeling in scope and complexity. It simulates the movement of individual vehicles based on traffic flow equations rather than driving rules, which requires less detail and computing time than a microscopic simulation and can be used over a wider area. MnDOT will use this model for applications such as staging construction seasons to minimize the disruption caused by multiple large projects, or coordinating traffic modeling across the road networks operated by MnDOT, counties and cities.
To assist in developing this system, MnDOT needed information about the capabilities of available modeling software packages in addition to the needs, desires and restrictions of the agencies and consultants who will be using the model.
What Was Our Goal?
The goal of this project was to better understand the capabilities of commercially avail-able modeling software packages to address MnDOT’s modeling and simulation needs.
What Did We Do?
Investigators interviewed stakeholders about their understanding of and need for mesoscopic traffic simulation and DTA. Stakeholders included individuals who have used or requested data from the Twin Cities Regional Travel Demand Forecasting Model maintained by the Metropolitan Council. Investigators also reviewed four case studies of mesoscopic DTA models used in Manhattan; San Francisco; Detroit; and Jacksonville, Florida.
To supplement the findings from the interviews and case studies, investigators conducted a comprehensive review of the claimed capabilities of six commercially avail-able traffic simulation software packages: TransModeler, Aimsun, DynusT/DynuStudio, Dynameq, Cube Avenue and Vissim. Investigators didn’t test the software, but instead reviewed manufacturers’ documentation and literature to identify limitations of their methods and whether those methods are applicable to MnDOT’s needs.
What Did We Learn?
To compare the capabilities of the various simulation software packages, investigators created a matrix that included comprehensive notations about a software package’s claimed features that may not fully meet MnDOT’s simulation needs. For example, some software packages claim to model actuated signals, but they create models based on Highway Capacity Manual assumptions rather than real-world conditions.
DynusT is the most commonly used simulation program, possibly because it is open-source and the easiest software to use, although it requires DynuStudio, a commercial graphical user interface and data management system. DynusT also has some limitations, such as not considering the individual lanes in each roadway segment, which would limit its effectiveness in modeling roads where individual lanes have imbalanced densities.
Most interviewees had only limited experience with mesoscopic modeling. Incorporating traffic signals in a simulation network is a significant challenge, according to interviewees, because currently a database of signal timings isn’t available.
While all four of the DTA case studies reviewed required more data, calibration and validation than older models, each of the developers reported that these challenges had been mitigated, and the models created could answer complex questions that previous models couldn’t.
Traffic simulation and modeling is a fast-developing field, particularly mesoscopic-scale modeling. Each of the software packages reviewed in this project has had at least two new versions in the past 18 months, and while their modeling approaches are fundamental to the software in some cases, in other cases capabilities will be added or improved as software develops.
The foundation of a mesoscopic model for the Twin Cities has been built and tested in Transmodeler (with significant pro bono work from the software developer). However, MnDOT has also used its existing DynusT model for several projects beyond its initial purpose, and the agency will use the information gathered in this project to determine which approach is more practical for MnDOT and its consultants based on cost, capabilities and data availability. Transmodeler is generally more powerful, but it will also incur greater costs, particularly since every consultant would need to acquire its own copy of the software.
This project developed a methodology using traffic data collected by the SMART-Signal system to identify intersections prone to red light running and, therefore, serious crashes. This methodology could help MnDOT prioritize intersections for safety improvements.
“The essence of this project was to develop a toolbox that traffic engineers can use to determine an intersection’s safety performance,” said Henry Liu, Research Professor, University of Michigan Transportation Research Institute.
Liu served as the study’s principal investigator.
“This research provides a way to classify intersections that have a higher potential for red light running,” Mick Rakauskas, Former Research Fellow, HumanFIRST Program, University of Minnesota
What Was the Need?
Engineers traditionally measure an intersection’s safety using the number of crashes that actually occur there. However, collisions are rare and somewhat random events, and it can take a long time to collect enough data to accurately assess a single location’s safety.
Traffic conflicts—“close calls” in which one or both drivers must brake, swerve or take some other evasive action to avoid a crash—happen much more often than collisions do. As a result, many research projects use traffic conflicts as an alternative measure of safety.
Red light running (RLR) is one of the most common and dangerous causes of traffic conflicts at signalized intersections. While not every RLR event leads to a collision, it is often the first step in a process that ends in one.
Additionally, crashes caused when drivers run red lights are typically right-angle crashes, which are frequently severe. About 45 percent of right-angle collisions result in injury compared to about 25 percent of other crash types. Reducing right-angle-crash frequency can therefore significantly improve overall road safety and reduce costs related to traffic collisions.
MnDOT’s Safety Group wanted to determine whether it was possible to objectively and automatically identify intersections where RLR events are most likely to occur. Developing a methodology to identify the most dangerous intersections would help MnDOT prioritize locations for safety improvements.
What Was Our Goal?
Several previous MnDOT research projects had developed the SMART-Signal system, an automatic system that collects data from traffic signal controllers at signalized intersections. MnDOT has installed the system at more than 100 intersections in the Twin Cities. This project sought to develop tools that use SMART-Signal data to evaluate safety performance at intersections.
What Did We Do?
Researchers analyzed SMART-Signal data collected at the intersection of Boone Avenue and Trunk Highway 55 (TH 55) in Golden Valley between December 2008 and September 2009. This intersection is equipped with both stop-bar detectors and advance detectors located about 400 feet upstream of the intersection. Researchers used stop-bar-actuation data and details about traffic signal phases to identify RLR events at the intersection.
However, since most intersections are equipped only with advance detectors, this method cannot be used to measure RLR events at all intersections. As an alternative, re-searchers used vehicle-speed and traffic-volume data from the advance detectors, along with recorded traffic-signal-phase information from SMART-Signal, to identify potential RLR events. They compared these potential events to actual RLR events identified using stop-bar data and developed a formula to predict whether an RLR event would occur. This formula can be applied at intersections of major and minor roads that are not equipped with stop-bar detectors.
Researchers then used data from a minor road to develop a method that identified whether an RLR event would lead to a traffic conflict. In this method, an intersection is first divided into four conflict zones (two in each direction). When a vehicle from the main road enters the intersection, the method enables researchers to calculate when the vehicle enters and leaves each of the conflict zones it passes through. Then they determine whether a vehicle from the minor road is in the same conflict zone. Using this methodology, researchers estimated the number of daily traffic conflicts at other inter-sections on TH 55. These estimates were based on data collected in 2009 and between 2012 and 2015.
Finally, researchers developed a regression model to evaluate whether adding the number of predicted traffic conflicts to a more standard model that used average annual daily traffic (AADT) would correlate with the number of actual collisions at that site. They evaluated the model using data from seven four-legged intersections and two T-intersections on TH 13 and TH 55.
What Did We Learn?
The formula for predicting RLR events matched observations 83.12% of the time, based on more than 2,000 data points.
The number of daily crossing conflicts at TH 55 intersections ranged from 7.9 (at Glenwood Avenue in 2009) to 51.2 (at Winnetka Avenue in 2013).
While limited data were available for the regression model (as no site had more than four years of SMART-Signal data available, and there were only 11 crashes in total), the model suggests that estimated average traffic conflicts and minor-road AADT both contribute to accurate prediction of right-angle-crash frequency, while major-road AADT does not. Due to the limited data available, however, these conclusions should be considered preliminary.
While there are currently no plans for follow-up studies, additional research efforts could include continuing to evaluate and improve the prediction model as more data are collected, and installing video cameras at intersections to validate the proposed methodologies.