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Modern roundabouts have been shown to be one of the safest methods of control for at-grade intersections. An evaluation of traffic safety at roundabouts published by MnDOT in 2017 shows substantial decreases in fatal and serious injury crashes at intersections after installation of a roundabout. Despite these benefits, concerns have been raised from citizens, commercial motor vehicle (CMV) drivers, and other stakeholders about the propensity for roundabouts to cause more rollover crashes for larger vehicles. The purpose of this evaluation is to further evaluate the safety of heavy commercial vehicles at roundabouts, investigate whether rollover crashes are more likely to occur at roundabouts, and break down the most common characteristics for CMV crashes at roundabouts in Minnesota. This report includes the results of an analysis comparing roundabouts to signalized intersections. The results of this evaluation indicate that roundabouts continue to be one of the safest forms of intersection control for heavy commercial vehicles, especially when compared to traffic signal control. Although roundabouts have experienced a greater number of rollover crashes compared to intersections with traffic signal control, their installation results in a decrease in high-severity crashes.

This report presents a study of the impact lane markings and signing have on driving behavior at a two-lane roundabout located in Richfield, Minnesota. After its completion in 2008, this roundabout sustained a suspiciously high amount of crashes. In response, through this study, engineers experimented with changes in the roundabout's signs and lane markings, as roundabout design regulations are relatively lax and nonspecific in contrast to those for standard signalized intersections. An observational study was conducted that reduced 216 hours of before and after video records of the roundabout into a database of all the violations committed by drivers. Along with the observational data, crash records were analyzed and demonstrated that improper turns and failing to properly yield account for the majority of collisions. The changes implemented in the approaches to the roundabout and specifically the extension of the solid line seems to have reinforced the message to the drivers that they must select the correct lane before approaching the roundabout entrance. Although choosing the correct lane does not directly address yielding violations, it does reduce the occurrence of drivers conducting an improper turn and to some extent reduces the need for a driver to change lanes within the roundabout. The implemented changes produced a 48% reduction in normalized occurrences of improper turns, and a 53% reduction in normalized occurrences of drivers choosing the incorrect lane a month after the changes, while a year later, these reductions were 44% and 50%, respectively.

Many cities in the United States are installing roundabouts instead of traditional intersections, due to evidence that roundabouts dramatically reduce fatal and severe injury crashes compared to traditional signalized intersections. However, the impact on pedestrian safety is not clear. This project was developed to investigate pedestrian accessibility in Minnesota urban roundabouts, addressing complaints from pedestrians regarding difficulties in crossing and safety. The methodology followed in this ongoing research is typical of other observational studies. A sufficiently large number of observations on the interactions between pedestrians or bicycles (peds/bikes) and vehicles at two modern urban roundabouts in the Twin Cities of Minneapolis and St. Paul in Minnesota were collected and reduced. These observations have supported a two phased analysis. Phase 1 involved the extraction of general information describing the crossing event, such as who yielded, the location of the crossing, or the number of subjects involved. Phase 2 looked deeper into these factors by considering the conditions inside the roundabout before the vehicle proceeds to the crossing and meets with the ped/bike. The results presented, although containing no surprises, do highlight and categorize the existence of friction between pedestrians and drivers at roundabout crossings. Also the identification of factors affecting driver yield behavior and pedestrian wait time do offer good background for modeling such interactions.

Appropriate deceleration on approaches to roundabouts is primarily accomplished through the use of applicable geometric design principles; however; traffic control devices (specifically signing and markings on the approach) also serve a vital role in communicating to the approaching driver what speed profile should be anticipated. This report provides a resource for engineers to identify and select appropriate speed-reduction treatments for high-speed approaches to roundabouts. The research examines best practices and research literature on speed-reduction techniques for high-speed approaches for all intersection types; as well as treatments for work zones and horizontal curves. Based on the findings from these efforts; the report summarizes a selection of treatments including traditional signs with and without beacons; pavement markings; illumination; speed-activated signs; and transition zones. Information on the effectiveness of these treatments; as well as potential costs of installation and maintenance; is provided for the practitioner to determine which treatment(s) best suit the site under consideration. Guidance is also provided for the methodology of conducting a speed study to determine the speed characteristics of a site; as well as links to resources for additional information. The project identifies a number of research needs specific to particular treatments as well as the general need for field research of the recommended countermeasures specifically on approaches to high-speed rural roundabouts.

When looking at measures of fatal and severe-injury crashes; roundabouts have demonstrated improved safety performance compared to traditional signalized intersections. Despite this; when it comes to less severe crashes; multilane roundabouts fail to provide a similar benefit. Previous research into this topic has identified behaviors that are associated with crashes in two-lane roundabouts; with yielding violations and turn violations generally being the largest contributors to crashes. This study sought to build on that work by expanding the data collection effort to include more sites and relate the frequency of individual behaviors to specific design features. Ultimately; four roundabouts were chosen for data collection and analysis; with two of them being full 2x2 roundabouts and the other two half-2x2. These locations were University Dr. S and 5th Ave. S in St. Cloud (half-2x2); 185th St. W and Kenwood Trail in Lakeville (full-2x2); and TH-22 and Adams St. (half-2x2) and TH-22 and Madison Ave. in Mankato (full-2x2). In the last two; changes in traffic control were implemented to reduce failure-to-yield crashes; and the study compared the driver behavior before and after the interventions. In general; the collective results show that the problems observed in the earlier site are present in all of the other sites with scale variations prompted by geometric and traffic control design elements. For example; in the St. Cloud roundabout; an increased rate of right-from-inner-lane turn violations were observed; which can be attributed to the sharper deflection angles present. Additionally; from the aforementioned roundabout as well as the one in Lakeville; it was concluded that turn violation rates are affected on the single or multilane geometry of the links approaching the roundabout. Single-lane roads result in fewer left-from-outer-lane violations. Overhead lane designation signs result in reduced turn violations similar to the earlier studies' interventions in the approach lane markings. Unfortunately; apart from confirming the trend; no successful design or intervention was discovered regarding yield violations.

This toolbox provides information to assist transportation agencies in utilizing roundabouts while evaluating traffic control at adjacent intersections, thereby avoiding adverse impacts on corridor performance. The information was developed by summarizing available information and conducting several evaluations. The toolbox addresses roundabouts as they relate to and/or impact the areas of comprehensive planning, mobility, unbalanced traffic flows, access management and other planning considerations.

The use of roundabouts in rural areas of the US is growing rapidly. For roundabouts constructed with concrete pavement, joint layout can be especially challenging. To reduce the need for sophisticated joint layouts, consideration is being given to constructing roundabouts without joints and instead using structural fiber-reinforced concrete (FRC) to bridge any cracks that might occur. In 2018, Minnesota’s first jointless FRC pavement roundabout was constructed at the intersection of Minnesota Trunk Highway 4 and County State Aid Highway 29. The National Road Research Alliance (NRRA) sponsored a study to document the construction and performance of Minnesota’s first jointless FRC roundabout. One of the key objectives was to carry out a three-year performance monitoring regimen of the roundabout to better understand its in-situ performance when exposed to traffic loading and environmental conditions. This report documents the third-year performance of the roundabout as per the requirements of Task 3 of the work plan.

Multi-lane roundabouts can be an effective intersection control and reduce injury crashes if the design and layout is carefully considered. Past experience has indicated that failure to yield and improper lane use violations can be over represented at multi-lane roundabouts, leading to a higher number of property damage crashes. Two LRRB funded studies have recently been conducted to attempt to address these violations. This document is a summary of findings from these studies and local agency experience to date.

Appropriate deceleration on approaches to roundabouts is primarily accomplished through the use of applicable geometric design principles. However, traffic control devices (specifically signing and makings on approach) also serve a vital role in communicating to the approaching driver what speed profile should be anticipated. This report provides a resource for engineers to identify and select appropriate speed-reduction treatments for high- speed approaches to roundabouts. The research examines best practices and techniques that are the most effective at lowering approach speed.

This quick reference guide accompanies 2022RIC03, "Strategies for Effective Roundabout Speed Reduction." It includes pictures of various methods of roundabout speed reduction, as well as information about which methods work best under which circumstances.