IIHS research bibliography

This study evaluated the impact of double-lane roundabout conversions on public attitudes, traffic performance, the environment, and safety at two intersections near Bellingham, Washington, and evaluated whether older drivers avoided the roundabouts by taking an alternative route. Driver support for the roundabouts increased from 34% before construction to 70% one year after. One year after construction, more than 40% of drivers did not believe the signs and pavement markings adequately conveyed information about appropriate speeds, right-of-way rules, or navigating the roundabouts in the presence of large trucks. After accounting for other roadway changes, substantial declines were attributed to the roundabout conversions in the delays and queue lengths on minor roads, the proportion of queued vehicles, and fuel consumption and vehicle emissions. Analyses of crash rates per entering vehicles found that the roundabout conversions were associated with reduced rates of injury and fatal crashes combined and increased rates of property damage-only crashes. Only the increase in the property damage-only crash rate at one roundabout was significant. The odds that drivers 70 and older traveled the study corridor versus an alternative route after the roundabout conversions was 0.32 times the odds before. These findings generally are consistent with prior research finding substantial traffic, environmental, and injury-reduction benefits of single-lane roundabouts. However, it seemed the greater complexity of double-lane roundabouts may present challenges as some confusion persisted one year after construction, there was evidence that some older drivers may have taken an alternative route to avoid them, and property damage-only crash rates increased.

Objectives: Each year in the United States more than 1.2 million injuries and 9,000 deaths occur in motor vehicle crashes at intersections. Previous research has found that construction of roundabouts in place of traditional intersections can decrease crash frequency and severity. Despite these safety benefits, some crashes still occur at roundabouts. The present study systematically reviewed police crash reports for a set of roundabouts in Maryland to develop a typology of crashes and identify potential countermeasures. Methods: A total of 283 crash reports were reviewed including 149 crashes at 29 single-lane roundabouts and 134 crashes at 9 double-lane roundabouts. Based on the police reports, crash types were developed and examined by type of roundabout (single-lane, double-lane), crash location within the roundabout (entrance, circular roadway, exit), and other variables. Field observations were conducted at 8 roundabouts with above-average crash histories to aid in identifying potential countermeasures. Results: About three quarters of the crashes involved only property damage. Of the injury crashes, 14% involved at least one disabling injury; the remaining crashes resulted in probable injuries (36%) or nonincapacitating injuries (49%). One common crash pattern at both single- and double-lane roundabouts involved vehicles colliding with the central island, which accounted for almost half of all single-vehicle run-off-road crashes. Other major crash types included rear-end and sideswipe collisions. About three quarters of all collisions occurred at entrances to roundabouts. Based on review of crash reports and visits to several roundabouts, high approach speeds were an important driver crash factor, and some drivers may not have seen the roundabout in time. Conclusions: Increasing the conspicuity of upcoming roundabouts through larger "roundabout ahead" and "yield" signs could reduce speeds by alerting drivers ahead of time, especially at night. Enhanced landscaping of central islands as well as reflective pavement markers and yield signs at the entrance to roundabouts also could help drivers recognize roundabouts and the need to yield to circulating traffic. Certain design features (e.g., entry deflection on approach roads) also may aid in reducing speeds.

Roundabouts can provide substantial safety and traffic flow benefits compared with traffic signals and stop signs and as a result are increasingly used in place of traditional intersections. However, construction of roundabouts can be hampered by the negative perceptions held by some drivers. Prior research has found that public support increases soon after roundabouts are built and drivers become familiar with them. The purpose of the current study was to measure longer-term changes in public opinion in six communities where stop signs or traffic signals were replaced with roundabouts. Telephone surveys were conducted approximately 6 weeks before, 6 weeks after, and 1 to 5 years after construction of the roundabouts. The proportion of drivers in favor of roundabouts ranged from 22% to 44% before construction compared with 48% to 67% soon after roundabouts were built and 57% to 87% after roundabouts were in place for at least 1 year. The majority of drivers of all ages favored roundabouts after they were in place for 1 year or more, although support was higher among younger drivers (ages 18 to 34) and lower among older drivers (65 and older). There were small but nonsignificant differences between the opinions of male and female drivers. Drivers who said the roundabouts improved safety or traffic flow, or both, had more favorable opinions of roundabouts 1 to 5 years after construction. Results indicate that public support continued to increase with time, presumably because drivers became more familiar and comfortable with this form of traffic control.

Roundabouts can provide substantial safety and traffic flow benefits compared with conventional intersections, but they often are opposed in the planning stage by local residents and elected officials who question their effectiveness. The purpose of the present study was to measure public opinion before and after construction of roundabouts in several communities and to evaluate the impact of roundabout construction on traffic flow. Three communities where stop-sign- or traffic-signal-controlled intersections were replaced with roundabouts in 2004 were the subjects of this research. Overall, 36 percent of drivers supported the roundabouts before construction compared with 50 percent shortly after construction. Roundabouts had very positive effects on traffic flow. Average intersection delays during peak hours at the three sites were reduced by 83-93 percent. Traffic congestion, as measured by the vehicle-to-capacity ratio, was reduced by 58-84 percent. These results provide further evidence that roundabouts can improve traffic flow and that public support for roundabouts increases after they are in place.

Traffic congestion and motor vehicle crashes are widespread problems, especially in urban areas. Opportunities to improve traffic flow and safety can be missed when traffic signals are installed at locations suitable for roundabouts. The present study examined ten signalized intersections in Northern Virginia that were newly constructed or recently modified. Standard traffic engineering algorithms were used to estimate the effects on traffic delays and motor vehicle crashes if these intersections had been constructed as roundabouts. It was estimated that roundabouts would have reduced vehicle delays by 62-74 percent, depending on the intersection, thus eliminating more than 300,000 hours of vehicle delay on an annual basis. Annual fuel consumption would have been reduced by more than 200,000 gallons, with commensurate reductions in vehicle emissions. Based on previous research on crash risk, it is estimated that construction of roundabouts in place of traffic signals could have prevented 62 crashes, 41 with injuries, between 1999 and 2003 at five of the intersections for which crash data were available. These results show the magnitude of the traffic flow and safety costs when traffic signals are installed at locations suitable for roundabouts.

Although modern roundabouts have been shown to improve safety and traffic flow, drivers often oppose their construction. The purpose of this study was to examine public opinion regarding roundabouts before and after construction in several U.S. communities and to evaluate their impact on traffic flow. The majority of drivers (55 percent) opposed roundabouts before construction, with most (41 percent) strongly opposed. After construction, the proportion of drivers strongly opposed to roundabouts declined to 15 percent, and the proportion favoring roundabouts increased from 31 to 63 percent. Roundabouts reduced overall vehicle delays, reduced the proportion of drivers that came to a stop, and reduced the level of traffic congestion. These results suggest that public support for roundabouts will build as drivers experience and adapt to this highly beneficial form of traffic control.

Objectives: This study estimated potential reductions in motor vehicle crashes and injuries associated with the use of roundabouts as an alternative to signal and stop sign control at intersections in the United States. Methods: An empiric Bayes procedure was used to estimate changes in motor vehicle crashes following conversion of 24 intersections from stop sign and traffic signal control to modern roundabouts. Results: There were highly significant reductions of 38% for all crash severities combined and of 76% for all injury crashes. Reductions in the numbers of fatal and incapacitating injury crashes were estimated at about 90%. Conclusions: Results are consistent with numerous international studies and suggest that roundabout installation should be strongly promoted as an effective safety treatment.

Modern roundabouts are designed to control traffic flow at intersections without the use of stop signs or traffic signals. U.S. experience with modern roundabouts is rather limited to date, but in recent years there has been growing interest in their potential benefits and a relatively large increase in roundabout construction. This interest has created a need for data regarding the safety effect of roundabouts. Changes in motor vehicle crashes following conversion of 23 intersections from stop sign and traffic signal control to modern roundabouts are evaluated. THe settings, located in seven states, are a mix of urban, suburban, and rural environments with the urban sample consisting of both single-lane and ultilane designs and the rural sample consisting of only single-lane designs. A before-after study was conducted suing the empirical Bayes procedure, which accounts for regression to the mean and traffic volume changes that usually accompany conversion of intersections to roundabouts. For the 23 intersections combined, this procedure estimated highly significant reductions of 40 percent for all crash severities combined and 80 percent for all injury crashes. Reductions in the numbers of fatal and incapacitating injury crashes were estimated to be about 90 percent. In general, the results are consistent with numerous international studies and suggest that roundabout installation should be strongly promoted as an effective safety treatment for intersections. Because the empirical Bayes approach is relatively new in safety analysis, the potential of this methodology in the evaluation of safety measures is demonstrated.