No abstract

Federal, state, and local agencies have identified increased intersection safety as an achievable objective in their campaigns for safer highways. Roundabouts could reduce the number of fatal and severe crashes. Roundabouts have been successful in Europe and Australia in both urban and rural environments for decades. The documented safety benefits associated with roundabouts address numerous concerns surrounding injury crashes at intersections; however, applications of roundabouts on two-lane, high-speed (40+ mph) rural highways have not been explored to the extent that urban roundabouts have in the United States. This research conducted a safety analysis of 17 intersections on high-speed rural roadways in the United States that were converted to roundabouts. The findings show the average injury crash frequency was reduced by 84%, average injury crash rate was reduced by 89%, angle crashes were reduced by 86%, and fatal crashes were reduced by 100%.

Reduced emissions and improved air quality are typically attributed to the use of roundabouts rather than traditional stop or signal control. Roundabouts are expected to reduce emissions as a result of reduced delays and stops. However, roundabouts slow all vehicles to speeds at which emissions may be higher, while signals stop and delay only a portion of vehicles. Roundabouts may also increase the amount of acceleration and deceleration for all vehicles. Because emissions are correlated to these modal events, their impacts should be considered in the evaluation of roundabouts. An on-road assessment of the emission impacts of two roundabouts compared with other types of traffic control was conducted with a vehicle instrumented with a portable emissions monitor. Emissions were measured along two corridors in Woodbury, Minnesota, with two drivers. The corridor at Bailey Road had two four-way stop-controlled intersections, one roundabout, and one signal. The corridor at Lake Road had one four-way stop, one roundabout, and one signal. Intersection spacing and traffic parameters were similar along each corridor. Emissions were compared for sections 750 ft upstream and 750 ft downstream of each intersection. Traffic conditions during data collection along both corridors were uncongested. Average emission rate by driver was compared for the three types of traffic control. Results suggested that emissions were highly dependent on driver behavior. The roundabouts did not necessarily have lower emissions than traditional stop or signal control.

No abstract

Roundabouts have proved to be effective in urban and suburban environments in the United States, but little has been reported on the effectiveness of roundabouts in rural environments on high-speed roadways. There is no question that roundabouts reduce the speeds of all vehicles at intersections and reduce the frequency of fatal and injury-related crashes. This research is the first comprehensive look at roundabouts in a rural environment with high-speed approaches. Nineteen intersections had ample comprehensive crash data to be evaluated and analyzed for safety performance. The findings validated the hypothesis that roundabouts in a rural environment as well as roundabouts in urban and suburban environments outperformed other intersection safety improvements. A before-and-after crash analysis was conducted for the 19 intersections by using a negative binomial regression model. Results showed statistically significant reductions for the total number of crashes (63%) and injury crashes (88%) when roundabouts were implemented. A before-and-after empirical Bayes estimation was also conducted, and the results were consistent, indicating a 62% to 67% reduction in total crashes and an 85% to 87% reduction in injury crashes at these rural intersections. Furthermore, results showed that injury-producing crash types, such as the angle crash, were reduced by 91%, and were statistically significant. Finally, this research produced crash prediction models at planning level for total and injury crashes at rural roundabouts on high-speed roadways. This supplements the models produced in NCHRP Report 572: Roundabouts in the United States and reported in the AASHTO Highway Safety Manual.