Mini-roundabouts are a type of roundabout characterized by a small diameter, and fully traversable central island and splitter islands. They are an alternative intersection design option in areas with constraints requiring additional land acquisition. They may be retrofitted within the existing intersection boundaries. Also, they are better suited for traffic calming and reducing delay, thereby, reducing emissions. They are suited to environments where speeds are relatively low and environmental constraints preclude the use of larger roundabouts with raised central islands. The standard-size roundabouts are safer than traditional minor road stop-controlled or signalized intersections, better suited for traffic calming, and reduce delay as well as emissions. However, the safety benefits associated with mini-roundabouts are not well documented and must be evaluated for planners and engineers to consider more mini-roundabout installations in the United States.
The focus of this research is on evaluating the safety effectiveness of converting a stop-controlled intersection with a speed limit ≥ 35 mph (56.3 kmph) to a mini-roundabout and examining the role of influencing factors on their safety effectiveness in the United States. The methodology includes : 1) identification of mini-roundabout installations in the United States, 2) before and after crash data and traffic volume data collection at selected mini-roundabout locations, 3) before and after analysis for determining safety benefits of mini-roundabouts, 4) safety effectiveness and crash modification factors (CMFs) computation for mini-roundabouts based on before and after crash data, and, 5) examining the effect of traffic characteristics, geometric characteristics, and on-network and off-network characteristics on mini-roundabout safety effectiveness and after period crashes. Crash, traffic volume, and geometry data for 25 mini-roundabouts in eight states was collected to conduct before-after analysis using the naive and Empirical Bayes (EB) method. Additionally, crash and traffic volume data for 723 reference intersections were gathered and used for computing the calibration factors and developing jurisdiction-specific safety performance functions (SPFs).
Results indicated a decrease in total crashes and FI crashes when TWSC/OWSC intersections were converted to mini-roundabouts. However, an increase in PDO crashes was observed. Likewise, an increase in total number of crashes, FI crashes, and PDO when AWSC intersections were converted to mini-roundabouts. Converting a TWSC/OWSC intersection to a mini-roundabout has better safety benefits than converting an AWSC intersection to a mini-roundabout. The number of crashes in the before period, cross-street traffic volume, speed limit at major street and cross-street, and intersection skewness have a statistically significant influence on the safety effectiveness of mini-roundabouts at a 90% confidence level.
These findings are useful to researchers and practitioners for conducting safety benefit analysis and making informed decisions pertaining to converting a stop-controlled intersection to a mini-roundabout.