Roundabouts are one of the most effective intersection control treatments available with the added benefit of calming traffic. They limit vehicle speeds to approximately 20 mph and can control vehicle speeds on four approaches simultaneously.
According to Federal Highway Administration (FHWA) study results, roundabouts achieve a 44% reduction in all crashes and a 72%-87% reduction in fatal/injury crashes when converting a two-way stop intersection to a roundabout. They also report a 48% reduction in all crashes and a 60%-78% reduction in fatal/injury crashes when converting a signalized intersection to a roundabout, and greatly reduce severity on those few crashes that do occur.
Roundabouts achieve significant crash reductions because they simplify motorist decision making and have fewer conflict points. At a four-way intersection, there are 32 possible conflict points between vehicles and only eight at roundabouts. See the diagrams to the right for a comparison of conflict points.
Angle and left turn crashes account for 63% of fatal crashes at an intersection. These are typically more severe because of the speed and manner of impact. A roundabout eliminates angle and left turn crashes by forcing vehicles to make a right turn in and a right turn out. Crashes that do occur in a roundabout are sideswipe in nature and less severe due to low vehicle speeds.
Roundabouts vs. Traffic Signals
Roundabouts have proven to be much safer than traffic signals. The projected injury crash rate for roundabouts is half that of traditional signals.
Modern Roundabouts vs Traffic Circles
There are many differences between roundabouts and traffic circles. Unlike traffic circles, roundabouts are used on higher volume streets to allocate right-of-way between competing intersection movements. Traffic circles have a large diameter, which contributes to high circulating speeds; roundabouts have a smaller diameter, promoting low circulating speeds. Roundabouts have lower entry speeds compared to traffic circles and feature a yield at every entry point, promoting low speed and no weaving.
Driving Modern Roundabouts
Research shows that drivers quickly adapt to the roundabout traffic flow. For instance, Vail and Avon, Colorado, both feature many high capacity roundabouts and are major tourist destinations with thousands of first-time roundabout drivers using the roundabout intersections each year. Despite large numbers of drivers who have not driven roundabouts previously, these intersections work well and do not confuse motorists. Proper use of signing and road striping at roundabouts assists motorists and minimizes the potential for confusion.