A roundabout is a type of circular intersection where traffic proceeds in a counterclockwise direction around a center island. This circular intersection can be used in place of standard stop-sign or signal-controlled intersections. However, not all circular intersections can be characterized as roundabouts. The three most common types of circular intersections are:
Lincoln Memorial Circle
More common before the 1960s, this type of intersection typically includes a large diameter, sometimes in excess of 300 feet. This results in speeds usually greater than 30 mph. In addition, rotaries are commonly controlled with the ‘yield to the right' rule giving priority to entering traffic. Lincoln Memorial Circle in Washington D.C. is an example of a rotary that provides access to Arlington Memorial Bridge and local roads.
Neighborhood Traffic Circles
Photo credit City of Vancouver, Canada
Are generally located at the intersection of two local streets and are used for traffic calming and/or aesthetics. Some traffic circles are placed on existing intersections without any modifications to the intersection making movements difficult for larger trucks. Approaches to the traffic circle may be uncontrolled or stop sign controlled. Left turns may also occur in a clockwise direction in front of the center island at some traffic circles. Traffic circles have been used by many cities in the past, but are increasingly rare today.
West Des Moines
Roundabouts employ specific design features to physically direct traffic into counterclockwise circulation around the center island. Each approach is under yield sign control. Speeds within the roundabout are generally 25 mph or less. Finally, left turning traffic must proceed around the center island, never in front. Rotary intersections and traffic circles are still found across the country and motorists have likely encountered at least one of these circular intersections while driving. Therefore, when roundabouts are mentioned, the majority of the driving public pictures these earlier types and problems associated with them. Although similar, the modern roundabout is a new version of circular intersections that eliminates most concerns related to rotaries and traffic circles.
To more clearly describe the key features of a modern roundabout, the following terms are used. These features are shown in the illustration* (1A) depicting a typical roundabout.
- Accessible Pedestrian Crossing – pedestrian crossings are provided a short distance (generally 1-2 vehicles lengths) from the roundabout on an approach; pedestrians should not be allowed to cross into the center island; a curb cut allows wheelchairs, strollers, bicycles, etc. easy access between the road crossing and the sidewalk
Apron – a slightly raised area around the center island that allows larger trucks, tractors and farm equipment, buses, etc. easier circulation in the roundabout; an apron is used instead of increasing the normal driving width to prevent smaller vehicles from achieving higher speeds through the roundabout
Bicycle Treatments – providing correct transitions from bicycle lanes allows cyclists to either ride through the roundabout with vehicles or walk their bikes using the pedestrian crossings; vehicle speeds average approximately 15 mph, which is comparable to bicycling speeds
- Center Island – a raised island around which traffic flows
- Circulatory Roadway – the driving lane or lanes around the center island; as previously mentioned, traffic proceeds in a counterclockwise direction entering and exiting only to the right.
Landscape Buffer – proper landscaping can prevent pedestrians from crossing into the center island while beautifying the intersection at the same time
- Splitter Island – a raised or painted island separating traffic entering and exiting the roundabout on each approach; the splitter island also serves to deflect and slow entering traffic, as well as providing a refuge for crossing pedestrians
- Yield Line – line at which entering traffic must give way to traffic circulating in the roundabout; if no traffic is in the roundabout, entering traffic may proceed without stopping
*1A - Illustration of a Roundabout