The Benefits of Roundabouts in Modern Traffic Systems

Modern roundabouts are a type of intersection that has a circular configuration. Curved approaches with a channelization by a splitter island, or a raised area that separates entering and exiting traffic, help slow down entering traffic. When approaching a roundabout, an entering vehicle yields to circulating traffic that travels around a center island in a counterclockwise direction. Studies have shown that circular intersections result in a safer, more efficient way to move traffic. According to the Federal Highway Administration, roundabouts reduce fatal and injury crashes by 82% compared to a two-way, stop-controlled intersection and by 78% compared to a signalized intersection.

There are different key components to a roundabout. The first component is called the central island. It is in the roundabout’s center where vehicles circulate. The central island typically has a landscaped, non-traversable area and a stamped concrete traversable section. This traversable section is called the interior truck apron. The truck apron is on a raised, rolled curb from the roundabout travel way. This allows tractor trailers and other large vehicles to make turns within the roundabout. Sometimes, truck aprons are needed on approaches. In this situation, these are called outside truck aprons.

The inscribed circle diameter (ICD) is the distance across the circle inscribed by the outer curb of the circulatory roadway. The ICD is used to describe the roundabout’s size.

The circulatory roadway is the paved path that vehicles drive within the roundabout.

Each leg of a roundabout has an entry and exit. The entry and exit curves are designed to control vehicle speeds.

A splitter island provides refuge for pedestrians who are crossing the road.

There are numerous types of roundabouts that have unique design elements to accommodate varying traffic situations, spatial limitations, and user requirements. Among the most common are:

• Single-lane Roundabouts: These are utilized for moderate traffic amounts. They are made to support one traffic lane circling the central island. Their ICD size typically falls in the range of 110 feet to 140 feet but can be larger. Single-lane roundabouts are the most common type of roundabout; they can be utilized in urban or rural environments and typically have three or four approaches. In unique settings, single-lane roundabouts can have five or more approaches.
• Multi-Lane Roundabouts: Specifically designed for higher capacity intersections, multi-lane roundabouts feature two or more traffic lanes entering and encircling the central island. Their ICD size typically ranges from 160 feet to 200 feet for a two-lane roundabout. A special design should be considered with multi-lane roundabouts, such as lane traffic control, speed controls, and avoiding entry path overlap which can cause sideswipe vehicle crashes. This is especially the case in two-by-two roundabouts.
• Mini Roundabouts: These operate similar to single-lane roundabouts, however, mini roundabouts are much smaller and have a fully traversable central island. Their ICD size is between 60 feet and 90 feet. Mini roundabouts are a great design option for urban settings where right-of-way is a major constraint.
• Peanut Roundabout: This alternative roundabout design operates very similarly to single-lane and multi-lane roundabouts, where entering traffic yields to traffic circulating. However, the circulating traffic must go around two closely spaced roundabout circles that are connected to each other by another curve or tangent. This makes the roundabout itself look peanut-shaped, hence the name. Peanut roundabouts are traditionally placed at intersections that have poor skew angles. Also, a peanut roundabout is a way to combine two closely spaced intersections into one.
• Dumbbell and Dog Bone/Teardrop Roundabouts: These roundabouts are typically built at diamond interchanges and are three-legged. They are two different roundabouts that are connected to each other by a tangent, which is longer in length than a peanut roundabout. A dumbbell roundabout features full circles at each roundabout, allowing all vehicles the ability to make a U-turn movement. A dog bone, or teardrop, roundabout is an unfinished or squashed circle that does not allow for continuous 360-degree travel within the circulatory roadway.
• Turbo Roundabouts: These roundabouts are multi-lane and use a spiral road geometry with physical channelization, or raised lane dividers, to maintain driver lane discipline within the roundabout. Also, turbo roundabouts have more of a tangential entry. These roundabouts originated in the Netherlands and are newer to the U.S. They eliminate the lane change side swipe conflicts in a two-by-two multi-lane roundabout by the raised lane dividers. The roundabout geometry spirals a driver out to the appropriate roundabout exit based on the lane the vehicle is in at entry.
• Other Nontraditional Roundabouts: Other unique roundabouts include a hamburger, a thru-about, and a compact roundabout.

When looking to improve traffic flow and reduce congestion, innovative roundabout engineering and design must be considered. Determining which roundabout type to use depends on several variables, such as traffic volume, right-of-way constraints, safety concerns, and the transportation project’s objectives. Every kind of roundabout is made to tackle certain design problems and offer the best possible vehicular safety.

Roundabouts are growing in popularity throughout the U.S. — it is estimated that there are as many as 13,000 roundabouts nationwide. Transportation agencies are realizing the numerous advantages of creating a roundabout intersection. Some of the most impactful roundabout benefits include:

• Safety: A roundabout’s greatest strength is improving safety. In comparison to a four-leg intersection that is either stop-controlled or signalized, roundabouts can dramatically lower accident frequency and severity by inducing lower vehicle speeds and reducing conflict points. A standard four-legged intersection has 32 vehicle-to-vehicle conflict points and 24 vehicle-to-pedestrian conflict points. In comparison, a four-legged roundabout has only eight vehicle-to-vehicle conflict points and eight vehicle-to-pedestrian conflict points. What a reduction! A study by the Insurance Institute for Highway Safety found roundabouts provide a 90% fatal crash reduction, 76% injury crash reduction, 30% to 40% pedestrian crash reduction, and 10% bicycle crash reduction. The reduction in pedestrian crashes is attributed to pedestrians only crossing one direction of traffic at a time. Roundabouts prevent dangerous T-bone crashes that are common at signalized and stop-controlled intersections.
• Capacity: By allowing traffic to flow constantly, roundabouts are designed to minimize halting vehicles. Single-lane roundabouts can accommodate up to 20,000 annual average daily traffic (AADT) and up to 1,300 vehicles per hour at peak times, whereas two-lane roundabouts can handle 25,000 to 45,000 AADT. Talk about efficiency!
• Speed, Speed, Speed: Did I mention speed? Roundabouts promote slower vehicle speeds. The roundabout geometry forces vehicles to slow down when entering the roundabout area. Vehicles entering a roundabout intersection are forced to slow down to 25 miles per hour or less. This, in turn, reduces drivers’ stopping sight distance, helping avoid crashes. Crashes are significantly lower compared to signalized and stop-controlled intersections.
• Aesthetics and Landscaping: A roundabout’s middle island offers a chance for aesthetic and landscaping improvements. Roundabouts allow cities and agencies to create an eye-catching focal point in rural, suburban, or urban settings.
• Lower Maintenance Costs: A signalized intersection requires electricity 24 hours per day. Signals also need maintenance for burned-out lights, loop detector replacement, pavement repairs, and other detector upgrades. Roundabouts only require streetlight electricity and landscaping maintenance costs. Although there might be more initial costs with a roundabout compared to a signalized intersection, long-term maintenance costs are lower overall.
• Less Environmental Impact: Because cars can travel through roundabouts by yielding, they frequently reduce idling time and fuel consumption, which lowers emissions.

Roundabouts are a great tool to help resolve challenging intersection designs due to a variety of factors, including increased traffic flow, enhanced safety, environmental considerations, and aesthetic benefits.

Brandon is a senior highway engineer based in our Columbus, Ohio, office. His fascination with roundabouts began in childhood when he drew maps and made a clay model of a roundabout. This early passion evolved into a professional career where he has contributed to over 30 conceptual roundabouts and more than 10 roundabouts currently in design, construction, or already built. Brandon’s lifelong interest in roads, sparked by family trips to Myrtle Beach, South Carolina, and a high school project on intersections, has driven his roundabout expertise. His dedication to improving traffic capacity and safety through roundabout design reflects his deep commitment to this innovative solution.