circular road with cars on it
In the United States, vehicles circle a modern roundabout in a counterclockwise direction, and incoming vehicles must yield to vehicles already in the roundabout. (Photograph by Brian McGuckin, Courtesy of the City of Carmel, Indiana)

To keep traffic moving safely and efficiently, engineers are increasingly turning to the modern roundabout, an upgrade on the traditional design that its supporters say is safer and more efficient than traditional traffic circles or signalized intersections. In addition to accommodating all sizes and types of vehicles as well as nonmotorized modes of movement, the roundabout is also credited with reducing greenhouse gas emissions.

In the 1985 comedy film National Lampoon’s European Vacation, the character played by actor Chevy Chase drives his car into a circular intersection in London and then gets stuck there for the rest of the day, driving around and around the circle’s raised central island because he cannot figure out how to navigate the lanes and exit. Although exaggerated for humorous effect, the movie’s scenario is familiar to some traffic engineers. They understand that a driver who is not experienced with circular intersections can indeed be confused by a poorly designed system and can essentially feel trapped when confronted by other vehicles in the circle. The results can range from travel delays and backed-up traffic to collisions, injuries, and even fatalities. 

Such problems have typically been associated with large circular intersections known as traffic circles, which in the United States are also called rotaries and can be found mostly in the eastern part of the country. Columbus Circle in New York City and Dupont Circle in Washington, D.C., are two well-known examples. There are also many small traffic-calming circles located primarily in residential areas. A key problem with the large traffic circles has been that some require the vehicles moving through the circle to yield to the vehicles trying to enter the circle; some traffic circles even have stop signs or traffic lights within the circle to force the circling traffic to stop and let more vehicles enter. 

To help resolve such difficulties, engineers in the early years of the 20th century developed a new type of circular intersection known as a roundabout. These systems began in Europe and Australia and have since spread throughout the world, changing and improving over time. (Authorities are considering major changes to the one showcased in European Vacation.)

The latest iteration of these traffic control systems — called a modern roundabout — began in the United Kingdom in the mid-1950s, according to “Roundabouts: A Direct Way to Safer Highways” by Leif Ourston and Joe Bared, which was published in the Autumn 1995 issue of Public Roads: A Journal of Highway Research, the quarterly magazine of the Federal Highway Administration. 

The first modern roundabouts constructed in the United States date to 1990 in Nevada, according to information compiled by the Insurance Institute of Highway Safety, based in Arlington, Virginia. Since then, the approach has been gaining traction, with the number of modern roundabouts in the United States increasing from roughly 150 in the late 1990s to more than 7,000 today, according to the Roundabouts Database compiled by the transportation engineering firm Kittelson & Associates Inc., of Portland, Oregon. Kittelson launched the database in 1997 and now provides the list as a service to the Transportation Research Board’s Standing Committee on Roundabouts. 

A modern roundabout generally features a smaller footprint than a traditional traffic circle — in one example from Ulster, New York, a 600 ft diameter traffic circle was replaced by a 200 ft diameter modern roundabout, and many modern roundabouts are even smaller, says Mark T. Johnson, P.E., the president and owner of MTJ Roundabout Engineering, in Madison, Wisconsin. Because traffic circles are larger, vehicles tend to drive at higher speeds and often need to change lanes — all of which makes maneuvering within them more challenging, according to Roundabouts: An Informational Guide, Second Edition (2010, Transportation Research Board’s National Cooperative Highway Research Program). Johnson was one of the authors of the guide.

Roundabouts are measured according to their inscribed circle diameter, which is the overall distance across the roundabout from outer edge to outer edge of the driving lanes. The inscribed diameter of so-called mini-roundabouts can measure between 45 and 90 ft, while single-lane roundabouts typically measure 90 to 180 ft across, and multilane roundabouts can reach diameters of 300 ft, according to the Informational Guide.

Many types to choose from

Mini-roundabouts — also known as compact roundabouts in some jurisdictions — are generally used in low-speed urban environments where right of way constraints preclude a larger circular intersection, according to the Informational Guide. They also differ from other roundabouts in that they feature fully traversable central islands, whereas single-lane and multilane conventional roundabouts generally feature raised islands that sometimes include traversable truck aprons around central islands, which can help large vehicles navigate the circular lanes. 

An important distinction between a modern roundabout and a traditional traffic circle is that the roundabout requires drivers who want to enter the circular intersection to yield to the vehicles already circling the roundabout. In the United States and other countries that drive on the right-hand side of the road, vehicles circle the roundabout in a counterclockwise direction; in the United Kingdom and other left-hand driving countries, the vehicles circle clockwise. Modern roundabouts can be constructed at single intersections, in pairs at nearby intersections, or in series at multiple intersections along a route.

photograph of a modern roundabout
Prominent signage and pavement markings help alert drivers to an upcoming roundabout. (Photograph by Brian McGuckin, Courtesy of the City of Carmel, Indiana)

Modern roundabouts also feature designs that help direct and slow the incoming traffic, including:

  • Configurations that deflect traffic around the central island of the roundabout. 
  • Flared entrance points that provide wider entry lanes that add capacity and accommodate large vehicles.
  • Raised or painted areas called splitter islands that separate the entering and exiting traffic and provide safe spaces for pedestrians to cross.

The “desirable maximum entry design speed” on mini-roundabouts ranges from 15 to 20 mph, while the entry speeds for single-lane roundabouts should not exceed 25 mph, and 30 mph is the top desired entry speed for multilane roundabouts, according to the Informational Guide. There are several ways to reduce the speed of vehicles entering a roundabout even — or especially — when the roads that lead to the roundabout have much higher speeds. These include signage, rumble strips, flashing lights, and various other visual cues that clearly alert drivers to the upcoming roundabout, says Nazir Lalani, P.E., M.ASCE, the president of Traffex Engineers Inc., of Ventura, California. Lalani has designed modern roundabouts in both the United States and the United Kingdom and has also presented various webinars for ASCE on the operational and safety differences between roundabouts and traffic circles. 

The geometry of the approaching roads and the roundabout itself are also critical to safe operation of a roundabout. The angle of entry into the roundabout should be close to 90 degrees to slow down the incoming traffic, Lalani says. If the angle is similar to that of a freeway entrance ramp, the vehicles can “come barreling up” at high speeds, too often leading to collisions and injuries, he explains.

Learning curve

Although existing traffic circles are sometimes converted to modern roundabouts, most often it is a traditional intersection — with a stop sign or traffic light — that gets changed to a roundabout, notes Wen Hu, Ph.D., the senior research transportation engineer at the Insurance Institute for Highway Safety. After such conversions, crash rates at those intersections tend to fall, according to an IIHS website about roundabouts. Citing several studies involving U.S. traffic crashes, the IIHS site reports a 72 to 80 percent decline in vehicular crashes that cause injuries and a 35 to 47 percent reduction in all crashes after an intersection is converted to a modern roundabout. European and Australian studies show similar results, the IIHS site notes.

figure depicting the elements of roundabout designEdit photo credit]
ELEMENTS OF ROUNDABOUT DESIGN (Image courtesy of USDOT Federal Highway Administration)

Some modern roundabouts feature multiple lanes of traffic — in London, there are many roundabouts with four lanes on which the traffic can resemble a Ben-Hur-style chariot race, says Lalani. But in the United States, one- and two-lane roundabouts are far more common “because as you increase the number of lanes going around a roundabout, it gets more complicated, especially at the exit lanes,” explains Lalani. 

Even two-lane roundabouts can be confusing for some drivers — at least initially. Although modern roundabouts have been shown to improve roadway safety overall, the single-lane versions often produce more immediate results than do two-lane roundabouts, says Hu. But IIHS research shows that safety also improves at two-lane roundabouts as drivers become more familiar with the new roundabout’s configuration, Hu explains. 

Still, “traffic engineers shouldn’t rely on drivers to learn how to navigate roundabouts on their own,” explained a July 2019 IIHS press release about Hu’s research. Instead, improved designs “could help people drive through a roundabout safely the first time they encounter it.” These measures could include more prominent signage and pavement markings or changes to the roundabout’s curvature or the sizing of splitter islands. Even “landscaping that limits drivers’ ability to see across the roundabout (could) promote slower speeds,” the press release suggests.

Central question

The issue of what to put or not put in the roundabout’s central island is debated among traffic engineers. In the mini-roundabouts, the central island is paved precisely so that vehicles — especially large trucks — can drive across the island when necessary. But for larger roundabouts, the central island is often raised several inches or more above the grade of the road and can feature either landscaping or other objects, including sculptures, monuments, and even fountains. Such visual obstructions can “make the drivers focus on the road in front of them instead of looking across the roundabout,” says Hu. 

It can be especially useful to block the view across the central island at night, when the headlights of vehicles approaching the roundabout might give drivers on the opposite side of the circular intersection the false notion that the road continues straight, notes Brian Walsh, P.E., the state traffic design and operations engineer at the Washington State Department of Transportation and the chair of the Transportation Research Board Standing Committee on Roundabouts and Other Intersection Design and Control Strategies.

That is why his department has created design guidance to encourage raising the elevation of some central islands with hill-like mounds — parabolic shapes made from river rocks and other natural materials — to heights of 6 ft or more to block the views of the headlights on approaching vehicles. Together with reflective signage and even illumination of the central island, the approach should help nighttime drivers “realize there’s something in the middle there, so they make better decisions on slowing down as they approach and deflecting or going around,” Walsh explains. “We want them to see that there’s (something) in the tangent road, that it isn’t straight.” The geometry ahead of the roundabout intersection can also help drivers see the curvilinear elements of the roundabout intersection.

Other traffic engineers contend that the central island should not have fixed objects such as statues or other hard structures because drivers — admittedly, not always sober or fully in control of their vehicles — are “having very serious collisions” with such obstacles in roundabouts, says Lalani. Instead, he recommends that there should only be “forgiving things” in the central island, such as low bushes that can be driven over or things that can be easily knocked aside. 

With respect to fixed objects within the central island, the context of the site matters, says Johnson. Fixed objects should be avoided in roundabouts located along high-speed roads — which he defines as roadways with posted speeds of 45 mph or higher. They are less of a concern along low-speed roads where “the probability of hitting the central island is much lower — and so it might be acceptable to risk putting in, for example, a statue or other fixed object” that may be used to represent the local culture, history, or art, Johnson explains.

One of the most unexpected things drivers can find in a roundabout’s central island is a set of train tracks. While some engineers and railroad officials oppose constructing a roundabout anywhere near a railroad crossing, there are successful examples. In both Salt Lake City and Mesa, Arizona, for instance, the track alignment for light-rail lines passes right through the center of certain roundabouts. And in the state of Washington, Walsh says there are two examples of railroads in close proximity to roundabouts where the roundabout operations provide better movement and lower the likelihood of cars being stopped on the tracks than was the case at the intersections that the roundabouts replaced.

Walsh’s agency is working on a design that will replace several signalized intersections with a series of roundabouts, including one that will be located about 20 ft from a railroad branch line serving an industrial park. The design will take into account methods to ensure that drivers stay off the tracks while potentially waiting for a gap in the roundabout, Walsh explains.

Widening the circle

Large trucks can be accommodated via design features and geometrics that often include a traversable apron that surrounds the central island’s inner edge, while multilane roundabouts “are designed either to allow large vehicles to track across more than one lane while entering, circulating, and exiting or to stay within their lane,” according to the Informational Guide

modern circular roadway or roundabout
Modern roundabouts can be constructed at single intersections, in pairs at nearby intersections, or in series at multiple intersections along a route. (Photograph courtesy of GHD)

Many of the first modern roundabouts did not include such aprons, notes Lalani, and had to be reconfigured later after numerous crashes had occurred. Measuring as much as 15 ft wide, the aprons are raised high enough to discourage passenger cars and other smaller vehicles from trying to drive over them but not so high as to hinder the large tires of trucks, Lalani explains. They are especially common in industrial areas or close to freeway interchanges where many trucks can be expected.

Early roundabouts were also designed with straight-faced curbs on the central and splitter island structures as much as 6 in. in height, which can be “hard on tires” if a truck strikes the curb, notes Walsh. Consequently, Washington’s transportation department designed a tapered curb that has become a standard element in its roundabout designs since 2007. 

Flared entrances are another measure used to accommodate the wider berth needed by large trucks and can also be designed to increase intersection capacity, Johnson notes. The approach has proved effective at reducing overall roadway improvement costs, he adds.

Although roundabouts can generally improve traffic conditions over traditional, signalized intersections, they can require tweaks or alterations as drivers adapt to them. In Carmel, Indiana, for instance, the first few roundabouts constructed had to be reworked because there were too many minor traffic accidents occurring, in part because people were driving through the roundabouts too quickly, says Jim Brainard, the city’s mayor. But after some relatively minor and inexpensive adjustments — mainly to the design angles — the roundabouts became much more efficient and effective. 

Since 1997, Carmel has constructed 133 roundabouts, with six more under construction. Brainard first saw such systems in the United Kingdom while a graduate student at Oxford University and quickly became a believer. Thanks in part to the city’s many roundabouts, the fatality accident rate in Carmel — measured by a five-year average from 2015 through 2019 — is just two per 100,000 people. In comparison, the fatality accident rate in nearby Indianapolis is nearly 12 per 100,000 despite the fact that the two cities experience the same weather and driving conditions, Brainard says.

Among other factors, the slower speeds that drivers generally use while traversing a roundabout often lead to greater safety, especially for pedestrians. “At slower speeds you get more reaction time,” Brainard notes.

But even at slower speeds, “we move 50 percent more cars per hour” compared with previous traffic flows that had to contend with delays at signalized intersections, Brainard says. For example, on a major road that runs through the center of Carmel — originally featuring four travel lanes and a center turning lane — the city replaced the traffic lights with roundabouts. As a result, the city removed one lane in each direction, added a bike path and a large median with trees, and enlarged the sidewalks “so pedestrians felt safer,” Brainard says. Yet the overall travel times did not decline. Individual vehicles might not move as quickly as they used to, Brainard says, “but you get to your destination in the same or less time because you don’t stop for lights.”

Life cycle costs

Roundabouts are not inexpensive — a single-lane roundabout costs roughly $1.2 to $1.8 million to construct while multilane roundabouts can cost more than $2 million each, according to estimates from the Wisconsin Department of Transportation. But they can provide substantial cost savings in other ways. Cities or counties no longer have to pay for the annual maintenance, electricity, and supplies for traffic lights at intersections that use roundabouts. And if a storm knocks out power, the roundabout keeps functioning; the city or county no longer needs to deploy police officers to direct traffic through intersections with dead traffic lights. 

At the same time, the designers of roundabouts should keep other costs in mind, especially the costs of maintaining the central island. Johnson, who worked at the Wisconsin Department of Transportation for four years before starting his roundabouts firm, realizes that most public agencies do not want to spend a lot of time or money mowing or weeding in the center of roundabouts. So he likes to select a landscape plan that is functional and economical, both in terms of safety and ongoing maintenance. 

There are also environmental benefits to roundabouts because fewer vehicles have to sit at traffic lights, idling and spewing greenhouse gases into the air. And fewer vehicles must come to dead stops at lights and then get moving again, which requires more fuel than if traffic had kept flowing as it generally would in a roundabout, Brainard says. To confirm such benefits, Brainard had the city engineer monitor vehicles at roundabouts and calculate the average fuel consumption involved. The engineer concluded that Carmel drivers save as much as $4 million in fuel each year at every intersection that was converted to a roundabout, Brainard says.

Sometimes, though, traffic must stop in a roundabout, even if just briefly. In Richland, Washington, for instance, a multilane roundabout was constructed in 2007 at the intersection of Steptoe Street and Columbia Park Trail, near a State Route 240 off-ramp — previously the site of a traffic bottleneck in the area, says Walsh. But when the new roundabout attracted more drivers making regional trips sooner than anticipated, traffic started to back up onto the off-ramp during peak times, which created the potential for high-speed, rear-end collisions on the freeway.

The Steptoe roundabout reached its capacity on a portion of the circle during just one hour or less of the day, Walsh explains. The solution was a signal metering system that controls entry into the roundabout at one location for just a short period to keep the traffic flowing on the off-ramp, Walsh says. After that brief period, the meter shuts off and the roundabout goes back to its normal operation. 

Although roundabouts can be controversial when first proposed for certain locations, IIHS studies show that opposition generally falls over time as the public becomes more familiar with the systems and learns how to navigate the lanes. In one study that Hu worked on, for instance, driver support for two roundabouts near Bellingham, Washington, increased from just 34 percent before construction to 70 percent more than a year after the roundabouts went into operation. 

The community engagement process has been the most difficult aspect of building roundabouts, especially for “those of us in the agency that saw the first ones proposed and constructed,” notes Walsh. But as more roundabouts are completed and the public becomes more receptive, newer projects benefit from the lessons learned during the earlier projects. 

Certain settings can be better or worse sites on which to build roundabouts — land that is too steep can be challenging as can locations where heavily traveled roads cross lightly traveled routes. And available right of way in developed areas “is a huge issue that often limits the ability to provide a well-designed roundabout without encroaching on adjacent private property — and the owners’ cooperation is not always guaranteed,” notes Lalani.

Still, roundabouts are turning up in places once considered unworkable. Historically, people worried that roundabouts could not be built safely in areas frequented by pedestrians or large trucks or in other supposedly impractical situations that have since proved to be quite practical and constructible after all. In December 2020, for instance, the world’s first undersea roundabout opened as part of an 11 km long tunnel connecting two islands belonging to Denmark’s Faroe Islands archipelago. 

On some projects “you have to do a lot more analysis to understand what is feasible and what isn’t feasible,” notes Johnson. But he is not a fan of blanket declarations that a roundabout will not work in this location or under those conditions. The key, Johnson explains, “is to look at (the situation) carefully and understand from a design and analysis perspective what you need to make it work safely.” 

This article first appeared in the March/April 2021 issue of Civil Engineering as "All about the Roundabout."

Cover photograph by Brian McGuckin, courtesy of the City of Carmel, Indiana