Traffic Flow That’s Just Right

Aug. 14, 2023
When to build roundabouts, and why they’re optimal for safety

By Noah Carmichael, PE, Contributing Author

For many drivers, roundabouts are anything but straightforward.

In communities that have adopted roundabouts, however, the learning curve for drivers has proven to be short and these circular intersections are a welcome change.

Although various forms of roundabouts have been around for more than 200 years, they became more commonplace in Europe and Australia in the 1960s after they were redesigned for improved safety. But it’s only been in the past 20 years that roundabouts gained popularity on this side of the Atlantic Ocean.

Wisconsin installed its first roundabout in 1999. Today, it has more than any other state with over 500, according to its Department of Transportation.

The mix of small- and mid-sized communities in Wisconsin might be one reason the state lends itself so easily to roundabouts. Replacing traditional intersections with roundabouts is ideal for situations where there is too much traffic for a stop sign, but not enough for a stop light. They also benefit communities experiencing growth, allowing additional traffic to flow through intersections without the cost and impact of adding lanes.

Installing a roundabout is far from a one-size-fits-all solution. That’s why engineers need to carefully evaluate any proposed intersection before creating a roundabout – so effective solutions are maximized for scenarios that need them most.


There are three key factors to consider when evaluating an intersection for a roundabout: traffic volume, land availability, and accident data.

Roundabouts are a classic case of Goldilocks and the Three Bears when it comes to traffic flow: Too much traffic, and the circle becomes clogged. Too little traffic, and it’s a waste of resources. Roundabouts work well when traffic flow is just right.

Simplicity is the best choice in road design. If a stop sign is sufficient, leave it there. If it needs something more, then placing a roundabout in that intersection is a perfect step up between a stop sign and a stop light.

Land availability is another consideration for roundabouts. The circular design takes up more real estate than a traditional intersection. In some cases, there isn’t the clearance to size the roundabout because buildings, homes, or other structures are in the way.

The diameter of the inscribed circle is determined by the vehicle type, traffic volumes, and speed limit. Roundabout diameters can vary widely based on these factors. Mini-roundabouts in urban, low-traffic areas may have a diameter as small as 45 feet.

On the other hand, a multiple-lane, higher-speed, and volume design could have a diameter of 200 feet or greater. While roundabouts require additional land because of the circular design, they allow for fewer travel lanes between intersections, offsetting the overall land impacts.

Analyzing historical accident data also plays a critical part of intersection design. Roundabouts should be considered at intersections with a high volume of accidents or fatalities. Statistically, the most dangerous move in driving is a left-hand turn. Making a left-hand turn in front of oncoming traffic is how the most serious crashes happen. A roundabout eliminates this turn.

Traffic within a roundabout moves slowly and in the same direction. The ideal vehicle speed for roundabouts is between 20 and 25 miles per hour, which reduces the likelihood of serious collisions. While roundabouts will have their share of sideswipe or rear-end accidents, catastrophic head-on collisions are highly unlikely.

Although roundabouts don’t reduce the overall number of crashes at an intersection, they are effective at decreasing the severity of accidents and improving the overall safety of the roadway corridor.

Nontraditional Designs 

Design considerations need to be given to unique roundabout setups. These include areas where streets enter at unusual angles, intersections with space constraints, or accommodating oversize, overweight (OSOW) vehicles. Flexibility is another benefit of the roundabout design.

Through creative design, engineers can develop a solution that maintains the intersection’s ability to flow traffic smoothly without unnecessary delay while addressing unique criteria.

One common design modification is the inclusion of a bypass lane. Typically, these lanes are added to intersections with a large volume of right-turning traffic. The right-turn bypass lane allows right-turning traffic to bypass the roundabout, providing additional capacity for the remaining traffic through the intersection.

Bypass lanes increase capacity without adding travel lanes to the roundabout. Intersections designed to accommodate OSOW vehicles are often modified to include truck aprons, mountable curbing, tapered center islands, or other geometric variances. For more complex situations, the engineer may look to include more “exotic” designs, such as the turbo roundabout, teardrop roundabout, or two-geometry roundabout. These designs can be incorporated to meet the most unusual circumstances.

Roundabouts also tend to be more pedestrian-friendly than traditional intersections. Since traffic flows in a single direction, vehicles are much easier for pedestrians to spot. Roundabouts also feature slower speeds, shortened crosswalks, and a pedestrian refuge in the splitter island. These design features add up to a safer environment for pedestrians and cyclists.

Roundabouts in busy commercial areas can present their own set of challenges. We’ve all visited areas with driveways every few feet leading into a range of big-box stores, office parks or strip malls. A successful design can’t block the entrance to a restaurant, but it also can’t accidentally send motorists into a grocery store parking lot rather than onto the correct road. Put special thought into roundabout design early in the process to make sure access points can be accommodated.

Roundabout Benefits

Roundabouts are a proven way to improve safety. The primary reason, of course, is speed. Cars simply can’t traverse the curves of a roundabout at the same speed they carry down a straightaway. Roundabout accidents occur, but they tend to be minor because of reduced vehicle speed and the simple geometrics of the collisions. 

According to the National Highway Traffic Safety Administration (NHTSA), one in five traffic fatalities happens at an intersection. Additionally, half of all serious traffic injuries come from intersection crashes. Simply put, lives can be saved by designing safer intersections.

Roundabouts are also more efficient. There’s not a forced stop, like at a traffic light. That keeps traffic flowing through intersections more smoothly, reducing delays

There’s less wear and tear on vehicles, too. Roundabouts are easier on a car’s brakes and transmission, while also showing improved emissions. The car isn’t coming to a complete stop, idling and restarting.

There are upsides for municipalities, too: Without upkeep for traffic signals a roundabout is an excellent way to reduce ongoing maintenance costs.

A roundabout isn’t always the ideal solution, however.

Consider an intersection with an average daily traffic (ADT) of 35,000 vehicles. It clearly exceeds the standards for a four-way stop intersection, but upon further review, there’s only enough right of way to create a single-lane roundabout. Even with the most creative roundabout design, the roadway ADT exceeds the 25,000 vehicles per day capacity of the intersection. Despite the many benefits of the circular design, a roundabout simply won’t function effectively, causing traffic backups at this intersection.

A critical component is evaluating a community’s projected growth over the roadway design period. It is more difficult to add capacity to a roundabout than a standard intersection. The circular traffic patterns add an additional level of complexity when attempting to complete the staged construction under traffic. Detours and impacts to the entire transportation network must be considered under this scenario. 

It is critical to reflect realistic or higher-than-expected growth rates when designing roundabouts. Even if there is plenty of available land, providing alternate or detour routes during construction could be challenging.

A Staple for the Future

It’s clear that roundabouts are here to stay in the United States. Used appropriately, they give communities the ability to graduate growing traffic levels while maintaining safe and smooth traffic flow.

For motorists, education on roundabouts comes with exposure. As roundabouts become more normalized, drivers become more adept at navigating them and moving flawlessly through intersections.

That gives engineers endless possibilities in their toolboxes moving forward, as new roundabout patterns emerge to handle increasingly complicated intersections. Expect to see more roundabouts throughout the U.S. in coming years. Those alternatives make roundabouts a possibility in areas where real estate limitations or skewed road angles come into play.

Engineers are continuously pushing the envelope on roadway and intersection design. It’s up to the designers to view each intersection as unique and build the custom plan that will best serve its community for years to come. R&B

Noah Carmichael, P.E., is Fehr Graham’s Lead Transportation Engineer. Reach him at [email protected] or 815.562.9087.  

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