Designing Roundabouts for Safer, Smoother Traffic Flow: When and Where They Work Best

Roundabouts are more than just a traffic trend—they’re a modern solution to some of our most persistent intersection challenges. From improving safety and reducing congestion to lowering emissions and enhancing community aesthetics, modern roundabouts offer a compelling alternative to traditional traffic signals and stop signs. But not every circular intersection is created equal. Understanding what sets modern roundabouts apart—and where they work best—can help cities and transportation departments make smarter, safer infrastructure choices.

Despite their increasing popularity, roundabouts often remain misunderstood. In this article, our team is drawing on their extensive experience to answer your most pressing questions on the topic.

What separates a modern roundabout from other circular intersections?
First, traffic in the circle always has the right-of-way. This allows traffic to flow continuously because circulating traffic clears out for new entering traffic. The second identifying characteristic is cars enter the roundabout with a yield condition. Traffic wanting to enter the circle must stop for those already in the roundabout. But if there is no traffic circulating, entering vehicles only slow down and do not stop at all. This design allows traffic to flow smoothly, up to a certain volume.

In which instances are roundabouts effective?
If an area has an intersection with lots of access points, multiple intersecting roads, or heavy traffic volume, a roundabout might be the best solution. They are also effective for intersections with safety issues, such as ones that don’t have any traffic control measures and connect with a busy side road where drivers are sitting for a long time. Adding a roundabout makes it safer to make left-turns and creates opportunities to improve flow.

What kinds of benefits do they offer?
Along with improving safety and traffic flow, roundabouts offer two other notable benefits. First, they add an attractive aesthetic element to communities. Often, the central island is landscaped or even used as a gateway to a community. This looks much better than signal poles and mast arms.

In addition, they are more environmentally friendly. They save gas, because cars don’t have to come to a complete stop at the intersection. They can just slow down. There is not as much acceleration as opposed to starting from a complete stop, resulting in lower gas used and emissions.

How do roundabouts improve safety?
They improve safety in a number of ways. To begin, a modern roundabout reduces the number of potential conflict points for vehicles to crash into each other. A normal intersection has 32 conflict points, while vehicles in a roundabout have eight.

Known as a traffic calming device, they are also designed to reduce speeds. This gives drivers more time to react to hazards, making their design inherently safer.

Finally, right angle collisions (“T-bones”), which often result in the most severe injury and property damage, are essentially eliminated. When accidents do occur, they are often side swipes at lower speeds and less detrimental angles, causing less severe injuries and reduced property damage.

How do roundabouts vary?
Roundabouts vary in a few ways:

• Size: For example, in rural areas, you may find larger ones to accommodate bigger vehicles. In contrast, you’ll see smaller roundabouts in urban areas due to size constraints.
• Shape: Not all roundabouts are a perfect circle. Some are ovals. Others, at an interstate interchange, might have a “teardrop,” “dumbbell,” or “dog bone shape.” If more than two roads intersect, they may look like a peanut.
• Number of Lanes: Depending on how much traffic volume an intersection sees, roundabouts can have one, two, or even three lanes (in rare occasions).

Despite those differences, the design principles remain the same no matter what. The circulating traffic will still always have the right-of-way and entering vehicles must yield.

Tell us about design considerations when choosing a roundabout.
There are several items to consider.

1. We recommend looking at the terrain to determine how hilly it may be. Roundabouts can be “tipped,” “tilted,” or “warped” to work in these areas, but not too much.
2. Another element is roundabouts usually require more space and right-of-way than a traditional four-way stop controlled intersection.
3. Lighting is also a factor. If there isn’t any already at the intersection, it must be added for nighttime driving
4. We also suggest looking at how closely the roundabout may be to a signalized intersection. If they are near one another, it may not work well. Traffic can begin to back up into the roundabout in this instance.

How do roundabout impact other site elements, like stormwater and landscaping?
While roundabouts don’t require any special stormwater infrastructure, they do affect other design elements. Landscaping is often a key consideration. Some cities like to use the land for signage, creating a gateway to their community. Others might add statues or plantings to enhance aesthetics and improve effectiveness. It may come as a surprise that roundabouts operate better when drivers can’t see across to the other side, as it can impact their decision-making.

If grass or plants are added, cities should plan for regular maintenance. To avoid this, some choose relatively lower maintenance solutions, such as paving the center of their islands. In those cases, a gate may be added to the middle to better accommodate vehicles hauling an oversized load. They can drive across the island if needed.

Although they can accommodate many pedestrians, roundabouts do come with some challenges for those who are visually impaired, because they don’t include crosswalk signals. In fact, there is some discussion within the industry about how to best handle it. However, a complete resolution has not been found.

Where don’t roundabouts work well?
Along with the site specifications mentioned above, there is a limit to how much traffic a roundabout can handle. A single lane roundabout can keep 25,000 vehicles per day moving. A dual lane can manage about 40,000. Once those traffic levels are met, the system begins to break down, and a signal may be the best solution.

We also find they aren’t as effective at intersections without balanced traffic volumes on the side roads. This is because, they slow the main roadway down for no real reason. However, if those connecting roadways are busy, they can be a great solution to improving traffic flow.

What should communities and Departments of Transportation (DOT) know before implementing one?
They should have a good understanding of the pros and cons to determine if it is the right solution for the intersection in question. Depending on the size of the roundabout, more right-of-way may be needed than what the community or DOT has.

Aside from design considerations, they should also plan on a public outreach component. It is not uncommon for roundabouts to initially be poorly received by the public, despite their increased prevalence. We recommend integrating an educational component into their communication plan.