The Circular Logic of Roundabouts

Touting 100 roundabout intersections, the City of Carmel, Indiana leads the nation in implementing this proven concept for improving roadway safety and reducing traffic congestion. GAI’s new Transportation Technical Leader Jay Vorisek, PE joins us as a guest blogger to discuss the safety and efficiency benefits of roundabout solutions, his experience with these efforts, and Carmel’s roundabout success story.

I became a big proponent of roundabouts after I was first introduced to this traffic-management concept in the mid-1990s. Since then, I’ve worked on more than 70 projects involving roundabouts, and I continue to plug roundabout benefits—from reducing crashes and their severity to mitigating traffic congestion and lowering fuel consumption. Roundabouts simply make sense for many communities, particularly in the area where I live in Indiana.

Roundabouts vs Traditional Signalized Intersections

Roundabouts provide safe and efficient means of controlling traffic right of way. When two vehicles approach an intersection at the same time, one has to give way to the other. In the U.S., we primarily use either stop signs or traffic signals to manage traffic flow—a roundabout offers a third way of controlling who has the right of way at an intersection.

Intersection conflict diagram
Traditional four-leg intersections have 32 vehicular conflict points. Roundabouts decrease conflict points to just eight.

Slow Down to Go Around

From an operational standpoint, roundabouts have proven safer than traditional intersections. Traffic has to slow down to move through a roundabout, so the severity of crashes is greatly reduced when compared to crashes that occur at signalized intersections, where vehicles are traveling at or in excess of the posted speed.

In addition to causing the driver to slow, roundabouts require him or her to be more engaged. At traditional signalized intersections, drivers move when the light turns green, often without thinking about other vehicles coming from either side. At a roundabout, entering drivers must look to the left to see if anybody is going to come into conflict with them. This makes the driver into an active participant instead of simply reacting to a signal.

Roundabouts also reduce opportunities for crashes. The Roundabout Information Guide, produced by the Federal Highway Administration in 2000 and later updated by the Transportation Research Board, shows in the above diagram how a four-leg single-lane roundabout has 75 percent fewer vehicle conflict points compared to a conventional intersection.

No More Sitting in Traffic

Roundabouts also move traffic much more efficiently. Even though traffic moves slower through roundabouts, it usually continues to flow without long backups. Traditional intersections can cause miles of congestion during peak rush hours.

Roundabout Rules: Slow Down, Look Left, Know Your Exit

Navigating a single-lane roundabout takes three simple steps:

1. Slow down
2. Yield to traffic coming from the left
3. Enter and proceed to your exit point

The rules are similar when navigating a multi-lane roundabout, but drivers need to choose an entrance lane based on where they are heading: use the left lane for a left turn and the right lane for a right turn.

Carmel, Indiana: A Roundabout Sensation

The City of Carmel, IN recently celebrated the construction of its 100th roundabout, the most of any city in the U.S. In 1996, Carmel’s Mayor Jim Brainard recognized the benefits of installing roundabouts over traditional intersections in his city. One roundabout succeeded another at all types of intersections, from low-volume residential streets to busy highway interchanges. Along with improved safety for all road users, the new intersections reduced delays, noise, and air pollution. As a bonus, the city estimates that the roundabouts save Carmel’s drivers 2.5 million gallons of gas per year.

By installing roundabouts, many of Carmel’s roads now allow traffic to flow smoothly with fewer lanes, allowing for wider sidewalks, tree-lined medians, and separate bike paths throughout the city. Such improvements elevated Carmel to one of the most livable cities in the U.S.—a safe, walkable, vibrant mixed-use community.


Pennsylvania Street Reconstruction

Carmel, IN

Pennsylvania Street

GAI contributed to Carmel’s roundabout success in an accelerated project that involved reconstructing a two-lane street into a four-lane, boulevard-style street, with a landscaped median and left turn lanes. A roundabout at 103rd Street facilitated traffic flow, and a second roundabout at 106th Street replaced an existing traffic signal.


Jay Vorisek, PE has nearly 30 years of engineering experience. A recognized transportation design leader in Indiana, he has been involved in the development of more than 70 roundabouts, including Indiana’s first multi-lane roundabout in Carmel.

For more information about GAI’s infrastructure services, projects, and trends, contact Indiana Transportation Director David Vorndran at 317.436.4822 and Transportation Technical Leader Jay Vorisek at 317.436.4846.

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