In the face of urban growth and rising traffic volumes, the search for innovative, efficient, and safe traffic management solutions has become paramount. Could roundabouts be part of the answer? Widely adopted in European cities, these structures are gradually making inroads in the United States, with 500 to 700 new installations annually.To understand the significance of roundabouts, let's examine their historical context. Grand traffic circles like New York City's Columbus Circle and Washington, D.C.'s Dupont Circle marked significant engineering achievements in their time. Designed for high-speed entries with multiple lanes, they also presented major challenges, including often complicated vehicle movement, collision risks, unclear yielding norms, inconsistent signage, and a lack of secure pedestrian crossings.

Recognizing value in the European model, U.S. states slowly began to adapt and Nevada pioneered the first modern roundabout in 1990. From modest beginnings, roundabout culture in the U.S. has seen significant expansion, growing from only 150 roundabouts in the 1990s to an estimated 9,000 today.

Unlike their predecessors, the core design principles of modern roundabouts prioritize safety and efficiency. The geometry of entry and exit lanes and sight distances are pivotal to these principles. Clear signage in the right places helps drivers know what is ahead, reducing sudden and dangerous moves. The addition of bike lanes and crosswalks forces drivers to slow down, improving safety.

Rising interest in slowing drivers and increasing the safety of pedestrians and bicycle riders has caused traffic engineers and urban planners to renew their efforts to refine the design of roundabouts. Today’s roundabouts typically fall into two categories: single-lane roundabouts, which are commonly found in areas with moderate traffic, and multilane roundabouts, which are designed to handle higher traffic volumes.

Single-lane roundabouts are simpler in design and easier for drivers to navigate due to the singular traffic lane. They are efficient for smoothly controlling the flow of vehicles in areas that do not experience high congestion.

Branching from standard designs, single-lane or mini-roundabouts provide many of the advantages of standard roundabouts while requiring less space. They have proven to be ideal for areas with existing low speeds where there's not enough room for bigger roundabouts with elevated centers. Mini-roundabouts are becoming increasingly prevalent in the U.S., with the highest adoption found in Maryland and Michigan.

While the U.S. has tended to favor single-lane roundabouts, multilane roundabouts are growing in popularity and use. Unlike their smaller counterparts, however, multilane roundabouts present more issues. The Federal Highway Administration (FHWA) Roundabout Technical Summary notes, for example, that when there is a significant gap between entry and exit points, vehicles entering the roundabout might find themselves driving parallel to those already circulating instead of crossing paths. Because of the design, drivers on the outer lane can mistakenly assume that those on the inner lane should first find a safe spot in the outer lane before exiting. This design ambiguity can cause drivers to think they are navigating a "concentric roundabout" (lanes move in parallel) when, in fact, it is a "crossing roundabout" (lanes cross over each other).

Considering a solution to multilane challenges, a Minnesota study identified two primary accident types in such roundabouts: one where incoming vehicles do not yield to those already inside, and another where drivers, having chosen the wrong initial lane, change lanes within the roundabout. Interestingly, unfamiliarity seemingly plays a key role in such situations. Many first-time drivers opt for the outer lane even if they intend to turn left because they associate the outer lane with exiting. A driving simulator study revealed a substantial portion of drivers struggle with lane choice in multilane roundabouts.

To counteract such issues, the FHWA has followed some other countries in considering turbo roundabouts in their “Advancing Turbo Roundabouts in the United States: Synthesis Report.” Aiming to resolve the typical crash causes associated with multilane roundabouts, the turbo roundabout's distinctive design ensures smoother traffic flow by using specific features such as a second lane opposite an entry point, requiring incoming vehicles to yield to two lanes of circulating traffic, and barriers to prevent mid-roundabout lane changes.

The turbo roundabout’s design is also compact, slowing vehicles, while the roads leading into it are perpendicular. Views for incoming vehicles are strategically blocked, and there is additional space provided for larger vehicles. Critically, this layout obliges drivers to choose their intended exit lane before even entering the roundabout.

Adapting from the lessons of the past and the innovations of today, urban planning is anything but static. It constantly evolves, reflecting the changing needs of society and advancements in engineering. With the advent of autonomous vehicles and smart traffic systems, roundabout design will continue to evolve. Considering the challenges of integrating these technologies with existing infrastructure, sensors, real-time traffic data, and vehicle-to-infrastructure communication will refine roundabout operations, maximizing efficiency and safety.

As traffic engineers and urban planners, we bear the responsibility for ushering in this new era of transportation. Continual innovation, informed design choices, and adaptability are imperative. Roundabouts and other innovations with proven efficiency and safety benefits will be at the forefront of this evolution. Our challenge is not just in perfecting the roundabout design of today, but in envisioning and shaping its role in the cities of tomorrow.

The Traffic Group is a Service-Disabled Veteran-Owned Small Business and a Maryland-based traffic engineering and transportation planning firm. For more information visit trafficgroup.com.