Roundabouts and Rotaries and Traffic Circles, Oh My!

By Jackson Radler

If most of your driving career has been in North America, it’s likely that circular traffic intersections are a bit unconventional. We Americans like intersections clean and efficient: with sharp 90° angles and computer-controlled lights maintaining an orderly flow of traffic. So, what’s the deal with these loopy curvy European contraptions? Are they actually better at getting traffic where it needs to go? What about safety? And while we’re at it, let’s go through a basic field guide so you can properly identify these circular contraptions out in the wild. You’ll be the life of the party once you start talking about the difference between roundabouts and a traffic circles, I promise.

Chapter 1: Are All Circles Created Equal?

Our story starts with one particular intersection: Columbus Circle (a traffic circle) in New York City (Figure 1, left). Its unveiling in 1905 started a traffic circle construction craze in cities across the United States. A traffic circles is a group of intersections (each equipped with traffic lights) connected by a circular road. Often serving more of an aesthetic role than a functional one, they can be found downtown in many cities, sporting large central islands adorned by monuments. While they look pretty, their large size promotes high speed and chaotic weaving of vehicles. What’s even more problematic is that priority is commonly given to entering vehicles, resulting in too many cars entering and the circle and becoming “locked up”.1

Another type of intersection introduced in the early 1900’s, the rotary, keeps the circular shape but does away with traffic lights. Rotaries function similarly to highway interchanges; they have wide lanes and often don’t require vehicles to yield. This facilitates entry and maneuvering at high speeds (40-60mph), which is fine for interstates but not for smaller roads and local streets.2

In the 1960’s, traffic engineers in the United Kingdom decided that they could do better than the Americans. They designed what would become the first modern roundabout (Figure 1, right). Central to this design was 1) requiring all entering vehicles to yield to circulating traffic and 2) making the circle smaller. Requiring vehicles to navigate tight turns forces them to slow down, and this is key to the function of a roundabout.1

Figure 1. Left: Columbus Circle, NYC. Notice it is actually 5 full intersections (with traffic lights) feeding into a circular road. Right: A modern roundabout. Significantly smaller and controlled only by yield signs.1

Chapter 2: Conflict

In a normal four-way intersection, turning vehicles must cross paths with the oncoming stream of traffic. In a roundabout, all maneuvers are either merging or diverging, no crossing required. This may sound familiar; it is the same principle behind highway on- and off-ramps. With all vehicles entering one stream of flow, collisions in a roundabout are typically low-angle ‘fender benders’. The design of roundabouts eliminates the possibility of severe head-on and right-angle (‘t-bone’) collisions, which are prevalent in four-way intersections (Figure 2).1

Figure 2. Primary Crash Types. The most severe types of collisions (head-on and t-bone) are eliminated in roundabouts.2

Beyond changing the angle at which collisions can occur, the geometry of a roundabout also reduces the number of potential conflict points, locations where travelers may cross paths in an intersection. With 75% fewer locations for drivers and pedestrians to watch out for potential obstacles (Figure 3), roundabouts offer a significant reduction in collisions.3 When traditional four-way intersections are converted to roundabouts, there is a 37% reduction in the total number of collisions on average.1

Figure 3. Conflict Points in Intersection Design.2

Chapter 3: Speed

When conflict does occur between a vehicle and a pedestrian, speed is a major factor in determining the probability of pedestrian fatality. At 30 mph, 40% of collisions are fatal for pedestrians, while at 20 mph only 5% are fatal (Figure 4).5 Roundabouts force drivers to reduce speed by virtue of their physical shape. This is a radically different mechanism from traffic lights or stop signs, which only provide an indication that vehicles should slow or stop without imposing any physical constraint. Traffic lights and stop signs rely on driver obedience, not a great system when lives are at stake.

Figure 4: Probability of pedestrian fatality if hit by a vehicle traveling at various speeds.5

Over the years, local governments converting intersections to roundabouts has provided a natural experiment to evaluate the safety benefits that roundabouts offer. In 2021, PennDOT released data collected over from 26 intersections that were converted to roundabouts (from stop sign or traffic light controlled). All intersections included in the analysis had police-submitted collision data spanning at least 3 years. The findings were as follows:

  • Fatalities were reduced by 100%
  • Serious injuries were reduced by 81%
  • Minor injuries were reduced by 36%
  • Total number of crashes was reduced by 22%
  • Crashes causing vehicle damage only (no injury) increased by 13%

These data indicate that roundabouts are not only effective in reducing the total number of collisions, but also in converting a proportion of injury-causing collisions to vehicle-damage only collisions.5 Such benefits are attributable to collisions occurring at lower speed (compared to conventional intersections), as mandated by a roundabout’s design.1

Chapter 4: Efficiency

So, all of that is great, but are roundabouts as good at moving vehicles through an intersection as traffic lights or stop signs? While efficiency varies depending on the specific intersection, the US Department of Transportation reports that roundabouts “typically operate with lower vehicle delays” than conventional intersections.1 One limitation of roundabouts is that vehicles from all roads are treated with equal priority, so they may not be as efficient for intersections between a high-volume road and a local street. However, roundabouts excel in off-peak conditions, as it is unnecessary for traffic to stop when the intersection is sparsely populated. Vehicle queues are also usually shorter, as traffic is allowed to move continuously, reducing the impact of backed-up cars on nearby streets. These shorter queues also offer a psychological benefit; drivers tend to tolerate slow but continuous progress better than waiting in a standstill.1

Conclusion

Americans have been slow to catch on, but many governmental agencies are now encouraging the use of roundabouts in intersection design. Following this trend, many conventional intersections (as well as rotaries and traffic circles) in the US have recently been converted to small, modern roundabouts (Figure 5). While roundabouts may not be perfectly suited for every application, the Federal Highway Administration recommends them as a “proven safety countermeasure” to promote lower speeds, and to improve safety for motorists and pedestrians alike.6

Figure 5: Conversion of a Rotary (large circle) to a modern Roundabout (small circle) in Kingston, NY. New York State Department of Transportation, https://www.dot.ny.gov/I587Intersection

TL:DR

The tight turns of a small, modern roundabout forces drivers to slow down, and this makes the intersection much safer for everyone. Traffic Circles and Rotaries are much larger circular intersections, and do not offer the same safety benefits.

Note from the author:

It is not appropriate to use the word “accident” when discussing motor vehicle crashes. This language implies that these events are unavoidable, and does not encourage work to increase safety. Instead, the Federal Highway Administration encourages use descriptive words such as “crash,” “collision,” and “injury”.7


References

  1. Roundabouts: An Informational Guide. US Department of Transportation Publication No. FHWA-RD-00-067
  2. massDOT Guidelines for the Planning and Design of Roundabouts. March 2022. https://www.mass.gov/doc/massdot-guidelines-for-the-planning-and-design-of-roundabouts/download
  3. NCHRP Report 572: Roundabouts in the United States. National Cooperative Highway Research Program, TRB, NAS, Washington, D.C., 2007.
  4. Leaf, W. A. and D. F. Preusser. Literature Review on Vehicle Travel Speeds and Pedestrian Injuries. Final Report DOT HS 809 021. National Highway Traffic Safety Administration, Department of Transportation, Washington, D.C., October 1999
  5. Pennsylvania Department of Transportation. Road Design: Roundabouts. https://www.penndot.pa.gov/ProjectAndPrograms/RoadDesignEnvironment/RoadDesign/Pages/Roundabouts.aspx
  6. Roundabouts, Intersection Safety. US Department of Transportation, Federal Highway Administration. https://safety.fhwa.dot.gov/intersection/roundabouts/index.cfm
  7. Reagle, 1997. Federal Motor Carrier Safety Administration, Newsroom. https://www.fmcsa.dot.gov/newsroom/crash-not-accident

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