B]Roundabouts are a safer alternative to traffic signals and stop signs[/B]. The tight circle of a roundabout forces drivers to slow down,
and the most severe types of intersection crashes — right-angle, left-turn and head-on collisions — are unlikely.
Roundabouts improve traffic flow and are better for the environment. Research shows that traffic flow improves after traditional intersections are converted to roundabouts. Less idling reduces vehicle emissions and fuel consumption.
Roundabouts generally are safer for pedestrians. Pedestrians walk on sidewalks around the perimeter and cross only one direction of traffic at a time. Crossing distances are relatively short, and traffic speeds are lower than at traditional intersections.
The first modern roundabouts in the United States were constructed in Nevada in 1990. Since then, many more have been built, although the precise number is unknown. Roundabouts are much more common in some other countries, including Australia, the United Kingdom and France.
Although some states and cities have been slow to build roundabouts, they are gaining more popularity in the United States.
Roundabouts are one of the evidence-based safety countermeasures recommended by the Federal Highway Administration.
Some states, such as New York and Virginia, have adopted "roundabout first" policies requiring that roundabouts be considered a preferred alternative when building new intersections or upgrading older ones if feasible (New York State Department of Transportation, 2011; Virginia State Department of Transportation, 2009).
Roundabouts are appropriate at many intersections, including high-crash locations and intersections with large traffic delays, complex geometry (more than four approach roads, for example), frequent left-turn movements, and relatively balanced traffic flows. Roundabouts can be constructed along congested arterials and at freeway exits and entrances, in lieu of traffic signals.
Sometimes space constraints or topography make it impossible to build a roundabout. Geometric design details vary from one site to another and must take into account traffic volumes, land use, topography and other factors. Roundabouts often require more space in the immediate vicinity of the intersection than comparable traditional intersections.
However, because roundabouts can reduce delays and queue lengths, they require less space on the approaching roads than comparable intersections controlled by stop signs or traffic signals.
An intersection with highly unbalanced traffic flows (that is, a very high traffic volume on the main street and very light traffic on the side street) may not be an ideal candidate for a roundabout. The same is true for isolated intersections in a network of traffic signals.
While the initial construction cost of a roundabout varies site by site, its maintenance usually is cheaper than for intersections with signals. The service life of a roundabout is significantly longer, approximately 25 years, compared with 10 years for a typical traffic signal (Rodegerdts et al., 2010).
At traditional intersections with stop signs or traffic signals, some of the most common types of crashes are right-angle, left-turn, and head-on collisions. These types of collisions can be severe because vehicles may be traveling through the intersection at high speeds. With roundabouts, these types of potentially serious crashes are essentially eliminated because vehicles travel in the same direction and at low speeds.
The vehicle-to-vehicle conflicts that do occur at roundabouts generally involve a vehicle merging into the circular roadway. In the case of multilane roundabouts, conflicts may also occur as vehicles exit.
Studies of intersections in the United States converted from traffic signals or stop signs to roundabouts have found reductions in injury crashes of 72%-80% and reductions in all crashes of 35%-47% (Retting et al., 2001; Eisenman et al., 2004; Rodegerdts et al., 2007).
A study of 19 higher-speed rural intersections (speed limits of 40 mph or higher) that originally had stop signs on the minor approaches and were converted to roundabouts found a 62% reduction in all crashes and an 85% reduction in injury crashes (Isebrands & Hallmark, 2012).
Studies of intersections in Europe and Australia that were converted to roundabouts have reported 25%-87% reductions in injury crashes and 36%-61% reductions in all crashes (Rodegerdts et al., 2010).
Based on the results of a 2004 study (Eisenman et al., 2004), it’s estimated that the conversion of 10% of the signalized intersections in the United States to roundabouts would have prevented approximately 51,000 crashes in 2018, including 231 fatal crashes and about 34,000 crashes involving injuries.
Most U.S. studies have focused primarily on single-lane roundabouts. When included, two-lane roundabouts have been associated with smaller reductions in crashes compared with single-lane roundabouts (Retting et al., 2001; Eisenman et al., 2004; Rodegerdts et al., 2007) or with increases in crashes (Isebrands & Hallmark, 2012; Wang & Cicchino, 2022).
A 2019 IIHS study, however, showed that the safety of two-lane roundabouts improves over time, as drivers become more familiar with them (Hu & Cicchino, 2019). The researchers looked at roundabouts built in Washington state between 2009 and 2015. They found that crashes at two-lane roundabouts decreased an average of 9% a year. At the same time, the odds that a crash at a two-lane roundabout involved an evident or incapacitating injury decreased by nearly one-third annually.
In addition to having fewer serious conflicts between vehicles than traditional intersections, roundabouts are generally safer for pedestrians as well. In a roundabout, pedestrians walk on sidewalks around the perimeter of the circular roadway. If they need to cross the roadway, they cross only one direction of traffic at a time. In addition, crossing distances are relatively short, and vehicle speeds tend to be low.
Studies in Europe indicate that, on average, converting conventional intersections to roundabouts can reduce pedestrian crashes by about 75% (Brilon et al., 1993; Schoon & van Minnen, 1994). Single-lane roundabouts, in particular, have been reported to involve substantially lower pedestrian crash rates than comparable intersections with traffic signals (Brude & Larsson, 2000).
Crossing at multi-lane roundabouts can be more difficult for pedestrians than crossing at single-lane roundabouts. A study found that motorists failed to yield to pedestrians 2-3 times more at multi-lane roundabouts than at single-lane roundabouts (Rodegerdts et al., 2007). Another study found that drivers exiting a roundabout were less likely to yield to pedestrians than drivers entering a roundabout (Hourdos et al., 2012).
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