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Jay Bohman WR 13300-11 Professor Clemmons Research Final Version 30 April 2016

Roundabouts: Finding the Right Way Introduction

Road transportation is a daily experience in many people’s lives, especially in

America. As a result, drivers, engineers, and city planners desire roadways that are safe,

efficient, and easy to use. Factors such as these must be considered for intersections that

require a new design solution, either because the existing model is inadequate or because

new roads are being developed. One unique type of intersection design is the modern

roundabout. They are characterized by traffic that travels around a central island and that

must yield to already circulating traffic (Rodegerdts et al. 1-3). While their usage in the

United States decreased after the mid-1950s, there are now thousands of roundabouts

incorporated into the nation’s automobile transportation system (FHWA 3).

In this paper, I argue that modern roundabouts are a highly favorable solution for

roadway intersections because of their benefits with regards to safety, environmental

factors, and traffic flow efficiency. I begin by addressing public opinion of roundabouts

and then explaining their safety benefits. Next, I demonstrate how roundabouts can

overcome commonly perceived drawbacks related to usage by pedestrians, cyclists,

oversize vehicles, and emergency vehicles. The penultimate sections are devoted to the

cost and land considerations and the environmental benefits associated with roundabouts.

Lastly, I show how roundabouts improve traffic flow efficiency and conclude that, all

factors considered, roundabouts are a highly favorable design solution.

Public Opinion

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Research has demonstrated a strong public sentiment against roundabouts. In their

survey of 11,972 Michigan residents, Savolainen et al. found that “51.1% of respondents

were either slightly or strongly opposed to roundabouts” (32). Various studies in

Vermont, Maine, and California have yielded similar or worse results (Savolainen et al.

25). Given these strong negative opinions, city planners and engineers may think it an

impractical endeavor to attempt to implement a roundabout in their own roadway system.

However, such public sentiments are usually drastically reversed once they are

implemented. In a survey of 44 states and 26 municipalities and counties known to have

roundabouts, undertaken by Jacquemart et al., “68 percent of the responses were negative

or very negative... before the construction of the roundabout” but “after construction, 73

percent indicated a positive or very positive attitude” (19). These results demonstrate that

poor public opinion should not prevent the consideration of roundabouts as a design

solution for roadway intersections.

Respondents to the aforementioned survey often voiced negative comments

revolving around drivers’ relative inexperience with roundabouts. In one case in Arizona,

“City Councilman Guy Phillips and others have argued that a roundabout will confuse

drivers,” initially rendering their use troublesome and unsafe (Duckett). It is true that

confusion may often result from drivers’ generally lacking experience with roundabouts,

but the way they function is simple and conducive to learning how to use them in a safe

manner.

For example, modern roundabouts include mechanisms to calm and direct traffic,

such as curved approaches and offset entryways, which greatly benefit learnability and

safety (FHWA 3). Additionally, potentially confused drivers can easily learn proper use

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by observing other drivers. Finally, roundabouts reduce the number of directions in which

a driver needs to watch for conflicting traffic. All of these characteristics render

inexperience a trivial factor in regards to the implementation and usability of

roundabouts.

Safety

Roadway safety is a primary concern for drivers, engineers, and city planners.

Both in theory and in practice, modern roundabouts are safer than conventional

intersections, such as those controlled by two-way stops, four-way stops, or light signals.

The foremost reason for their improved safety is their inherent reduction of automobile

speed which allows for increased driver reaction time and a diminished chance of severe

crashes that could result in fatalities or serious injuries. Furthermore, the characteristic

traffic pattern of guided circulation virtually eliminates the otherwise routine possibility

of head-on and right angle crashes.

The most decisive theoretical safety improvement of roundabouts over traditional

intersections is the drastic decrease in conflict points. As defined in the Transportation

Research Board’s report on roundabouts, “a conflict point is a location where the paths of

two motor vehicles … diverge, merge, or cross each other” (Rodegerdts et al. 5-5). As

demonstrated by a diagram from this report (see fig. 1), a conventional single-lane four-

way intersection has thirty-two conflict points whereas a roundabout only has eight.

While stop signs and traffic lights attempt to regulate these points, the physical geometry

of roundabouts is definitively “more effective than the reliance on driver obedience of

traffic control devices” (Rodegerdts et al. 5-7).

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Figure 1

In addition to the theoretical safety benefits, numerous in-depth before-and-after

studies have concluded that roundabouts are safer in practice, reducing the number of

crashes and injury-causing crashes. In their article, Retting et al. cite a 47% and 71%

reduction of each, respectively, at 181 Dutch intersections that were converted from

signal or stop sign control to a roundabout (628). A similar 74 percent reduction in

injury-causing crashes was reported for a study of 73 converted intersections in Australia

(628).

Retting et al. set out to measure this data for intersections in the United States and

used an even more powerful statistical analysis tool to account for variations in traffic

volume before and after the conversion (628). Overall, their procedure estimated “highly

significant reductions of 38% for all crash severities combined and 76% for injury

crashes” (630). Such considerable safety benefits, clearly caused by the geometry and

functioning of roundabouts, demonstrate how roundabouts are a highly favorable design

solution compared to conventional intersections.

Usability for Unique Traffic

A common concern about roundabouts is their usability for unique types of

traffic: pedestrians, cyclists, oversize vehicles, and emergency vehicles. As Goel posits in

his article against roundabouts, the lack of controlled stops for automobile traffic seems

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to render pedestrian crossing more dangerous than at traditional intersections (“Rise of

the Roundabouts”). However, properly designed modern roundabouts include “splitter

islands” between lanes of approaching roadways, splitting traffic into entering and exiting

lanes (Rodegerdts 5-12). Because of this feature, pedestrians have the safety benefit of

individually considering each lane of traffic in which vehicles can only come from one

direction.

Reduced vehicle speed is another way roundabouts provide better safety than

traditional intersections for pedestrians and cyclists alike. Here again, Goel argues that, of

all users, “cyclists suffer the most because of blind spots on a roundabout” (“Rise of the

Roundabouts”). When bikers ride on the outer edge of the circulatory roadway, they

certainly do face increased danger from exiting vehicles. However, low automobile

speeds allow cyclists to merge comfortably with vehicle traffic and enjoy the safety

benefits detailed in the previous section. Consequently, roundabouts are a suitable design

solution for both bikers and pedestrians.

Accommodating oversize and emergency vehicles is another common concern of

roundabouts. Large vehicles often have a wide turning radius and low ground clearance

above the roadway surface, both of which pose potential problems at roundabouts. For

their report, Russell et al. surveyed characteristics of roundabouts in all fifty United

States in an effort to address such difficulties. They conclude that, with a proper

knowledge of “what [oversize vehicles] need to be accommodated, and their turning

characteristics, any knowledgeable designer can do it” by implementing two simple

measures (“Accommodating” vi). These are the inclusion of a truck apron, that is, a

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drivable portion of the roundabout’s center island, and a maximum height of three inches

for splitter islands, truck aprons, and curbs (207).

Such design features also greatly aid usability for emergency vehicles by

providing additional traversable surfaces for their own use and room for regular traffic to

clear the way. While Goel contends that “roundabouts are not suitable for “platooned”

traffic flow,” like ambulances, factors such as lower speeds and single-direction traffic

allow them to pass with increased safety compared to other intersections (“Rise of the

Roundabouts”; FHWA 6). Overall, deliberate planning and the inherent functioning of

roundabouts allow easy accommodation of unique traffic as a highly favorable

intersection design.

Land and Cost Considerations

Land use and cost are two prevailing concerns that prevent the implementation of

roundabouts. Goel puts this quite simply in his article, stating that “roundabouts eat up a

lot of public space” and are “an expensive solution for traffic control” (“Rise of

Roundabouts”). These observations are a fair assessment only in the case of replacing

adequately functioning intersections with roundabouts. In the case of intersections that

require a new design solution, however, land use and cost must be considered on a case-

by-case basis. Be that as it may, some general observations can be made. First of all, as

explained in the Transportation Research Board’s report, “roundabouts may require more

pavement area at the intersection compared to a traffic signal, but less on the approaches

and exits” (Rodegerdts et al. 3-33). Therefore, most of the so-perceived “eaten up public

space” is simply moved in order to build a safer intersection. Overall, the slight increase

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of land area is but a minor concern. Beyond land usage, the cost of roundabouts is a

slightly more complex issue.

As described in the Federal Highway Administration’s technical summary, a

simple analysis of construction costs shows that roundabouts are more expensive than

signalized intersections in “retrofit situation[s]” but “comparable in new installations”

(25). However, a more systematic benefit-cost analysis of intersection improvement

projects “recognizes that not all of the benefits and costs of an alternative can be

quantified by pure construction costs” (FHWA 25). Some other cost-related factors that

engineers and city planners should consider are the previously examined safety benefits,

operation and maintenance costs, and the topics of the next sections of this paper:

environmental and traffic efficiency benefits.

To expand on the middle point, roundabouts typically have lower operation and

maintenance costs than signalized intersections for two reasons: they do not have traffic

lights to “power, maintain, and keep current in terms of signal timing,” and they “can

often serve for longer periods of time between major upgrades” (FHWA 25). Although

the land usage and costs of intersection designs must be assessed on a case-by-case basis,

many factors of a cost-benefit analysis clearly favor the implementation of roundabouts.

Environmental Benefits

Studies show that reducing greenhouse gas emissions, a major modern society,

can be aided by the implementation of roundabouts. In their article, Mandavilli et al. cite

numerous studies of vehicular emissions at intersections, noting that roundabouts reduced

HC emissions by an average of 33% in five studies, CO by 36% in six studies, and NOx

by 21% in six studies (136). In their own study, Mandavilli et al. examined six

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intersections in the United States, gathering traffic data from 7:00 a.m. to 1:00 p.m. and

from 1:00 p.m. to 7:00 p.m. on normal weekdays before and after conversion to a

roundabout. Their analysis found statistically significant reductions of CO, CO2, NOx,

and HC emissions in both the a.m. and p.m. periods with average reductions of 33%,

46%, 34%, and 53%, respectively (140). These drastic results demonstrate the

environmental benefit that roundabouts have over other intersection designs.

Greenhouse gas emission reductions result from roundabouts’ characteristic

traffic pattern which reduces “delays, queues, and proportion of vehicles stopped”

(Mandavilli et al. 141). Detailed descriptions of these measurements will be given in the

following section. Most notably, unlike conventional intersections, roundabouts do not

force vehicles to stop, especially during off-peak hours. Mandavilli et al. reference

studies which show that idle vehicles “emit about seven times as much CO as vehicles

traveling at ten miles per hour” and “4.5 times greater than a vehicle moving at five miles

per hour” (136). The aforementioned roundabout studies are certainly supported by this

fact. Overall, as Mandavilli et al. conclude in their article, “it is reasonable to suggest that

a modern roundabout may be the best alternative to reduce vehicular emissions” at

locations where roundabouts are a feasible design solution (141).

Traffic Efficiency

In the survey undertaken by Jacquemart et al., described in this paper’s section on

public opinion, uncertainty about traffic efficiency was one of the most prevalently

reported reasons for not implementing roundabouts (20). Perhaps they seem congestion-

inducing due to their characteristically slower vehicle speeds or their ability to hold more

vehicles than similarly sized traditional intersections. Whatever the reason for this faulty

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uncertainty, research has demonstrated that there are “statistically significant reductions

in delay, queueing, and proportion of vehicles stopped … after the installation of a

modern roundabout” (Russell et al., “Operational Performance” ii). These metrics

summarize six measures of effectiveness used to analyze traffic flow efficiency. Before

detailing these measures in their own study, Russell et al. review results from previous

research on traffic flow and operational evaluation of roundabouts. A Kansas State

University study compared the operational performance of the first modern roundabout in

Kansas to four comparable two-way and four-way stop controlled intersections using a

computer program (17). The report concluded that roundabouts performed better and

“helped to establish that even at relatively low traffic volumes, roundabouts could be

more efficient than [stop-controlled intersections]” (18). Another study yielded similar

results for before and after periods at three modern roundabout locations with a 20%

reduction in delays and 14 to 37% reduction in proportion of vehicles stopping across the

sites (18).

In their own study, Russell et al. compared the before and after operational

performance of eleven modern roundabouts with eight other traffic control devices. They

collected field data with 360° video cameras and then recorded traffic movements

manually from the film. This data was entered into computer software which calculated

the six measures of effectiveness that the authors believe are “directly relate[d] to the

operational effects of the roadway” (“Operational Performance” 28). The first is the 95th

Percentile Queue Length: the length such that 95% of average queues are less than it. The

second is Degree of Saturation, or the ratio of traffic volume to the intersection’s

capacity. The third is the Average Intersection Delay for all vehicles entering the

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intersection and the fourth is the average delay of the busiest approach. Finally, the fifth

is the Proportion of Vehicles Stopped on approach, and the sixth is this proportion at the

busiest approach (29).

In the study undertaken by Russell et al., intersections had a statistically

significantly better performance in all six of these metrics after the implementation of a

roundabout by at least 42% and up to 71% (“Operational Performance” 124). Most

notably, the average delay decreased from 20.2 to 8 seconds and the proportion of

vehicles stopped decreased from 58 to 29 (124). Because the eleven studied roundabouts

had a range of different traffic conditions, it is statistically reasonable to conclude that the

implementation of modern roundabouts significantly improves operational traffic flow

compared to traditional intersection designs (130). Traffic efficiency is certainly one of

the primary considerations at an intersection for drivers, engineers, and city planners. In

this respect, roundabouts definitively excel as a design solution.

Conclusion

Because of their benefits with regards to safety, environmental factors, and traffic

flow efficiency, roundabouts are a highly favorable solution for intersection designs.

Though they often face public disapproval, qualms about their usability, and misgivings

about their use of resources, they can easily be designed to overcome these difficulties.

As a result, engineers and city planners should continue to implement roundabouts

frequently in America’s roadway system. Ultimately, because roundabouts are safe,

ecological, and efficient intersection design, drivers can be assured that they are an

excellent solution for their daily transportation.

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Works Cited

Duckett, Beth. “Scottsdale’s 1st arterial traffic roundabout draws criticism.” The Arizona

Republic. Gannett Co. 25 Jan. 2013. Web. 30 March 2016.

Federal Highway Administration. Roundabouts. Washington, D.C: U.S. Department of

Transportation, 2010. Print.

Goel, Tarun. “Rise of the Roundabouts.” Bright Hub Engineering. Bright Hub Inc., 15

July 2011. Web. 20 March 2016.

Jacquemart, Georges. Modern Roundabout Practice in the United States. Washington,

D.C: National Academy Press, 1998. Print.

Mandavilli, Srinivas, et al. "Environmental Impact of Modern Roundabouts."

International Journal of Industrial Ergonomics 38.2 (2008): 135-42. Print.

Retting, Richard A., et al. “Crash and Injury Reduction Following Installation of

Roundabouts in the United States.” American Journal of Public Health. 91.4

(2001): 628-31. Print.

Rodegerdts, Lee, et al. Roundabouts: An Informational Guide. 2nd ed. Washington, D.C:

Transportation Research Board, 2010. Print.

Russell, Eugene R., Sr., et al. Accommodating Oversize/Overweight Vehicles at

Roundabouts. Topeka: Kansas Department of Transportation, 2013. Print.

Russell, Eugene R., Sr., et al. Operational Performance of Kansas Roundabouts: Phase

II. Topeka: Kansas Department of Transportation, 2004. Print.

Savolainen, Peter T., et al. “Examining Statewide Public Perceptions of Roundabouts

Through a Web-Based Survey.” Transportation Research Record: Journal of the

Transportation Research Board no. 2312 (2012): 25-33. Print.