On this page...
Over the last 25 years, roundabouts have been used in the United States to reduce crashes, traffic delays, fuel consumption, air pollution, and construction and maintenance costs.
Roundabouts often move more traffic compared to other intersection options and enhance intersection beauty. They have also been used to manage speed in residential neighborhoods and are accepted as one of the safest types of intersection designs. Roundabouts are specifically designed for urban, suburban, and rural locations to accommodate various vehicle types.
The Iowa DOT and other communities in Iowa use roundabouts in certain situations to enhance safety and reduce delays encountered by the motoring public.
Roundabouts have been used effectively throughout the world for many years.
Back to topWhat Is a Roundabout?
A modern roundabout is an unsignalized, circular intersection engineered to maximize safety and minimize traffic delay. Over the last few decades, tens of thousands of roundabouts have been installed in Europe, Australia and other parts of the world. Recently, they have gained support in the United States, including communities in Iowa. From 1999 to 2009, at least 33 modern roundabouts were constructed in Iowa.
Drivers in Iowa are becoming more comfortable using roundabouts in the communities that have them. In the cities and counties where roundabouts have been built, even in locations where the public had been hesitant about accepting them initially, roundabouts ultimately have been accepted enthusiastically because of the increased safety they provide, along with traffic calming, and environmental and aesthetic benefits.
Roundabouts differ from the old, larger traffic circles and rotaries in three major areas.
- A roundabout is generally smaller in diameter, requiring lower traveling speeds.
- At roundabouts, the entering traffic yields the right-of-way to the circulating traffic. This yield-at-entry rule keeps traffic from locking up and allows the free flow movement.
- The splitter and center islands of a roundabout deflect entering traffic and reinforce the yielding process.
The majority of modern roundabouts operate very efficiently with single-lane approaches and a single circulating lane. In cases where certain movements need to carry higher traffic volumes, roundabouts can be designed with multiple lanes. Modern designs include signing, pavement markings and design features that can guide multiple lanes through a roundabout without the old problem of becoming “trapped” in the center lane.
Benefits of Going in Circles
While roundabouts are a relatively new type of intersection in Iowa, they are becoming more common as evidence of their benefits grows. Improved traffic flow, aesthetics and cost savings make roundabouts a good idea, and the safety gains are compelling.
Fewer Crashes and Less Severe Crashes
Roundabouts benefit from good geometry, exhibiting only a fraction of the troublesome crash patterns typical of right-angle intersections. A typical four-legged intersection has 32 vehicle-to-vehicle conflict points and 24 vehicle-to-pedestrian conflict points. By comparison, a four-legged roundabout has only eight vehicle-to-vehicle conflict points and eight vehicle-to-pedestrian conflict points. This is an approximate 70 percent reduction in conflict points. In addition, since all vehicles are traveling in the same direction and at a lower speed in a roundabout, crashes are generally rear end or sideswipe in nature. Left-hand, right-angle (T-bone) and head-on crashes are virtually eliminated by a roundabout. The illustrations at the top of this page show the conflict points of a standard intersection and a typical roundabout. Studies by the Insurance Institute for Highway Safety show that roundabouts provide a:
- 90 percent reduction in fatal crashes;
- 76 percent reduction in injury crashes;
- 30 to 40 percent reduction in pedestrian crashes; and
- 10 percent reduction in bicycle crashes
Lower Vehicle Speeds
A standard stop sign or traffic signal controlled intersection always has at least one direction of traffic stopped. A roundabout uses yield-at-entry traffic control to eliminate stopping when it is not required.
Less Vehicle Pollution
Vehicles entering a roundabout must yield at entry, but are not required to stop if the roundabout is clear. This eliminates some stop-and-go traffic associated with stop sign or traffic signal controlled intersections. This leads to fewer vehicles idling while stopped at an intersection.
Lower Maintenance Costs As Compared to a Traffic Signal
A traffic signal requires electricity 24 hours a day. In addition, the signals need maintenance by field personnel for burned out lights, loop detector replacement, etc. A typical roundabout generally only needs electricity for streetlights at night and maintenance for landscaping, if included.
Increased Landscaping Opportunities
A standard intersection requires a large paved area to accommodate all the turning movements. A roundabout provides opportunity to landscape the center island, providing green space within the intersection.
Pedestrians Cross One Direction of Traffic at a Time
Pedestrians need only cross one direction of traffic at a time at each roundabout approach, as compared with two-way and all-way stop-controlled intersections. The conflict locations between vehicles and pedestrians are generally not affected by the presence of a roundabout, although conflicting vehicles come from a more defined path at roundabouts. In addition, the speeds of motorists entering and exiting a roundabout are reduced with good design. As with other crossings that require acceptance of gaps in traffic flow, roundabouts still present visually-impaired pedestrians with unique challenges.
Back to topDriver Education
All motorists have encountered and are familiar with standard intersections controlled by yield signs, stop signs or traffic signals. When approaching, the motorist instantly knows from the signing or traffic signal whether to stop before proceeding. If stopped, pedestrians may cross the street in front of the vehicle and vehicles on the cross street proceed. Once able to proceed through the intersection, the motorists may go directly left, straight, right, or, in some cases, perform a U-turn.
If an intersection is uncontrolled (no stop signs or signal), a yield-to-the-right rule is generally followed giving priority to the vehicle on your right.
A roundabout requires a different technique to properly drive. Since this is a change from standard intersections, many motorists experience some degree of anxiety during their first few encounters with roundabouts. However, the steps to correctly maneuver through a roundabout are easy to master.
Back to topDefinitions
Back to topDispelling Common Myths About Roundabouts
List items for Myths About Roundabouts
The traditional roundabout, or circulatory intersection, suffers from a poor reputation in the United States. The word "roundabout" frequently conjures up memories of a bad driving experience abroad or in the eastern United States. However, modern roundabout design has been greatly improved from previous adaptations in Europe and along the East Coast.
The large diameter, high-speed, multi-lane roundabout has evolved into a smaller diameter, slow-speed, one- or two-lane design that provides similar capacity to that of a traffic signal. Listed below are some of the most common myths and current facts about modern roundabouts.
Roundabouts keep traffic moving. The major delay on a person's morning or evening commute is usually the time spent sitting at traffic signals. Eliminating the need to stop and wait reduces delay.
Using a roundabout is much the same as making a right turn from a stop sign. At a traffic signal, a right-turning driver stops at the stop bar, looks for conflicting traffic coming from the left, chooses an acceptable gap in the traffic flow, and then turns right onto the cross street. At a modern roundabout, the oncoming driver approaches the yield line, looks for conflicting traffic coming from the left, chooses an acceptable gap in the traffic flow, and then enters the roundabout with a right turn at the yield sign. Once inside the roundabout, a driver continues circling counter-clockwise until reaching the desired exit. Exit maneuvers are also right turns.
Roundabouts are pedestrian friendly. The splitter islands (see illustration above) provide a space for pedestrians in the middle of each crossing. Therefore, pedestrians only need to cross one direction of traffic at a time. The pedestrian crosswalks are set at least one full car length back from the yield line. That way, pedestrians do not have to cross in front of drivers that are looking for their gap in traffic. Experience has shown that the stopped vehicle one car length back from the yield line is more aware of pedestrians.
According to the Insurance Institute for Highway Safety, modern roundabouts reduce motor vehicle crashes. Their July 2001 Status Report noted "most serious kinds of crashes at conventional intersections are virtually eliminated by roundabouts. Crashes that do occur tend to be minor because traffic speeds are slower." The study reviewed 24 intersections around the United States that have been converted from stop signs or traffic signals to modern roundabouts. At those intersections, all crashes were reduced by 39 percent. Serious crashes were reduced by 76 percent. At the time of the study, there had been no fatalities at any of the new roundabouts. So, the study estimates that fatal or incapacitating injuries will be reduced by 90 percent at those intersections.
Modern roundabouts are usually less expensive than signalized intersections for two primary reasons:
- Expensive traffic signal equipment, as well as maintenance of that equipment, is not needed; and
- Under certain traffic conditions, the free flow movement of the roundabout is able to reduce the capacity needs of adjoining roadways; thus, fewer traffic lanes may be needed. Roundabouts usually do not require separate left- and right-turn lanes, which also helps lower costs of intersection approaches.
Since roundabouts are currently rare around the United States, all types of drivers may experience initial confusion upon their first encounter. However, as roundabouts become more common and motorists become more familiar with their operation, the initial confusion will be significantly reduced. Most people quickly learn their operation. Plus, because of the low speeds, there is generally much less risk of a crash or injury compared to a traditional intersection.
Roundabouts have design features specifically intended to accommodate trucks, buses, tractors, and larger vehicles. The main characteristic is a truck apron, a slightly raised area around the center island allowing larger trucks easier circulation in the roundabout. It is typically 3 to 4 inches higher than the paved roadway. A truck apron is used instead of increasing the normal driving width to prevent smaller vehicles from achieving higher speeds through the roundabout. With a properly designed truck apron, a roundabout is able to accommodate all types of larger vehicles.
Electronic Resources & Links
Back to topDesign & Traffic Safety Resources
- Iowa Department of Transportation
- Iowa DOT's Traffic Engineering Assistance Program
The Iowa Department of Transportation offers no-cost, expert roundabout reviews during the feasibility, planning, design or operation of roundabouts in Iowa. The DOT is presently using a nationally-known and respected roundabout consulting firm to help ensure early success for roundabouts in Iowa, until such time that in-state experience and expertise are developed.
See the Complimentary Roundabout Design Review Service brochure.
Requests For Services
Iowa DOT's Office of Traffic and Safety
Additional Design Resources
Iowa DOT's Office of Design
The district local systems engineers are listed in the DOT's Need Answers publication.
Iowa DOT's Office of Traffic & Safety
Roundabout Demonstrations
To learn more about driving through a roundabout, take a test drive by visiting one of these demonstration sites.
- City of Johnston, Iowa - NW 62nd Avenue Roundabout Corridor Road Construction Project
- City of Coralville, Iowa - What Is a Roundabout?
- Fort Worth, Texas - Using a Roundabout
- Sugarland, Texas - How Do You Use a Roundabout?
- Minnesota - How About a Roundabout?
- Missouri Department of Transportation - All About a Roundabout
- Florida - Modern Roundabouts - Pedestrian, Bicyclists and Motorist