Roundabouts
Frequently Asked Questions and Use

(Resources: Insurance Institute for Highway Safety, American Planning Association’s Transportation Planning Division, Alaska Roundabouts, and Arizona Department of Transportation)

1 | What is a roundabout?
The modern roundabout is a circular intersection with design features that promote safe and efficient traffic flow.

Vehicles travel counterclockwise around a raised center island, with entering traffic yielding the right-of-way to circulating traffic. Entering vehicles negotiate a curve sharp enough to slow speeds to about 15-20 mph. Within the roundabout and as vehicles exit, slow speeds are maintained by the deflection of traffic around the center island and the relatively tight radius of the roundabout and exit lanes. Slow speeds aid in the smooth movement of vehicles into, around, and out of a roundabout. Drivers approaching a roundabout must reduce their speeds, look for potential conflicts with vehicles already in the circle, and be prepared to stop for pedestrians and bicyclists. Once in the roundabout, drivers proceed to the appropriate exit, following the guidance provided by traffic signs and pavement markings.

Modern roundabout

Older traffic circle

2 | How common are roundabouts in the United States?
The exact number of roundabouts in the United States is unknown, but approximately 1,000 and at least 48 public, over 100 in North Carolina have been built since their inception in 1990.

By comparison, there are about 20,000 roundabouts in France, 15,000 in Australia, and 10,000 in the United Kingdom. States that have active programs to construct roundabouts include Alaska, California, Colorado, Connecticut, Florida, Hawaii, Indiana, Kansas, Maryland, Michigan, Minnesota, Mississippi, Nevada, New Hampshire, New York, North Carolina, Oregon, South Carolina, Utah, Vermont, Virginia, Washington, and Wisconsin.

3 | How do roundabouts differ from older traffic circles and rotaries?
Although both traffic circles and modern roundabouts utilize round circulating roadways, there are major differences in the design and operation of these two types of intersections. The main distinguishing factors include:

4 | Do drivers favor roundabouts?
Drivers may be skeptical, or even opposed, to roundabouts when they are proposed. However, opinions seem to change once drivers become familiar with roundabouts.

A 2002 Institute study in three communities where roundabouts replaced stop sign-controlled intersections found 31 percent of drivers supported the roundabouts before construction compared with 63 percent shortly after. A more recent study surveyed drivers in three additional communities where roundabouts replaced stop signs or traffic signals. Overall, 36 percent of drivers supported the roundabouts before construction compared with 50 percent shortly after. Follow-up surveys conducted in these six communities after roundabouts had been in place for more than one year found the level of public support increased to about 70 percent on average.

5 | How do roundabouts affect traffic flow?
Roundabouts keep traffic moving by eliminating the need to stop and wait for long periods of time.

A good example that illustrates this situation is the Northwestern Connector study in the Detroit suburbs. Completed in 2001, this study compared upgraded traffic signals and modern roundabouts in detail at fourteen highly congested intersections. This comparison indicated that modern roundabouts would decrease motorists’ total annual delay by more than 50 percent compared to upgraded traffic signals. During the morning and afternoon rush hours, modern roundabouts would reduce motorists’ delays by about 70 percent on average compared to upgraded traffic signals.

A study of three locations in New Hampshire, New York, and Washington, where roundabouts replaced traffic signals or stop signs, found an 89 percent average reduction in vehicle delays and a 56 percent average reduction in vehicle stops. A study of 11 intersections in Kansas found a 65 percent average reduction in delays and a 52 percent average reduction in vehicle stops after roundabouts were installed.

6 | How do roundabouts affect safety?
Studies have shown that roundabouts typically improve safety, particularly by reducing overall crash rates and the severity of collisions.

Several features of roundabouts promote safety. 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 essentially are eliminated because vehicles travel in the same direction. Installing roundabouts in place of traffic signals can also reduce the likelihood of rear-end crashes and their severity by removing the incentive for drivers to speed up as they approach green lights and by reducing abrupt stops at red lights. The vehicle-to-vehicle conflicts that occur at roundabouts generally involve a vehicle merging into the circular roadway, with both vehicles traveling at low speeds — generally less than 20 mph.

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.

7 | How do roundabouts affect older drivers?
Roundabouts eliminate the primary risk factors associated with older drivers and intersection crashes by forcing traffic to slow down and move in only one direction.

Age-related declines in vision, hearing, and cognitive functions, as well as physical impairments, may affect some older adults' driving ability. . . . In 2004, about half of drivers 80 and older in fatal crashes were involved in multiple-vehicle intersection crashes, compared with 24 percent among drivers younger than 70. Older drivers' intersection crashes often are due to their failure to yield the right-of-way. Particular problems for older drivers at traditional intersections include left turns and entering busy thoroughfares from cross streets. Roundabouts eliminate these situations entirely.

8 | Are roundabouts safe for pedestrians?
Roundabouts are generally safer for pedestrians than traditional intersections.

In a roundabout, pedestrians walk on sidewalks around the perimeter of the circulatory roadway. If it is necessary for pedestrians to cross the roadway, they cross only one direction of traffic at a time. In addition, crossing distances are relatively short, and traffic speeds are lower than at traditional intersections. Studies in Europe indicate that, on average, converting conventional intersections to roundabouts can reduce pedestrian crashes by about 75 percent. Single-lane roundabouts, in particular, have been reported to involve substantially lower pedestrian crash rates than comparable intersections with traffic signals.

Those pedestrian injuries that do occur tend to be less serious due to the relatively low speeds encountered at modern roundabouts. Legitimate concerns have been raised regarding the ability of blind pedestrians to negotiate roundabouts, and this topic is under consideration within the United States transportation community.

9 | Can modern roundabouts safely accommodate bicycles?
Modern roundabouts can easily accommodate cyclists when properly negotiated.

Cyclists have two choices when negotiating a roundabout. The more avid and skilled cyclists can choose to negotiate the roundabout much like a vehicle would: merge into a traffic lane before the bike lane ends; ride close to the middle of the lane to prevent vehicles from passing and cutting the cyclist off; enter the roundabout after yielding to vehicles within the roundabout; circulate the roundabout being careful to watch for vehicles waiting to enter the roundabout; and exit the roundabout as a normal vehicle would do. Bicyclists not wanting to enter the roundabout can enter the sidewalk using the ramps where the bike lane ends, and proceed around the roundabout as a pedestrian, walking their bicycle.

10 | What are appropriate locations for roundabouts?
Roundabouts are appropriate at most 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, in lieu of road widening, and can be appropriate in lieu of traffic signals at freeway exits and entrances.

11 | What types of intersections may not be good candidates for roundabouts?
Roundabouts are not appropriate everywhere.

Intersections that may not be good candidates include those with topographic or site constraints that limit the ability to provide appropriate geometry, those with highly unbalanced traffic flows (that is, very high traffic volumes on the main street and very light traffic on the side street), and isolated intersections in a network of traffic signals.

12 | Do roundabouts require more space than traditional intersections?
Roundabouts do not necessarily require more space than traditional intersections.

Geometric design details vary from site to site and must take into account traffic volumes, land use, topography, and other factors. Because they can process traffic more efficiently than traffic signals and stop signs, roundabouts typically require fewer traffic lanes to accommodate the same amount of traffic.

In some cases, roundabouts can require more space than stop signs or traffic signals at the actual intersection to accommodate the central island and circulating lanes, but approaches to roundabouts typically require fewer traffic lanes and less right-of-way than those at traditional intersections.

The following example from Asheville, North Carolina, illustrates that roundabout dimensions can be compatible with those of traditional intersections.

Before

After

13 | Are there other benefits?
Roundabouts also reduce vehicle emissions and fuel consumption because vehicles no longer idle at signalized intersections. Additionally, roundabouts provide opportunities for landscaping and other aesthetic enhancements.

In one study, replacing a signalized intersection with a roundabout reduced carbon monoxide emissions by 29 percent and nitrous oxide emissions by 21 percent.

In another study, replacing traffic signals and stop signs with roundabouts reduced carbon monoxide emissions by 32 percent, nitrous oxide emissions by 34 percent, carbon dioxide emissions by 37 percent, and hydrocarbon emissions by 42 percent. Constructing roundabouts in place of traffic signals can reduce fuel consumption by about 30 percent. At 10 intersections studied in Virginia, this amounted to more than 200,000 gallons of fuel per year.

14 | Can roundabouts accommodate larger vehicles?
Yes. To accommodate vehicles with large turning radii such as fire trucks, buses, and tractor-trailers, roundabouts provide an area between the circulatory roadway and the central island, known as a truck apron, over which the rear wheels of these vehicles can safely track.

The truck apron generally is composed of a different material texture than the paved surface, such as brick or cobble stones, and is slightly raised to discourage routine use by smaller vehicles.

15 | What are the impediments to building roundabouts?
The primary impediments to building roundabouts are negative public perception and traditional design practices.

Despite the safety and other benefits of roundabouts, as well as the high levels of public acceptance once they are built, some states and cities have been slow to build roundabouts, and some are even opposed to building them. The principal impediment is the negative perception held by some drivers and elected officials. Transportation agencies also have long been accustomed to installing traffic signals, and it can take time for deeply rooted design practices to change.

16 | Are modern roundabouts more expensive than traffic signals?
Roundabouts may have higher initial costs than traditional intersections.

In some situations, modern roundabouts require more right-of-way at intersections than traffic signals, resulting in higher initial costs. However, in other cases, traffic signals require numerous and lengthy turn lanes (for storage) to achieve acceptable delays. In these situations, traffic signals may require more total right-of-way than roundabouts.

Modern roundabouts also move traffic much more efficiently than traffic signals. Because of this benefit, the streets between roundabouts do not have to be constructed with as many lanes as the streets between signals. Because the modern roundabout allows funds to be expended where they are needed most - at the intersections - costs can be reduced on the streets themselves. Other long-term cost savings result because roundabouts do not require expensive signal equipment or future maintenance of that equipment.

Another factor worthy of consideration is the long-term cost savings to society as a result of the reduction in crash rates from implementing roundabouts.

17 | Are there additional concerns with building roundabouts?

(Resource: Genesee Transportation Council, Metropolitan Planning Organization for the Genesee-Finger Lakes Region, NY)

How to Use a Roundabout
(Resource: City of Colorado Springs, Transportation/Traffic Engineering Division)

Modern roundabouts are different from traffic signals and will require drivers to learn how they operate. Experience in the United States has shown that motorists quickly adapt to this new type of intersection. Proper use of signing and striping at roundabouts assists motorists and minimizes the potential for confusion.

1	When approaching a roundabout, follow the designation signs.
2	Slow down and yield to pedestrians in the crosswalk.
3	Look to the left. Traffic already within the roundabout always has the right-of-way.
4	Approach the yield line and enter the roundabout when there is an adequate gap in the circulating traffic flow.
5	If another car is waiting at the yield line ahead of you, do not stop in the crosswalk. Keep the crosswalk clear for pedestrians.
6	Bicyclists are permitted to ride within the roundabout and will be riding in the lane just as other vehicles do. Do not pass a bicycle in a roundabout.
7	Once you have entered the roundabout, you have the right-of-way. Proceed counterclockwise to your exit.
8	As you approach your exit, turn on your right turn signal.
9	Exit the roundabout, yielding to pedestrians in the crosswalk.