Roundabouts

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What is a Roundabout?

Modern roundabouts were developed in the United Kingdom in the 1960s and now are widely used in many countries. The modern roundabout is a circular intersection with design features that promote safe and efficient traffic flow.

At roundabouts in the United States, vehicles travel counterclockwise around a raised center island, with entering traffic yielding the right-of-way to circulating traffic. In urban settings, entering vehicles negotiate a curve sharp enough to slow speeds to about 15-20 mph; in rural settings, entering vehicles may be held to somewhat higher speeds (25-30 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

roundabout2                   roundabout3              Roundabout4 

 

How do roundabouts affect safety?

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 in urban areas and less than 25-30 mph in rural areas.

Below is a figure showing signs and lane markings used at roundabouts.

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Despite the demonstrated safety benefits of roundabouts, some crashes still occur; the two most common crash types are “failure to yield” and multi-lane roundabouts “include improper lane selection”.

Proper care should be exercised when entering the roundabout.  The vehicle in already in the roundabout has the right of way.

Are there other benefits?

Because roundabouts improve the efficiency of traffic flow, they also reduce vehicle emissions and fuel consumption. 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. And roundabouts can enhance aesthetics by providing landscaping opportunities.

Can roundabouts accommodate larger vehicles?

Yes. To accommodate vehicles with large turning radii such as 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 colored material than the paved surface, usually a reddish colored concrete, to discourage routine use by smaller vehicles.

How do roundabouts affect older drivers?

Age-related declines in vision, hearing, and cognitive functions, as well as physical impairments, may affect some older adults' driving ability. Intersections can be especially challenging for older drivers. Relative to other age groups, senior drivers are over-involved in crashes occurring at intersections.

In 2006, forty percent of drivers 70 and older in fatal crashes were involved in multiple-vehicle intersection crashes, compared with 22 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.

A recent study in six communities where roundabouts replaced traditional intersections found that about two-thirds of drivers 65 and older supported the roundabouts. Although safety effects of roundabouts specifically for older drivers are unknown, the 2001 Institute study of 23 intersections converted from traffic signals or stop signs to roundabouts reported the average age of crash-involved drivers did not increase following the installation of roundabouts, suggesting roundabouts may not pose a problem for older drivers.

Are roundabouts safe for pedestrians?

Roundabouts generally are 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.

Do drivers favor roundabouts?

Drivers may be skeptical or even opposed to roundabouts when they are proposed. However, opinions quickly change when drivers become familiar with roundabouts. A 2002 Institute study in three communities where single-lane roundabouts replaced stop sign-controlled intersections found 31 percent of drivers supported the roundabouts before construction compared with 63 percent shortly after. Another study surveyed drivers in three additional communities where single-lane 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.

Is there any other areas installing roundabouts?

The first modern roundabouts in the United States were constructed in Nevada in 1990. Since that time, although the precise number of roundabouts is unknown, approximately 1,000 have been built. 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.

Conclusion:

We hope that this brochure has been informative and has illustrated some of the pros and cons associated with stop signs. These devices can produce negative impacts on the environment and waste unnecessary tax reserves.

Therefore, they should only be installed when warranted by a competent engineering study. If you have questions or suggestions concerning traffic, please call the Engineering Department at (701) 787-3720.