February 2014

  1. What is a roundabout?

    The modern roundabout is a circular intersection with design features that promote safe and efficient traffic flow. It was developed in the United Kingdom in the 1960s and now is widely used in many countries, and increasingly in the United States.

    At roundabouts in the U.S., 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 (30-35 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.

    Roundabout right turn
    Roundabout straight ahead
    Roundabout left turn

    Common traffic maneuvers at roundabouts


     
  2. How do roundabouts differ from older traffic circles?

    Modern roundabouts are much smaller than older traffic circles — also known as rotaries — and roundabouts require vehicles to negotiate a sharper curve to enter. These differences make travel speeds in roundabouts slower than speeds in traffic circles. Because of the higher speeds in older circles, many are equipped with traffic signals or stop signs to help reduce potential crashes. In addition, some older traffic circles and rotaries operate according to the traditional "yield-to-the-right" rule, with circulating traffic yielding to entering traffic.

    Modern roundabout

    Modern roundabout

    Older traffic circle

    Older traffic circle


     
  3. How do roundabouts affect safety?

    Roundabouts promote safety in several ways. 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 and at low speeds  generally less than 20 mph in urban areas and less than 30-35 mph in rural areas. 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 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 percent and reductions in all crashes of 35-47 percent. Persaud, B.N.; Retting, R.A.; Garder, P.E.; and Lord, D. 2001. Safety effect of roundabout conversions in the United States: empirical Bayes observational before-after study. Transportation Research Record 1751:1-8. Eisenman, S.; Josselyn, J.; List, G.; Persaud, B.; Lyon, C.; Robinson, B.; Blogg, M.; Waltman, E.; and Troutbeck, R. 2004. Operational and safety performance of modern roundabouts and other intersection types. Final Report, SPR Project C-01-47. Albany, NY: New York State Department of Transportation. Rodegerdts, L.; Blogg, M.; Wemple, E.; Myers, E.; Kyte, M.; Dixon, M.; List, G.; Flannery, A.; Troutbeck, R.; Brilon, W.; Wu, N.; Persaud, B.; Lyon, C.; Harkey, D.; and Carter, D. 2007. Roundabouts in the United States. National Cooperative Highway Research Program Report no. 572. Washington, DC: Transportation Research Board. 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 percent reduction in all crashes and a 85 percent reduction in injury crashes. Isebrands, H. and Hallmark, S. 2012. Statistical analysis and development of crash prediction model for roundabouts on high-speed rural roadways. Transportation Research Record 2312:3-13. Studies of intersections in Europe and Australia that were converted to roundabouts have reported 25-87 percent reductions in injury crashes and 36-61 percent reductions in all crashes.  Rodegerdts, L.; Bansen, J.; Tiesler, C.; Knudsen, J.; Myers, E.; Johnson, M.; Moule, M.; Persaud, B.; Lyon, C.; Hallmark, S.; Isebrands, H.; Crown, R.; Guichet, B.; and O’Brien, A. 2010. Roundabouts: an informational guide. National Cooperative Highway Research Program Report no. 672. Washington, DC: Transportation Research Board.

    Based on the results of a 2004 Institute study, Eisenman, S.; Josselyn, J.; List, G.; Persaud, B.; Lyon, C.; Robinson, B.; Blogg, M.; Waltman, E.; and Troutbeck, R. 2004. Operational and safety performance of modern roundabouts and other intersection types. Final Report, SPR Project C-01-47. Albany, NY: New York State Department of Transportation. it is estimated that the conversion of 10 percent of the signalized intersections in the United States to roundabouts would have prevented approximately 46,000 crashes in 2012, including 184 fatal crashes and 31,000 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 Persaud, B.N.; Retting, R.A.; Garder, P.E.; and Lord, D. 2001. Safety effect of roundabout conversions in the United States: empirical Bayes observational before-after study. Transportation Research Record 1751:1-8. Eisenman, S.; Josselyn, J.; List, G.; Persaud, B.; Lyon, C.; Robinson, B.; Blogg, M.; Waltman, E.; and Troutbeck, R. 2004. Operational and safety performance of modern roundabouts and other intersection types. Final Report, SPR Project C-01-47. Albany, NY: New York State Department of Transportation. Rodegerdts, L.; Blogg, M.; Wemple, E.; Myers, E.; Kyte, M.; Dixon, M.; List, G.; Flannery, A.; Troutbeck, R.; Brilon, W.; Wu, N.; Persaud, B.; Lyon, C.; Harkey, D.; and Carter, D. 2007. Roundabouts in the United States. National Cooperative Highway Research Program Report no. 572. Washington, DC: Transportation Research Board. or with increases in crashes. Isebrands, H. and Hallmark, S. 2012. Statistical analysis and development of crash prediction model for roundabouts on high-speed rural roadways. Transportation Research Record 2312:3-13. An Institute study of two-lane roundabout conversions at two intersections near Bellingham, Wash., found the rate of injury and fatal crashes combined fell to zero at one intersection; at the other, it was 34 percent lower than it would have been without the conversion. Hu, W.; McCartt, A.T.; Jermakian, J.S.; and Mandavilli, S. 2013. Public opinion, traffic performance, the environment, and safety after the construction of double-lane roundabouts. Arlington, VA: Insurance Institute for Highway Safety.  The rate of crashes with property damage only was 6 times as high as it would have been at one intersection and 13 percent higher at the other. Only the sixfold increase was significant.

  4. What types of crashes typically occur at roundabouts?

    Despite the demonstrated safety benefits of roundabouts, some crashes still occur. An Institute study of crashes at 38 roundabouts in Maryland found that four crash types (run-off-road, rear-end, sideswipe, and entering-circulating) accounted for almost all crashes. Mandavilli, S.; McCartt, A.; and Retting, R.A. 2009. Crash patterns and potential engineering countermeasures at Maryland roundabouts. Traffic Injury Prevention 10(1):44-50. A common crash type at both single-lane and two-lane roundabouts involved vehicles colliding with the central island. These crashes, which often involved unsafe speeds, accounted for almost half of all single-vehicle run-off-road crashes. Collisions occurred more frequently at entrances to roundabouts than within the circulatory roadway or at exits. About three-quarters of the crashes involved property damage only. There were no right-angle or head-on collisions, potentially severe crash types that commonly occur at traditional intersections. The researchers concluded that unsafe speeds were an important crash factor. Some drivers may not have seen the roundabout in time to slow down sufficiently.

    A review of crashes at 39 roundabout in the United States found that entering-circulating, exiting-circulating and rear-end collisions were the most common crash types. Rodegerdts, L.; Blogg, M.; Wemple, E.; Myers, E.; Kyte, M.; Dixon, M.; List, G.; Flannery, A.; Troutbeck, R.; Brilon, W.; Wu, N.; Persaud, B.; Lyon, C.; Harkey, D.; and Carter, D. 2007. Roundabouts in the United States. National Cooperative Highway Research Program Report no. 572. Washington, DC: Transportation Research Board. A large majority of crashes at the single-lane roundabouts were entering-circulating crashes. At multi-lane roundabouts, the majority of crashes were exiting-circulating.

  5. What design features can help minimize crashes at roundabouts?

    Design features that encourage drivers to slow down are the key to optimizing roundabout safety. Traffic signs (e.g., speed limit signs well in advance of roundabouts and larger "roundabout ahead" and yield signs), pavement markings and lighting should be adequate so that the drivers are aware that they are approaching a roundabout and alerted to reduce their travel speeds. Center island landscaping can promote slower speeds and focus drivers' attention on the roadway close to them by limiting their through vision. Islands separating the approach and exit lanes, known as splitter islands, should extend far enough from the roundabout to provide pedestrian refuge and to delineate the roundabout. Other design features such as adequate curvature of approach roads far enough in advance of roundabouts and the alignment of approaching roads with the center island also may aid in reducing speeds. 

    The increased complexity of multilane roundabouts present more challenges for motorists. A study of a pair of two-lane roundabouts near Bellingham, Wash., found that confusion about some aspects of navigating the roundabouts persisted one year after the construction ended. Hu, W.; McCartt, A.T.; Jermakian, J.S.; and Mandavilli, S. 2013. Public opinion, traffic performance, the environment, and safety after the construction of double-lane roundabouts. Arlington, VA: Insurance Institute for Highway Safety.  More than 40 percent of drivers said it wasn't clear from signs and pavement markings what speed to drive, which lane has the right of way when exiting or that they shouldn't drive next to large trucks in the roundabouts. At multilane roundabouts, signs and lane marking should remind drivers of the correct yielding patterns and help them choose the appropriate lane. At two-lane roundabouts, for example, signs need to convey clearly that entering traffic must yield to both lanes of traffic.

    The photos below show sample guide signs and lane markings used at roundabouts.


    Yield sign


    Guide signs


    Lane markings


     
  6. How do roundabouts affect traffic flow?

    Several studies conducted by IIHS and others have reported significant improvements in traffic flow following conversion of traditional intersections to roundabouts. Most research has focused primarily on conversions of traditional intersections to single-lane roundabouts. A study of three intersections in Kansas, Maryland and Nevada where roundabouts replaced stop signs found that vehicle delays were reduced 13-23 percent and the proportion of vehicles that stopped was reduced 14-37 percent. Retting, R.A.; Luttrell, G.; and Russell, E.R. 2002. Public opinion and traffic flow impacts of newly installed modern roundabouts in the United States. ITE Journal 72:30-32,37. A study of three locations in New Hampshire, New York and Washington state 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. Retting, R.A.; Mandavilli, S.; Russell, E.R.; and McCartt, A.T. 2006. Roundabouts, traffic flow and public opinion. Traffic Engineering and Control 47(7):268-72. 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. Russell, E.R.; Mandavilli, S.; and Rys, M.J. 2004. Operational performance of Kansas roundabouts: phase II. Report no. K-TRAN KSU-02-04, Final Report 01-04. Manhattan, KS: Department of Civil Engineering, Kansas State University.

    A 2013 Institute study of two-lane roundabout conversions at two intersections near Bellingham, Wash., found substantial declines in vehicle delays on the minor roads (33 percent and 90 percent) and the proportion of vehicles waiting in queues (35 percent and 43 percent). Overall intersections delays increased (12 percent and 22 percent), due to slightly longer delays on the major approaches as vehicles slowed to enter the roundabouts. Hu, W.; McCartt, A.T.; Jermakian, J.S.; and Mandavilli, S. 2013. Public opinion, traffic performance, the environment, and safety after the construction of double-lane roundabouts. Arlington, VA: Insurance Institute for Highway Safety. 

    Because roundabouts improve the efficiency of traffic flow, they also reduce vehicle emissions and fuel consumption. Installing roundabouts in place of traffic signals or stop signs has been found to reduce carbon monoxide emissions by 15-45 percent, nitrous oxide emissions by 21-44 percent, carbon dioxide emissions by 23-37 percent and hydrocarbon emissions by 0-42 percent. Hu, W.; McCartt, A.T.; Jermakian, J.S.; and Mandavilli, S. 2013. Public opinion, traffic performance, the environment, and safety after the construction of double-lane roundabouts. Arlington, VA: Insurance Institute for Highway Safety.  Várhelyi, A. 2002. The effects of small roundabouts on emissions and fuel consumption: a case study. Transportation Research Part D: Transport and Environment 7(1):65-71. Mandavilli, S.; Russell, E.R.; and Rys, M. 2004. Modern roundabouts in United States: an efficient intersection alternative for reducing vehicular emissions. Poster presentation at the 83rd Annual Meeting of the Transportation Research Board, Washington DC.  Constructing roundabouts in place of traffic signals or stop signs reduced fuel consumption by an estimated 23-34 percent. Hu, W.; McCartt, A.T.; Jermakian, J.S.; and Mandavilli, S. 2013. Public opinion, traffic performance, the environment, and safety after the construction of double-lane roundabouts. Arlington, VA: Insurance Institute for Highway Safety.  Várhelyi, A. 2002. The effects of small roundabouts on emissions and fuel consumption: a case study. Transportation Research Part D: Transport and Environment 7(1):65-71. Niittymäki, J. and Höglund P.G. 1999. Estimating vehicle emissions and air pollution related to driving patterns and traffic calming. Presented at the Urban Transport Systems Conference, Lund, Sweden.

    A 2005 Institute study documented missed opportunities to improve traffic flow and safety at 10 urban intersections suitable for roundabouts where either traffic signals were installed or major modifications were made to 10 intersections with signals. Bergh, C.; Retting, R.A.; and Myers, E.J. 2005. Continued reliance on traffic signals: the cost of missed opportunities to improve traffic flow and safety at urban intersections. Arlington, VA: Insurance Institute for Highway Safety. It was estimated that the use of roundabouts instead of traffic signals at these intersections would have reduced vehicle delays by 62-74 percent. This is equivalent to approximately 325,000 fewer hours of vehicle delay on an annual basis. Based on the results of this study, it is estimated that the conversion of 10 percent of the signalized intersections in the United States to roundabouts would have reduced vehicle delays by more than 900 million hours and fuel consumption by more than 600 million gallons in 2012.

  7. 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 paved with materials like brick or cobblestone that have a different texture than the roadway to discourage smaller vehicles from using it.

  8. 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 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 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. Brilon, W.; Stuwe, B.; and Drews, O. 1993. Sicherheit und Leistungsfahigkeit von Kreisverkehrsplatzen (Safety and capacity of roundabouts). FE Nr 77359/91. Bochum, Germany: Lehrstuhl fur Verkehrswesen, Ruhr-Universitat Bochum. Cited by: Elvik R. Effects on road safety of converting intersections to roundabouts: a review of evidence from non-US studies. Transportation Research Record 1847:1-10. Schoon, C. and van Minnen, J. 1994. The safety of roundabouts in the Netherlands. Traffic Engineering and Control 35(3):142-8. Single-lane roundabouts, in particular, have been reported to involve substantially lower pedestrian crash rates than comparable intersections with traffic signals. Brude, U. and Larsson, J. 2000. What roundabout design provides the highest possible safety? Nordic Road & Transport Research  2000(2):17-21.

    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, L.; Blogg, M.; Wemple, E.; Myers, E.; Kyte, M.; Dixon, M.; List, G.; Flannery, A.; Troutbeck, R.; Brilon, W.; Wu, N.; Persaud, B.; Lyon, C.; Harkey, D.; and Carter, D. 2007. Roundabouts in the United States. National Cooperative Highway Research Program Report no. 572. Washington, DC: Transportation Research Board. Another study found that drivers exiting a roundabout were less likely to yield to pedestrians than drivers entering a roundabout. Hourdos, J.; Richfield, V.; and Shauer, M. 2012. Investigation of pedestrian/bicyclist risk in Minnesota roundabout crossings. Report no. MN/RC 2012-28. St. Paul, MN: Minnesota Department of Transportation.

  9. Do drivers like roundabouts?

    Drivers may be skeptical of or even opposed to roundabouts when they are proposed. However, several Institute studies show that opinions quickly change when drivers become familiar with them. 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. Retting, R.A.; Luttrell, G.; and Russell, E.R. 2002. Public opinion and traffic flow impacts of newly installed modern roundabouts in the United States. ITE Journal 72:30-32,37. Another study surveyed drivers in three additional communities where a one- or two-lane roundabout replaced stop signs or traffic signals. Overall, 36 percent of drivers supported the roundabouts before construction compared with 50 percent shortly after. Retting, R.A.; Mandavilli, S.; Russell, E.R.; and McCartt, A.T. 2006. Roundabouts, traffic flow and public opinion. Traffic Engineering and Control 47(7):268-72. 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. Retting, R.A.; Kyrychenko, S.Y.; and McCartt, A.T. 2007. Long-term trends in public opinion following construction of roundabouts. Transportation Research Record 2019:219-24.

    The additional travel lanes in multilane roundabouts increase the complexity of the driving task. Still, a study of a pair of two-lane roundabout conversions near Bellingham, Wash., found that the proportion of drivers who favored the roundabouts increased from 34 percent before construction to 51 percent six months after and 70 percent more than one year after. Hu, W.; McCartt, A.T.; Jermakian, J.S.; and Mandavilli, S. 2013. Public opinion, traffic performance, the environment, and safety after the construction of double-lane roundabouts. Arlington, VA: Insurance Institute for Highway Safety. 

  10. How do roundabouts affect older drivers?

    Older drivers are more likely than other drivers to be wary of roundabouts, but they also are particularly likely to benefit from them in terms of improved safety. Relative to other age groups, senior drivers are over involved in crashes occurring at intersections. In 2012, 37 percent of fatal passenger vehicle crashes involving drivers 70 and older were intersection crashes, compared with 24 percent of fatal crashes of drivers younger than 70.

    Older drivers' intersection crashes often are due to their failure to yield the right-of-way. Mayhew, D.R.; Simpson, H.M.; and Ferguson, S.A. 2006. Collisions involving senior drivers: high-risk conditions and locations. Traffic Injury Prevention 7(2):117-24. Braitman, K.A.; Kirley, B.B.; Chaudhary, N.K.; and Ferguson, S.A. 2007. Factors leading to older drivers' intersection crashes. Traffic Injury Prevention 8(3):267-74. Since all traffic flows in the same direction at roundabouts and more slowly than at traditional intersections, the consequence for failing to yield is likely less severe at roundabouts. Particular problems for older drivers at traditional intersections include left turns and entering busy thoroughfares from cross streets. Roundabouts eliminate these situations entirely. 

    Although safety effects of roundabouts specifically for older drivers are unknown, a 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. This suggests roundabouts don’t pose a problem for older drivers. Persaud, B.N.; Retting, R.A.; Garder, P.E.; and Lord, D. 2001. Safety effect of roundabout conversions in the United States: empirical Bayes observational before-after study. Transportation Research Record 1751:1-8.

    A study in six communities where roundabouts replaced traditional intersections found that about two-thirds of drivers 65 and older supported the roundabouts. Retting, R.A.; Kyrychenko, S.Y.; and McCartt, A.T. 2007. Long-term trends in public opinion following construction of roundabouts. Transportation Research Record 2019:219-24. A study of two intersections converted to roundabouts near Bellingham, Wash., found that about two-thirds of driers 70 and older favored the roundabouts one year after construction. Hu, W.; McCartt, A.T.; Jermakian, J.S.; and Mandavilli, S. 2013. Public opinion, traffic performance, the environment, and safety after the construction of double-lane roundabouts. Arlington, VA: Insurance Institute for Highway Safety.  In both studies, the older drivers were less likely to favor roundabouts than younger drivers.

    Observations of drivers in the study in Washington found that older drivers (those with an estimated age of 70 or older) were significantly less likely to travel on the roundabout corridor versus an alternative parallel route without roundabouts after the roundabout conversions. Hu, W.; McCartt, A.T.; Jermakian, J.S.; and Mandavilli, S. 2013. Public opinion, traffic performance, the environment, and safety after the construction of double-lane roundabouts. Arlington, VA: Insurance Institute for Highway Safety.  However, the proportions of older drivers on either road were very small before and after the roundabout conversion. In another study, design elements that improve the path and operational guidance were found to increase the comfort, confidence and perception of safety for drivers ages 65 and older. Lord, D.; Schalkwyk, I.; Chrysler, S.; and Staplin, L. 2007. A strategy to reduce older driver injuries at intersections using more accommodating roundabout design practices. Accident Analysis and Prevention 39(3):427-32. For example, a yield sign could have a plaque underneath reading "to traffic in circle", and an advanced warning sign could have a plaque with the word "roundabout".

  11. How common are roundabouts in the United States?

    The first modern roundabouts in the United States were constructed in Nevada in 1990. Since then, more than 1,800 have been built, although the precise number is unknown. Kittleson and Associates. Roundabout Inventory Database. Available: http://roundabout.kittelson.com/Roundabouts. Accessed: February 11, 2014. 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 nine evidence-based safety countermeasures recommended by the Federal Highway Administration. Federal Highway Administration 2012. Guidance memorandum on promoting the implementation of proven safety countermeasures. Washington, DC: U.S. Department of Transportation. 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. Highway Design Manual. New York State Department of Transportation. Virginia State Department of Transportation 2009. Road Design Manual. Virginia State Department of Transportation.

  12. What kinds of intersections are good candidates for roundabouts? What kinds aren't?

    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 site to site 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 stop sign- or traffic signal-controlled intersections.

    Intersections 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 often are not ideal candidates for roundabouts.

  13. Are roundabouts more expensive than 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 signal.  Rodegerdts, L.; Bansen, J.; Tiesler, C.; Knudsen, J.; Myers, E.; Johnson, M.; Moule, M.; Persaud, B.; Lyon, C.; Hallmark, S.; Isebrands, H.; Crown, R.; Guichet, B.; and O’Brien, A. 2010. Roundabouts: an informational guide. National Cooperative Highway Research Program Report no. 672. Washington, DC: Transportation Research Board.

  14. What is a mini-roundabout?

    A mini-roundabout has a smaller diameter and central island than a full-size roundabout. Mini-roundabouts are typically used in low-speed urban environments where there is not enough space to build a larger roundabout. In a mini-roundabout the central island is designed so that larger vehicles can drive over it, if necessary, and the splitter island on the approach and exit roads typically is not raised. Mini-roundabouts accommodate pedestrians well because of the short crossing distances and very low vehicle speeds on entering and exiting approaches. Developed in the United Kingdom, mini-roundabouts are used in many countries around the world and are increasingly being built in the United States.  Rodegerdts, L.; Bansen, J.; Tiesler, C.; Knudsen, J.; Myers, E.; Johnson, M.; Moule, M.; Persaud, B.; Lyon, C.; Hallmark, S.; Isebrands, H.; Crown, R.; Guichet, B.; and O’Brien, A. 2010. Roundabouts: an informational guide. National Cooperative Highway Research Program Report no. 672. Washington, DC: Transportation Research Board. Federal Highway Administration. 2010. Mini-roundabouts: technical summary. Report no. FHWA-SA-10-007. Washington, DC: U.S. Department of Transportation.