Roundabout Benefits

Safety studies from the Insurance Institute for Highway Safety and the National Cooperative Highway Research Program (NCHRP) 572*, with both studies indicating roundabouts reduce the severity of crashes at intersections where stop signs or signals were previously used for traffic control.

*NCHRP Report 572: Roundabouts in the United States. National Cooperative Highway Research Program, TRB, NAS, Washington DC, 2007.

Several reasons why roundabouts are safer:

  • Less potential for serious crashes - since vehicles all travel around the center island in the same direction, head-on and left-hand turn (T-bone) collisions are eliminated.
  • Low travel speeds - because drivers must yield to traffic before entering a roundabout, they naturally slow down. The few collisions that occur in roundabouts are typically minor with few injuries, since they occur at low speeds of 15 - 20 miles per hour.
  • No red lights to run - roundabouts are designed to keep traffic flowing without requiring vehicles to stop, so the incentive for drivers to speed up to make it through a yellow or red light is removed.
  • Reduce delay and improve traffic flow

Contrary to many peoples' perceptions, roundabouts actually move traffic through an intersection faster, and with less congestion on approaching roads. Roundabouts promote a continuous flow of traffic. Unlike intersections with traffic signals, you don't have to wait for a green light at a roundabout to get through the intersection. Traffic is not required to stop - only yield - so the intersection can handle more traffic in the same amount of time.

Roundabout studies by Kansas State University have measured traffic flow at intersections before and after conversion to roundabouts. In each case, installing a roundabout led to a 20 percent reduction in delays. The proportion of vehicles that had to stop - just long enough for a gap in traffic - was also reduced.

Conflict Points
Source: http://safety.fhwa.dot.gov/intersection/roundabouts/presentations/safety_aspects/short.cfm

Save money

The cost difference between building a roundabout and a traffic signal is comparable. A roundabout may need more property within the actual intersection, but takes up less space on the streets approaching the roundabout. Roundabouts usually require less overall property to build than a signal with turn lanes because traffic doesn't have to line up and wait for a green light. In addition to reducing congestion and increasing safety, roundabouts eliminate hardware, maintenance and electrical costs associated with traffic signals, which can amount to approximately $5,000 per year. However, there are typically more overhead lights and additional maintenance with the central island landscaping or grass mowing at a roundabout. Many communities are also favorable to the aesthetics of a well-designed and landscaped roundabout. There is typically little difference in the overall cost and maintenance between a signalized intersection and a roundabout.

Good locations for roundabouts

Roundabouts are safe and efficient, but they are not the ideal solution for every intersection. SDDOT looks at several factors when deciding to build a roundabout at an intersection. Engineers consider these characteristics when determining the best solution for a particular intersection:

  • Crash history
    • Data about the number of accidents
    • Type of crashes
    • Speeds
    • Other contributing factors
  • Intersection operation
    • The level of current and projected travel delay being experienced, and backups on each leg of the intersection.
    • Types of vehicles using the intersection - This is especially important for intersections frequently used by large trucks.
  • Cost
    • Societal cost of crashes
    • Right-of-way (land purchase) requirements
    • Long-term maintenance needs

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.

  • 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.
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