Several studies conducted by IIHS and others have reported significant improvements in traffic flow following conversion of traditional intersections to 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 et al., 2002).
- 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 et al., 2006).
- 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 et al., 2004).
- An Institute study of two-lane roundabout conversions at two intersections near Bellingham, Washington, 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) (Hu et al., 2014). 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.
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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 23-34 percent and hydrocarbon emissions by 0-40 percent (Hu et al., 2014; Várhelyi, 2002).
Constructing roundabouts in place of traffic signals or stop signs reduced fuel consumption by an estimated 23-34 percent (Hu et al., 2014; Várhelyi, 2002; Höglund & Niittymäki, 1999).
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 et al., 2005). It was estimated that the use of roundabouts instead of traffic signals at these intersections would have reduced vehicle delays by 62-74 percent.
Based on the results of that study, we estimate that the conversion of 10 percent of the signalized intersections in the United States to roundabouts would have reduced vehicle delays by more than 981 million hours and fuel consumption by more than 654 million gallons in 2018.
Reprinted with permission from the Insurance Institute for Highway Safety. Please visit here for the original article.
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