What are the benefits?
Roundabouts are used to improve safety, increase intersection capacity and efficiency, reduce environmental impacts, and enhance community values. Additional benefits include lower costs over other types of intersections and greater design flexibility. Explore the benefits of roundabouts in-depth below.
SAFETY: ROUNDABOUTS VS. TRADITIONAL INTERSECTIONS
Reduction in number and severity of crashes
Several features of the modern roundabout promote safety. The safety benefits alone are the primary reason roundabouts are being installed throughout the country and the world. According to a study by the Institute for Highway Safety of locations where stop signs or traffic lights were replaced by roundabouts, all crashes were reduced by 37 percent and
serious crashes fell by 75 percent, including a 90 percent reduction in fatalities. The study also found a 40 percent reduction in pedestrian collisions.
At traditional intersections, 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 to "beat the red light." With roundabouts, these types of potentially serious crashes are eliminated because vehicles travel in the same direction.
Reduced Conflict Points
Traditional intersections have 32+ conflict points between vehicles (cars, trucks, bikes) and 16+ conflict points between vehicles and pedestrians. When a roundabout is installed, the number of conflict points is reduced to eight. The conflicts that do occur at roundabouts generally involve a vehicle merging into the circular roadway, with both vehicles traveling at low speeds – generally less than 25 mph in urban areas -- resulting in collisions that are low in severity. Installing roundabouts in place of traffic signals also reduces the likelihood of rear-end crashes by reducing abrupt stops at red lights.
Pedestrian and Bicycle Safety
Pedestrians and bicyclists need only cross one direction of traffic at a time at each approach as they traverse roundabouts, as compared with unsignalized intersections. Crossing distances are relatively short, and traffic speeds are lower than at traditional intersections. In addition, the speeds of motorists entering and exiting a roundabout are reduced with good design. Learn more about how people on bicycles can use roundabouts (link to new page).
INCREASED INTERSECTION CAPACITY AND EFFICIENCY
Under many traffic conditions, an unsignalized roundabout can operate with less delay to users than traffic signal control or all-way stop control.
Unlike all-way stop intersections, a roundabout does not require a complete stop by all entering vehicles, which reduces both individual delay and delays resulting from vehicle queues. Many drivers adjust their speed to take advantage of approaching gaps in circulating traffic, particularly in the off-peak period. If there is no traffic in the roundabout, they don't have to stop at all.
A roundabout can also operate more efficiently than a signalized intersection because drivers are able to enter from different approaches at the same time when traffic is clear without the delay incurred while waiting for the traffic signal to change (i.e. no yellow or red times).
Results from studies at Kansas State University (KSU) on several modern roundabouts show that modern roundabouts have greater overall operational efficiency than all other forms of traffic control. In one example, a KSU study analyzed changes in before and after delay and percent stopping.
|Measure of Effectiveness||Before||After||%Diff||Description of Measure|
|Average Intersection Delay (Seconds/Veh)||20||8||-65%||Average vehicle delay for all vehicles entering the intersection|
|Max Approach Delay (Seconds/Veh)||34||10||-71%||Average vehicle delay for the approach with highest delay|
|95% Queue Length (Feet)||190||104||-53%||Value below which 95% of all observed lengths of vehicles being backed fall|
|Proportion Stopped–Intersection (%)||58||29||-52%||Proportion of vehicles approaching intersection and required to stop|
|Max Proportion Stopped (%)||62||37||-62%||Highest proportion of vehicles stopped on one approach|
|Degree Of Saturation–Intersection (v/c)||0.463||0.223||-53%||Measure of congestion on the roadway that is being used by traffic|
It adds up...
A roundabout serving 15,000 vehicles a day saves, annually, a total of 15,000 gallons of fuel, compared to the same intersection controlled by a traffic light, according to analysis from the Washington State Department of Transportation.
A key finding of the Insurance Institute for Highway Safety in one study is that vehicle delays at the 10 intersections would have been reduced by 62-74 percent, saving 325,000 hours of motorists' time annually. Fuel consumption would have gone down by about 235,000 gallons per year, and there would have been commensurate reductions in vehicle emissions. Assuming $18 per hour in worker wages and $3.50 per gallon of gas is a community cost of $6.7 million.
REDUCED ENVIRONMENTAL IMPACT
By reducing vehicular delay through increased efficiency, the community will improve air quality through lowered emissions.
The potential reduction in fuel consumption can increase in air quality. Another KSU study found a significant reduction in emissions in before and after measurements.
|Measures of Effectiveness||Before||After||% Change|
|Carbon Monoxide (CO) Kg/hr||10.79||7.26||-33%|
|Carbon dioxide (CO2) Kg/hr||237.30||127.59||-46%|
|Oxides of Nitrogen (NOx) Kg/hr||0.348||0.225||-35%|
|Hydrocarbons (HC) Kg/hr||0.446||0.21||-53%|
Community members may be skeptical of or even opposed to roundabouts when they are proposed. However, several Insurance Institute for Highway Safety (IIHS) studies show that opinions quickly change when drivers become familiar with them.
- In three communities where single-lane roundabouts replaced stop sign-controlled intersections, 31 percent of drivers supported the roundabouts before construction, compared with 63 percent shortly after (Retting et al., 2002).
- In three other communities where a one- or two-lane roundabout replaced stop signs or traffic signals, 36 percent of drivers supported the roundabouts before construction compared with 50 percent shortly after (Retting et al., 2006).
- Follow-up surveys conducted in the above 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 et al., 2007).
- When two intersections near Bellingham, Washington, were converted to two-lane roundabouts, support for the roundabouts went from 34 percent before construction to 51 percent six months after and 70 percent more than one year after (Hu et al., 2014).