Roundabouts are one of the most effective intersection control treatments available with the added benefit of calming traffic. They limit vehicle speeds to approximately 20 mph and can control vehicle speeds on four approaches simultaneously.
According to Federal Highway Administration (FHWA) study results, roundabouts achieve a 44% reduction in all crashes and a 72%-87% reduction in fatal/injury crashes when converting a two-way stop intersection to a roundabout. They also report a 48% reduction in all crashes and a 60%-78% reduction in fatal/injury crashes when converting a signalized intersection to a roundabout, and greatly reduce severity on those few crashes that do occur.
Roundabouts achieve significant crash reductions because they simplify motorist decision making and have fewer conflict points. At a four-way intersection, there are 32 possible conflict points between vehicles and only eight at roundabouts. See the diagrams to the right for a comparison of conflict points.
Angle and left turn crashes account for 63% of fatal crashes at an intersection. These are typically more severe because of the speed and manner of impact. A roundabout eliminates angle and left turn crashes by forcing vehicles to make a right turn in and a right turn out. Crashes that do occur in a roundabout are sideswipe in nature and less severe due to low vehicle speeds.
Roundabouts versus Traffic Signals
Roundabouts have proven to be much safer than traffic signals. The projected injury crash rate for roundabouts is half that of traditional signals.
Roundabouts versus Traffic Circles
There are many differences between roundabouts and traffic circles. Unlike traffic circles, roundabouts are used on higher volume streets to allocate right-of-way between competing intersection movements. Traffic circles have a large diameter, which contributes to high circulating speeds; roundabouts have a smaller diameter, promoting low circulating speeds. Roundabouts have lower entry speeds compared to traffic circles and feature a yield at every entry point, promoting low speed and no weaving.
Research shows that drivers quickly adapt to the roundabout traffic flow. For instance, Vail and Avon, Colorado, both feature many high capacity roundabouts and are major tourist destinations with thousands of first-time roundabout drivers using the roundabout intersections each year. Despite large numbers of drivers who have not driven roundabouts previously, these intersections work well and do not confuse motorists. Proper use of signing and road striping at roundabouts assists motorists and minimizes the potential for confusion.
Frequently Asked Questions
What is the purpose of a roundabout?
Roundabouts are designed to be safer and more efficient than a traditional intersection. The geometry creates a low speed (20-30mph) environment inside the circulatory roadway, as well as at the entry and exit locations. The geometry also prevents high angle crashes such as "T-bone" and left turn angle crashes. Lower angle, low speed crashes tend to be less severe than higher angle, high speed crashes.
More efficient operation results from the yield at entry - drivers only have to watch for traffic from the left, and if there is an adequate gap available, they can enter the roundabout without stopping. Once in the roundabout, drivers have the right-of-way, so they will not have to stop or yield to exit. If the driver does need to yield at entry to traffic inside the roundabout, their delays are brief and typically less than the time they would have been delayed at a traffic signal.
How much traffic can a roundabout accommodate?
According to Roundabouts: An Informational Guide from the Federal Highway Administration, the maximum Average Daily Traffic (ADT) for a single-lane, four-leg roundabout is greater than 20,000 vehicles per day. For double-lane roundabouts, 40,000 to 50,000 vehicles per day can be accommodated, depending on the traffic patterns.
How do semis, oversized loads, farm equipment, and other large vehicles navigate roundabouts?
The design of the intersection will allow oversized loads and other large vehicles to navigate the roundabout while still providing adequate visual and physical indicators to guide and slow passenger vehicles. One way this is accomplished is with truck aprons - an area between the central island and the traveled way that is mountable by larger vehicles but not used by passenger vehicles.
Do roundabouts have an elevation or slope?
Like all roads, roundabouts have some "slope" to drain rain water off of the roadway. Like all "flat" roads, the slope is very slight and typically unnoticeable to roadway users.
What about drivers who are not familiar with roundabouts?
Roundabouts are designed to be simple to use. The geometry cues drivers to slow down, allowing more time for decisions. Once the driver reaches the yield line, he/she yields to traffic already in the roundabout. The only decision remaining is if the driver wants to take the first exit to turn right, the second exit to continue straight, the third exit to turn left, or the fourth exit to make a U-turn.
Are roundabouts worth the cost to install them?
ODOT, like other state and local agencies, is operating with reduced funding and is very conscious of the need to utilize funding in a manner that produces the greatest benefit for the least cost. While roundabouts may often be more expensive, they do cost less than the average cost of a single fatal car crash (taking into account lost earnings, lost household production, property damage, medical costs, and other factors). AAA estimates that a single fatal motor vehicle crash costs the nation $6 million. ODOT considers roundabouts to be very cost-effective if they reduce fatalities and injuries.
How do roundabouts affect air quality?
Replacement of signalized intersections with roundabouts has been found to reduce vehicle emissions and fuel consumption by 30% or more. This is due to the reduction in idle time by vehicles waiting for the light to change.