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Roundabouts

Roundabouts

Roundabouts are one of the most effective intersection improvements available.

Roundabouts are growing in popularity as more and more transportation agencies recognize the increased safety and traffic-moving efficiency they bring to intersections. 

Safety

The most common justification for a roundabout is safety. This is because roundabouts only have 8 potential conflict points vs. 32 at a traditional intersection. Studies by the Federal Highway Administration (FHWA) show that roundabouts achieve a 44% reduction in crashes and reduce serious injury and deadly crashes by nearly 90% at two-way stop intersections. When roundabouts replace a traffic signal, FHWA found a 48% reduction in crashes and nearly 80% drop in serious injury and deadly crashes.

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.

Efficiency

Roundabouts can move traffic more efficiently which reduces delays and fuel consumption. This is because traffic generally doesn’t need to come to a full stop at the intersection.

A study by the Insurance Institute for Highway Safety (IIHS) 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 981 million hours and fuel consumption by more than 654 million gallons in 2018.

Larger Vehicles

Ohio is a state that grows things and moves things. There are often concerns about how large farm machinery or semi-trucks will navigate roundabouts. 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.

Public Opinion

Roundabouts typically aren’t the most popular solution with the public. Much of that is due to a lack of education about their benefits and them being a relatively new traffic pattern for many areas of Ohio. Public reaction usually flips to positive within a year or two of them being installed and open to traffic.

An IIHS study of three communities where single-lane roundabouts replaced stop sign-controlled intersections found that only 31% of drivers supported the change before construction. However, after more than a year support soared to 70%.

Navigation

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.