What is a Roundabout?

Modern roundabouts are becoming mainstream throughout Ohio and the United States. These circular intersections efficiently move traffic counterclockwise around a center island, offering efficiency, economic savings, and increased safety benefits to drivers and communities.  An intersection’s traffic volume and size are a few factors considered when deciding a roundabout is the preferred choice over a traffic light-controlled option in a location.

Efficiency

• Roundabouts are more efficient than traditional intersections.
  • Eliminate wasted time spent at red lights during off-peak hours.
  • 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.
  • 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.

Economic Savings

• Roundabouts cost less long-term than traditional traffic signal intersections.
  • Reduce road electricity and maintenance costs.
  • Less expensive to operate than traditional intersections with traffic signals.
  • Provide longer service life:
    • Roundabouts = 25-year service life
    • Signal equipment = 15-year service life

Safety Benefits

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. 

Roundabouts versus Traffic Signals

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. The diagram below illustrates the difference in conflict points between a conventional intersection and an equivalent single-lane roundabout. There are 32 conflict points in the conventional intersection (8 merging, 8 diverging, 16 crossing) and only eight total conflict points in an equivalent roundabout (4 merging, 4 diverging). Additionally, the type of conflicts in a roundabout are of the same-direction variety, which result in substantially less severe collisions with a lower likelihood of injury or death. 

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. 

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Pedestrian-versus-vehicle conflict points are also significantly fewer in a roundabout (as shown in the below diagram) with only eight as opposed to 32 at a conventional intersection.

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 often include stop signs or traffic signals and 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. Roundabouts are designed for lower-speed driving with all traffic moving in the same direction, eliminating left turns across traffic and creating fewer stress points that can cause crashes.

Frequently Asked Questions

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.

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, they yield 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.

Roundabout Tips to Remember

• Take it slow. Roundabouts are intentionally designed to slow vehicle speeds to 15-20 mph.
• Look left and yield to all pedestrians and vehicles already inside the roundabout.
• Stay in your lane. Do not change lanes within the roundabout. As with any other type of intersection, you must be in the proper lane before entering.
• Keep moving. Once in the roundabout, you have the right-of-way. Always stop for pedestrians.
• Clear the roundabout for emergency vehicles. Exit the roundabout, then pull to the right. Do not stop within the roundabout.

Printable Resources and Videos

Roundabout Video (Credit: City of Mason)

Rules of the Roundabout Video (Credit: City of Montgomery)

Roundabouts Video (Credit: West Chester Township)

Roundabout Article: Roundabouts becoming part of driving landscape to improve safety, reduce travel time (Credit: Dayton Daily News)

Roundabouts vs. Traffic Lights Video

Benefits of Roundabouts Video

Why the US Hates Roundabouts Video

The Ohio Department of Transportation Videos

View the Ohio Department of Transportation’s roundabout rules videos, with the three levels of movement on roundabouts:

• Roundabout Rules 1: Single Lane Roundabout
• Roundabout Rules 1B: Single Lane with a Lane Shift
• Roundabout Rules 2: Multi-lane Roundabouts
• Roundabout Rules 2B: Multi-lane with Pedestrians and a Lane Shift
• Roundabout Rules 3: Multi-lane with Different Lane Configurations
• Roundabout Rules: Overview