Understanding Roundabout Design 

Roundabouts are increasingly popular in traffic and safety engineering projects due to their proven efficiency and safety benefits. If you’re new to the concept, understanding the basics of roundabout design can help you appreciate why they may be favored over traditional intersection configurations.  

What is a Roundabout? 

A roundabout is a type of circular intersection where traffic flows counterclockwise around a central island. Unlike traffic circles or rotaries, roundabouts are designed to slow down vehicles, reduce conflict points, and improve traffic flow. Traffic entering a roundabout is always required to yield to the circulating traffic. 

Key Elements of Roundabout Design 

Central Island 

The central island is the circular area in the middle of the roundabout. It is usually landscaped to enhance aesthetics and make the roundabout more visible to approaching drivers. The size and design of the central island can vary depending on the specific requirements of the project (i.e., design vehicle). 

Entry and Exit Lanes 

Entry lanes are the approach roads leading into the roundabout. They are designed to include roadway geometry to physically slow down approaching traffic. Exit lanes are the roads leading out of the roundabout. Proper design ensures smooth and safe entry and exit for all types of vehicles. 

Circulatory Roadway 

This is the lane(s) that circles around the central island. It is designed to accommodate vehicles of all sizes, including large trucks and emergency vehicles. The width of the circulatory roadway can vary based on traffic volume and types of vehicles expected. Dedicated bicycle lanes are never included adjacent to the circulatory roadway. Rather, bicyclists are given the choice to mix with vehicles or exit the roadway prior to entering the roundabout and navigate the intersection as a pedestrian. 

Pedestrian Crossings 

Pedestrian safety is a critical aspect of roundabout design. Crosswalks are usually placed one car length from the entry (to prevent blocking) and exit points where vehicle speeds are typically lower. Raised pedestrian crossings and refuge islands are typically included to allow for two-stage crossings thereby enhancing safety. 

Benefits of Roundabout Design 

Improved Safety 

Roundabouts significantly reduce the likelihood of severe crashes due to the lower speeds associated with these facilities. Further, traditional intersections have multiple conflict points where accidents can occur. In contrast, roundabouts reduce the number of conflict points and eliminate the possibility of head-on and high-speed right-angle collisions. 

Efficient Traffic Flow 

Roundabouts keep traffic moving smoothly and typically result in reduced congestion. Because vehicles only have to yield rather than stop, delays are minimized. This is particularly beneficial for side street approaches during off-peak traffic hours, when mainline volumes are lower. 

Cost-Effective 

In the long run, roundabouts can be more cost-effective than signalized intersections. They require less maintenance and can operate efficiently without the need for traffic signal maintenance and electricity. 

Environmental Benefits 

Roundabouts can reduce vehicle idling and improve fuel efficiency, leading to lower emissions. This can make  them an environmentally friendly alternative to traffic signal control. 

Federal Highway Administration (FHWA) Guidelines for Roundabout Design 

The FHWA provides detailed guidelines for roundabout design, which serve as a valuable resource for engineers. These guidelines cover various aspects, such as: 

• Geometric Design: Recommendations for the size, shape, and configuration of roundabouts. 

• Traffic Control: Guidelines for yield signage, pavement markings, and other traffic control devices. 

• Safety Considerations: Strategies for reducing conflict points and enhancing visibility. 

• Accessibility: Guidance on accommodating pedestrians and cyclists, including those with disabilities. 

By following these guidelines, engineers can design roundabouts that are efficient, safe, and suitable for the specific needs of a community. 

Common Challenges and Solutions 

Space Constraints 

One of the main challenges in roundabout design is space. In urban areas, acquiring enough land can be difficult. In such cases, a mini-roundabout or a compact roundabout might be a feasible alternative. 

High Traffic Volume 

Roundabouts are generally effective for moderate traffic volumes. For high traffic volumes, a multi-lane roundabout or a signalized roundabout may be necessary to ensure smooth traffic flow. 

Accommodating Large Vehicles 

Designing a roundabout that can accommodate large vehicles like trucks and buses can be challenging. The use of truck aprons—paved areas around the central island—can help large vehicles navigate the roundabout without compromising safety. 

Public Acceptance 

Introducing roundabouts in communities unfamiliar with them can lead to resistance. Public education campaigns and community engagement can help in gaining public acceptance and ensuring the successful implementation of roundabouts. 

Where We Make An Impact

Roundabouts offer numerous benefits, including improved safety, efficient traffic flow, cost savings, and environmental benefits. By following guidelines from FHWA or state specific engineers can design roundabouts that meet the specific needs of their locations while ensuring safety and efficiency. McClure is ready to help with your next roundabout project, providing expertise to create solutions that enhance traffic flow and safety.

Understanding Roundabout Design 

Roundabouts are increasingly popular in traffic and safety engineering projects due to their proven efficiency and safety benefits. If you’re new to the concept, understanding the basics of roundabout design can help you appreciate why they may be favored over traditional intersection configurations.  

What is a Roundabout? 

A roundabout is a type of circular intersection where traffic flows counterclockwise around a central island. Unlike traffic circles or rotaries, roundabouts are designed to slow down vehicles, reduce conflict points, and improve traffic flow. Traffic entering a roundabout is always required to yield to the circulating traffic. 

Key Elements of Roundabout Design 

Central Island 

The central island is the circular area in the middle of the roundabout. It is usually landscaped to enhance aesthetics and make the roundabout more visible to approaching drivers. The size and design of the central island can vary depending on the specific requirements of the project (i.e., design vehicle). 

Entry and Exit Lanes 

Entry lanes are the approach roads leading into the roundabout. They are designed to include roadway geometry to physically slow down approaching traffic. Exit lanes are the roads leading out of the roundabout. Proper design ensures smooth and safe entry and exit for all types of vehicles. 

Circulatory Roadway 

This is the lane(s) that circles around the central island. It is designed to accommodate vehicles of all sizes, including large trucks and emergency vehicles. The width of the circulatory roadway can vary based on traffic volume and types of vehicles expected. Dedicated bicycle lanes are never included adjacent to the circulatory roadway. Rather, bicyclists are given the choice to mix with vehicles or exit the roadway prior to entering the roundabout and navigate the intersection as a pedestrian. 

Pedestrian Crossings 

Pedestrian safety is a critical aspect of roundabout design. Crosswalks are usually placed one car length from the entry (to prevent blocking) and exit points where vehicle speeds are typically lower. Raised pedestrian crossings and refuge islands are typically included to allow for two-stage crossings thereby enhancing safety. 

Benefits of Roundabout Design 

Improved Safety 

Roundabouts significantly reduce the likelihood of severe crashes due to the lower speeds associated with these facilities. Further, traditional intersections have multiple conflict points where accidents can occur. In contrast, roundabouts reduce the number of conflict points and eliminate the possibility of head-on and high-speed right-angle collisions. 

Efficient Traffic Flow 

Roundabouts keep traffic moving smoothly and typically result in reduced congestion. Because vehicles only have to yield rather than stop, delays are minimized. This is particularly beneficial for side street approaches during off-peak traffic hours, when mainline volumes are lower. 

Cost-Effective 

In the long run, roundabouts can be more cost-effective than signalized intersections. They require less maintenance and can operate efficiently without the need for traffic signal maintenance and electricity. 

Environmental Benefits 

Roundabouts can reduce vehicle idling and improve fuel efficiency, leading to lower emissions. This can make  them an environmentally friendly alternative to traffic signal control. 

Federal Highway Administration (FHWA) Guidelines for Roundabout Design 

The FHWA provides detailed guidelines for roundabout design, which serve as a valuable resource for engineers. These guidelines cover various aspects, such as: 

• Geometric Design: Recommendations for the size, shape, and configuration of roundabouts. 

• Traffic Control: Guidelines for yield signage, pavement markings, and other traffic control devices. 

• Safety Considerations: Strategies for reducing conflict points and enhancing visibility. 

• Accessibility: Guidance on accommodating pedestrians and cyclists, including those with disabilities. 

By following these guidelines, engineers can design roundabouts that are efficient, safe, and suitable for the specific needs of a community. 

Common Challenges and Solutions 

Space Constraints 

One of the main challenges in roundabout design is space. In urban areas, acquiring enough land can be difficult. In such cases, a mini-roundabout or a compact roundabout might be a feasible alternative. 

High Traffic Volume 

Roundabouts are generally effective for moderate traffic volumes. For high traffic volumes, a multi-lane roundabout or a signalized roundabout may be necessary to ensure smooth traffic flow. 

Accommodating Large Vehicles 

Designing a roundabout that can accommodate large vehicles like trucks and buses can be challenging. The use of truck aprons—paved areas around the central island—can help large vehicles navigate the roundabout without compromising safety. 

Public Acceptance 

Introducing roundabouts in communities unfamiliar with them can lead to resistance. Public education campaigns and community engagement can help in gaining public acceptance and ensuring the successful implementation of roundabouts. 

WHERE WE MAKE AN IMPACT