Roundabouts

Innovative Intersections

Drivers on a roundabout intersection from overhead.

What is a roundabout?

A modern roundabout is a circular intersection where traffic moves counterclockwise around a central island. Vehicles entering the roundabout must yield to those already circulating.

Roundabouts can be used in both urban and rural settings and come in three configurations: mini, single-lane, and multi-lane.

Mini roundabout

Single-lane roundabout

Multi-lane roundabout

Design principles

The modern roundabout is characterized by the following basic principles:

Shape
Generally circular shape with counterclockwise traffic flow

Yield-at-Entry
Vehicles approaching the roundabout must wait for a gap in the circulating flow, or yield, before entering the circle

Slow speeds
Through geometric curvature and deflection, the traffic is directed or channeled to the right with a curved entry path into the circulatory roadway.

Design characteristics

Roundabouts are a versatile choice for intersection control because the geometry can be modified to suit the constraints of each intersection. While roundabout geometry is flexible, there are certain design characteristics that all roundabouts share and some that are site dependent.

Common to all roundabouts

Central island: Directs vehicles around circulating lane

Splitter islands: Separates approach and exit lanes in advance of the intersection

Lighting, signing, & pavement markings: Improves intersection visibility and provides needed information to intersection users.

Site dependent

Truck aprons: Accommodates large vehicle trailers

Crosswalks, sidewalks, & shared use paths: Accommodates pedestrians and bicyclists

Landscaping & hardscaping: Can be used to enhance intersection visibility and beauty

Roundabout overview

What are the benefits of roundabouts?

Enhanced safety

Reduced number of conflict points.

Roundabouts reduce fatalities by 90% by eliminating crossing conflict points and head-on crashes.
Roundabouts significantly reduce the total number of conflict points in an intersection, which means fewer opportunities for crashes.

Source: National Academy of Sciences (NAS)

Intersection with 32 conflict points versus roundabout with 8 conflict points

Conventional intersection vs roundabout

Conflict points	Conventional intersection	Roundabout
Diverging	8	4
Merging	8	4
Crossing	16	0
Total	32	8

Slower vehicle speeds

Slower vehicle speeds give drivers more time to make decisions.
Slower vehicle speeds improve pedestrian and cyclist outcomes when a crash does occur.

Source: USDOT

Operational performance

Roundabouts improve traffic flow by using a yield-on-entry system.
Drivers only need to come to a stop when there isn’t enough room for them to safely enter the intersection.
Less stopping means fewer delays and less congestion.

Vehicle 1: Safely enters intersection.
Vehicle 2: Safely enters the intersection.
Vehicle 3: Already in the intersection, approaching vehicles must yield.
Vehicle 4: Must yield to Vehicle 3 to enter safely.
Vehicle 5: Approaching the intersection but is still far enough away to not affect the other drivers.

Resiliency

Roundabouts operate without power, functioning during outages and freeing emergency crews to focus on signalized intersections.

Placemaking opportunities

Roundabouts offer spaces for landscaping, hardscaping, signage, or public art, allowing for collaboration with local agencies to enhance community character.

Environmental considerations

Roundabouts allow continuous traffic flow, reducing the time vehicles spend idling.
This leads to lower vehicle emissions and fuel consumption contributing to a cleaner environment.

Are roundabouts appropriate everywhere?

Roundabouts are excellent solutions for many intersection challenges, making them increasingly popular. However, they might not be the best option in every situation. Common concerns are listed below, but most can be addressed with careful design and proper outreach. Comprehensive planning and design are essential to ensure roundabouts are suitable for a specific intersection.