Innovative Intersections and Interchanges

 

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Single Roundabout icon
Innovative Intersection: Single Roundabout
US 9 at Troy Schenectady Road, Latham, N.Y.

What Is A Single Roundabout Interchange?

  • A grade-separated interchange where all ramps begin or end at a single roundabout on the arterial
  • Serves as the overpass or underpass of a freeway
  • A circular unsignalized intersection where traffic flows in one direction around a central island. Traffic entering the roundabout must yield to traffic already inside
  • Can accommodate more than four intersecting roadways

When Should It Be Considered?

  • With heavy turning volumes onto and off of the freeway ramps
  • In urban areas with moderate traffic volumes
  • Where right of way is limited
  • At heavily-used freeway off-ramps where vehicles tend to back up onto the freeway

Benefits

  • Improved safety: Reduces the number of points where vehicles cross paths and eliminates the potential for right-angle and head-on crashes
  • Increased efficiency: Decreases the delay for ramp traffic and eliminates the need for traffic signals
  • Continuous flow: Yield-controlled design minimizes backups on the freeway, reducing the potential for high-speed rear-end crashes

How to Navigate

Below shows how to navigate a single roundabout interchange. Click the image to view a larger version.

Single Roundabout Interchange navigation diagram

Conflict Points

The number of conflict points (locations where vehicle travel paths intersect) is one metric that can be used to evaluate the safety of an innovative intersection or interchange.

There are three categories: crossing, merging or diverging.

In general, merging and diverging conflict points — where vehicles are moving in the same direction — are associated with less severe crash types than crossing conflict points where vehicles are moving in opposite directions.

The diagrams below compare possible vehicle travel movements and associated conflict points at a conventional diamond interchange to a single roundabout interchange.

These diagrams represent a general case, with one travel lane in each direction, and do not take into account pedestrian or bicycle movements at an intersection or interchange.

When compared to a conventional diamond interchange, a single roundabout interchange has no crossing conflict points, two fewer merging conflict points, and two fewer diverging crossing points.

Conventional Diamond Interchange: Conflict Points

Conventional Intersection conflict diagram.

Legend

Diagram Legend. Filled circle is diverging, half-filled is merging, empty is crossing.
Conflict Type Count
Crossing 6
Merging 8
Diverging 8
Total:

22 Conflicts

Single Roundabout Interchange: Conflict Points

Single Roundabout conflict diagram.

Legend

Diagram Legend. Filled circle is diverging, half-filled is merging, empty is crossing.
Conflict Type Count
Crossing 0
Merging 6
Diverging 6
Total:

12 Conflicts

Resources

Virginia Department of Transportation

Federal Highway Administration

Page last modified: Nov. 7, 2019