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Multi-Modal Handbook

Roundabouts

Roundabouts

Roundabouts are a subset of circular intersections with specific design and traffic control features. These features include yield control of all entering traffic, channelized approaches, and geometric curvature and features to induce desirable vehicular speeds (Roundabouts: An Informational Guide, Second Edition, 2010, Transportation Research Board, p.1-4).

Standards

PennDOT: Follow the recommendations contained in the Transportation Research Board's National Cooperative Highway Research Program (NCHRP) Report 672: Roundabouts: An Informational Guide.

Comments

Roundabouts reduce traffic conflicts (for example, left turns) that are frequent causes of crashes at traditional intersections. Unlike a traffic circle or a rotary, a roundabout's incoming traffic yields to the circulating traffic. (FHWA)

The following is an excerpt from the Federal Highway Administration's Technical Summary on Roundabouts (FHWA-SA-10-006) regarding the benefits of roundabouts:

Roundabouts are becoming more popular based on the multiple opportunities to improve safety and operational efficiency, and provide other benefits. Of course, roundabouts are not always feasible and do not always provide the optimal solution for every problem. The benefits of roundabout intersections, and some constraining factors, are described below.

Existing Roundabout in Unionville, Chester County.

The following table (Exhibit 1-9: Roundabout Category Comparison from the National Cooperative Highway Research Program (NCHRP) Report 672: Roundabouts: An Informational Guide — page 1-12) summarizes the basic geometry and capacities of roundabouts to provide for a quick assessment on the applicability of a roundabout for any proposed location.

Design Element Mini-Roundabout Single-Lane Roundabout Multilane Roundabout
Desirable maximum entry design speed 15 to 20 mph
(25 to 30 km/h)
20 to 25 mph
(30 to 40 km/h)
25 to 30 mph
(40 to 50 km/h)
Maximum number of entering lanes per approach 1 1 2+
Typical inscribed circle diameter 45 to 90 ft
(13 to 27 m)
90 to 180 ft
(27 to 55 m))
150 to 300 ft
(46 to 91 m)
Central island treatment Fully traversable Raised (may have traversable apron) Raised (may have traversable apron)
Typical daily service volumes on 4-leg roundabout below which may be expected to operate without requiring a detailed capacity analysis (veh/day)* Up to approximately 15,000 Up to approximately 25,000 Up to approximatly 45,000 for two-lane roundabout
*Operational analysis needed to verify upper limit for specific applications or for roundabouts with more than two lanes or four legs.

Recommendations

Roundabout in the Uptown Worthington development in East Whiteland Township, Chester County, PA.

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The entire Multi-Modal Handbook PDF may be downloaded here.