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Sustainable Infrastructure
through Innovative Engineering

Since 1975

Wallace Montgomery is a recognized leader in the sustainable development of transportation and public works facilities. Respected for our efforts to enhance and protect the environment, the firm has delivered high-quality services to a diverse group of clients since its founding in 1975.

WM specializes in planning, engineering, and construction management for government agencies, as well as the private sector. Our portfolio includes projects involving highways, streets, bridges, transit, facilities, traffic features, utilities, parks, trails, and building structures. The successful completion of these jobs has come from embracing the firm’s core values that emphasize communication, client service, and quality work. Wallace Montgomery’s mission will always be to stay focused on these principles and dedicated to meeting our clients’ needs.

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close overlay Roundabout Facts

Roundabout Basics

Roundabouts have been in operation throughout the United States for 25 years. They remain one of FHWA’s Every Day Counts (EDC) innovations to improve the safety performance of intersections and interchanges. WM’s roundabout experience dates back to the beginning of the roundabout revolution! We provide policy, planning and design services for roundabout projects of all complexity - from the smallest mini-roundabout to complex roundabout corridors and interchanges.

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Good roundabout design avoids lane changes within the circulatory roadway. Spiral lane markings are often used on multilane roundabouts to effect lane shifts while maintaining lane continuity.

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Flare is a method for providing additional capacity at a roundabout. In this case, a one-lane approach is widened to two lanes approaching the roundabout.

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The proper application of Deflection is a key to maximizing safety at roundabouts. Deflection is developed by providing the appropriate entry path curvature necessary to reproduce approach speeds.

Studies in the UK show that too much deflection can result in rear end crashes on the approaches and that too little deflection can result in failure to yield crashes at the roundabout.

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Lane widths typically range between 13’ and 20’ - and are also site specific. Considerations influencing the selection of appropriate lane widths include:

  • number of lanes,
  • requisite design vehicles (e.g. wide loads, farm equipment, etc.),
  • design vehicle operation (i.e. is side by side operation required), and
  • the presence of pedestrians.
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Roundabout diameters typically range between 90’ for small single lane roundabouts and 225’ for multi-lane configurations - and are generally site specific. Considerations influencing the selection of an appropriate diameter include:

  • approach geometry
  • number of legs
  • number of lanes
  • requisite design vehicles (e.g. tractor trailers, buses, farm equipment, etc.
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Designers sometimes introduce reversing curvature on higher speed approaches to highlight the change in context and the need to reduce speed. Curbing and landscaping can accentuate the effect as well.

Southern Roundable | Hampstead Bypass | Hampstead, MD | Source: Google Earth

Roundabout Fact #1

Estimates suggest that the US will top the 5,000 roundabout milestone sometime in 2016. While the US lags far behind other countries (e.g. the UK, France, and Australia) in terms of total roundabouts and roundabouts per capita, their popularity continues to grow across NA. There is at least one roundabout in every state in the country.

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Roundabout Fact #2

All roundabouts are circular intersections - but not all circular intersections are roundabouts! Roundabouts are often confused with other circular intersections like New Jersey style traffic circles, rotaries, town square circles, and traffic calming circles.

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Roundabout Fact #3

FHWA considers the Roundabout to be one of nine proven safety countermeasures. The other eight countermeasures include:

  • Corridor Access Management,
  • Backplates on Signal Heads,
  • Longitudinal Rumble Strips/Stripes,
  • Enhanced Delineation & Friction for Horizontal Curves,
  • Safety Edges,
  • Medians and Pedestrian Refuge Islands,
  • Pedestrian Hybrid Beacons, and
  • Road Diets.

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Roundabout Fact #4

Since the first roundabout was built in the United States more than 25 years ago, there have been a total of 32 known fatal crashes at roundabouts nationally (all roundabouts). A breakdown of the crashes is as follows:

  • 50% involved impared drivers,
  • 34% involved motorcyclists,
  • 81% involved single vehicles,
  • 59% involved fixed objects (curbs, retaining walls, poles, etc.),
  • 44% involved excessive speeds,
  • 1 fatal crash involved a bicyclist, and
  • 0 (zero) fatal crashes involved pedestrians.
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Roundabout Fact #5

Roundabouts usually provide more capacity than similarly sized signalized intersections. Planners can use roundabouts in sequence along corridor to implement a “wide nodes/narrow roads” approach - achieving operational and safety goals while minimizing the number of lanes between intersections.

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Contact Us
Andrew Duerr, PE
Associate
(717) 590-1408
aduerr@wallacemontgomery.com