Roundabouts: An Informational Guide (2nd Edition)

Library
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Transportation Research Board (National Cooperative Highway Research Program) 2010

Roundabouts: An Informational Guide (2nd Edition) PDF (21.6 MB)

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Mike: I haven’t read this yet, but my experience on the Vineyard, where
a roundabout solved a longtime traffic problem, at least for the autos,
tells me the following: 1) roundabouts are great if they replace stop
signs or lights in an intersection with /only two roads crossing at
right angles*/; 2) It can still be treacherous for a bike to negotiate
even such a modest roundabout; 3) roundabouts are horrible when four or
five lanes of traffic descend on it and must funnel into it; witness the
roundabout south of Route 9 with Hammond Street, Hammond Pond Pkwy,
Brookline Street and two other thoroughfares all converging.
Be well and stay safe.
Bob Jampol

** the junction of Edgartown-Vineyard Haven Road and Airport-Barnes Road.**

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Hi Bob,

Thanks for your comments. This voluminous guide and other information is helpful to add context to issues you raise. Some thoughts:

  1. Irregular intersections are a mess in general for all road users. They take up a lot of space and introduce score of possible conflict points spread our throughout the intersection. These conflicts pose serious dangers to cyclists and pedestrians. Roundabouts drastically reduce conflicts by forcing traffic into a single stream. I think roundabouts offer a solution to these imperfect intersections. An intersection with an extra leg is an inefficient and dangerous tangle; a roundabout with an extra leg is just a roundabout.
  2. Cyclists can find roundabouts intimidating. However, speed and vehicle conflicts rather than overtaking/rear-ending are greater risks to the safety of bicyclists. Roundabouts reduce speeds and conflict areas. In the Auburn/Comm Ave case, there will be opportunities for to transition to a multi-use path, hopefully using buttonless detection of bicyclists.
  3. The old MDC’s roundabouts are much larger and higher speed than modern roundabouts. They don’t use modern design principles. Just like any other intersection, it isn’t accurate to lump them all together. The roundabout proposed at Auburn and Comm Ave. would have a diameter about one-third of the roundabout you refer to, and would have a pretty conventional geometry. That means slower vehicle speeds and less time in the roundabout.
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Hi Bob.

As Mike points out, while the terms are used interchangeably, roundabouts and rotaries are traditionally two very different things. Rotaries typically imply much larger circle radii and higher speeds, built more like highway interchanges, often with necessary lane changes. Most of the old traffic circles along VFW and other MDC and State roads fit that pattern. By contrast, modern “roundabouts” use a much smaller circle and a tighter inflection angle entering the roundabout which drastically reduces speed. We don’t have many good examples around here. The best ones are on private developments: Ikea in Stoughton and at the Natick Mall. Horace James Circle (Hammond Pond Parkway) is one of the worst kinds of rotaries, modified to be a bit more roundabout-like several years ago, but it still has very troublesome geometry. I’m very concerned that people will immediately compare the Auburn Street proposal with the notorious “bad” rotaries around here, since that’s what people have known for so many years. That’s going to be a challenge.

Safety is improved by roundabouts because vehicle conflicts are reduced, especially the most dangerous types: right angles (think t-bones) And best of all, there are no left turns! There are also dutch designs for roundabouts with separated bike lanes, but they take up a lot more space than we usually have around here.