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As an author published in the area of traffic safety and a former Transportation Board member, I would feel remiss in not pointing out numerous errors in Joel Williams Thursday “ViewPoint.”
What has surprised me in the discussion of local roundabouts is the obvious, successful, example that already exists. I was a Transportation Board member when the alternatives for the intersection at North Mesa and San Ildefonso were discussed.
The stop signs on North Mesa and San Ildefonso were causing considerable backups, especially a conflict of school buses and commuter traffic. I don’t know whether county engineers were considering a roundabout before I proposed it at T-Board, but it had certainly not been brought up for discussion at that point. The only realistic alternative was a 4-way traffic signal and I think most users of that intersection would realize the disadvantages of that.
about works, in part, because it is well designed. There are two through-traffic lanes, one from Barranca Mesa and one to North Mesa, so that, at commuter times when the traffic is primarily in one direction there is a minimal conflict.
Williams does not account in any way for the complexity of roundabout design instead citing a single figure from a much more comprehensive document http://www.trb.org/Main/Public/Blurbs/153636.aspx).
The data uses the “Gap Acceptance Method” of mathematical modeling, one of many models in the document. The figure is described by the document as “simple graphs that can be used to obtain an estimate of roundabout capacity, when a high degree of accuracy is not required.”
The model fails to account in any way for geometry and only in the most basic fashion for the inequalities of traffic flow between Trinity and its side streets. In short, the number of vehicles per hour cited is unlikely to any way accurately reflect the real situation of a well designed traffic system on Trinity Drive.
Since the rest of the arguments in the column assume that his number is correct, these arguments may be similarly dismissed, however the further factual errors should be corrected.
While less common in the U.S., roundabouts are widely used in Europe and, contrary to Williams’ assertion, are used on freeways and all other roads. They are used in part because, as the NCHRP study that Williams cites so prominently points out, the delay through a roundabout in real studies, not theoretical models, is less than a 4-way signal.
These figures strongly suggest that the average traffic delay for a single lane roundabout (about 10 seconds at 1,700 vehicles/hour) is less than that for a two-lane signalized intersection (about 15 seconds at the same traffic count). Less delay translates to more vehicles per hour.
Skepticism of “experts” as Mr. Williams encourages is healthy, but cherry-picking a document to support an opinion shows neither expertise or good use of science.