Carbondale may end up with ‘offset-left’ roundabout design | PostIndependent.com
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Carbondale may end up with ‘offset-left’ roundabout design

John Colson
Post Independent Staff
Glenwood Springs, CO Colorado
Illustration courtesy Ourston Roundabout EngineeriA diagram depicting the proposed "offset-left" design for a roundabout at Main Street and Highway 133 in Carbondale. Blue lines extending forward from the "cars" indicate the sight-lines from driver to pedestrian crosswalks.
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CARBONDALE, Colorado – Carbondale motorists, as well as pedestrians and bicyclists, may one day be negotiating the cutting edge in roundabout design, according to state highway planners.

If all goes as planned, construction on the new traffic-control device, and other improvements to the highway, could begin in spring 2014.

The Colorado Department of Transportation (CDOT) is recommending that the Highway 133/Main Street roundabout be built as an “offset-left” design, which highway planners feel would work better for Carbondale than the more common “radial” designs that have been used for other Roaring Fork Valley communities.



The design brings four single lanes of traffic (two each from Main Street and Highway 133) into the roundabout at offset angles in relation to the central island (see illustration).

The only nearby example of this particular design, relatively speaking, according to highway officials, is in the town of Edwards, in Eagle County.



According to the town’s consultants, Ourston Roundabout Engineering, the design slows traffic as it enters the roundabout, which is where vehicles and pedestrians most frequently interact.

The town’s Board of Trustees, at a meeting on March 19, concurred with CDOT’s recommendation, according to Town Administrator Jay Harrington.

“I don’t know if I’d call it a decision,” Harrington said of the trustees’ conclusions, which followed a discussion with CDOT engineer Roland Wagner.

The decision on the design, Harrington said, is up to the highway planners, who feel the offset-left arrangement has clearer sight-lines so motorists can see pedestrians and bicyclists trying to get across either Main Street or the highway.

Harrington said the trustees seemed to accept CDOT’s reasoning, though they asked many questions about the interaction between vehicles and pedestrians or cyclists.

“The board basically asked that CDOT do everything it could to make this as bicycle and pedestrian friendly as possible,” Harrington said.

Public Works Director Larry Ballenger, who has been intensively involved in the planning for redesigning Highway 133 from the Roaring Fork River bridge to Main Street, said that there is some concern that the offset-left design may allow cars to reach higher speeds as they leave the roundabout than would be possible with a radial design.

If that is the case, he said, “the speeds are just going to have to be posted” in plain view of motorists.

Another meeting between the trustees and CDOT is expected this spring, as the highway planners work on the design.

“We are moving forward, starting to get into the details of the design,” Ballenger said.

One initial part of the roundabout design, the possibility of a pedestrian/bicycle walkway running under the roundabout, has been abandoned, said Harrington.

The $2 million estimated cost, along with concerns about the feasibility of relocating utilities and questions about whether such tunnels actually get used much, took the idea off the table, Harrington said.

jcolson@postindependent.com


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