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Subproblem 6a - Page 3 of 3

ID# C306A03

Sub-problem 6a: Feasibility of Conversion to a Roundabout

The traffic volumes at a three-legged roundabout are much easier to compute than those at a four-legged roundabout, because of the non-existent movements. The computations will be simplified even further here because of the existing channelization that effectively removes the EB and NB right turns from the operation. Assuming no U-turns, Exhibit 3-43 shows the computed entry and circulating volumes at each entry point.

Exhibit 3-43. Roundabout Entry and Circulating Volumes

Entry Point

Entry Volume

Circulating Volume

Eastbound approach

EB Through (2,010)

WB Left (120)

Northbound approach

Northbound Left (257)

EB Through (2,010)

Westbound approach

WB Through + WB Left

(358 + 120 = 478)

NB Left (257)

At this point it is clear that the circulating volume, 2,010 vph, opposing the traffic entering on the northbound approach exceeds the stated upper limit of 1,200 vph. We will therefore abandon the HCM analysis at this point with the conclusion that the HCM is not able to provide an indication that a single-lane roundabout could accommodate the traffic.

This does not in itself indicate that a roundabout should be conclusively dismissed as a legitimate method of accommodating traffic at this intersection. It simply demonstrates that the HCM is not able to confirm that a single-lane roundabout would be a feasible solution to the problem. A more comprehensive roundabout feasibility study, probably involving a multi-lane roundabout, would be required for the operating agency if they were to pursue the notion of a roundabout at this location. Outside of the HCM, a very good sourcebook for conducting this type of analysis is Roundabouts: An Informational Guide.

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