Subproblem 6a - Page 1 of 3
Feasibility of Conversion to a Roundabout
Roundabouts have become an
increasingly popular intersection treatment in the United States. By their
nature, roundabouts reduce the number of conflicts between competing movements
at intersections. Recognizing this, the 2000 edition of the HCM includes a
section in Chapter 17, identified as Part C, that addresses the capacity of
roundabouts. The procedures in that section will be applied here to
illustrate their details. Note that the HCM procedures address only the
capacity of a roundabout, which is only one element that must be considered
in establishing the suitability of this treatment alternative. No advice is
given in the HCM on other factors that could influence the suitability
The HCM section on
roundabouts begins by acknowledging the limited scope of the procedures. The
Only single lane roundabouts are covered in the chapter.
The HCM suggests that
a comprehensive roundabout analysis model should be used if capacity
values are required for multiple-lane roundabouts.
The results of the computations are limited to capacity
estimates. No delay or level of service values are produced.
The procedure is not recommended for cases where the
circulating flow exceeds 1,200 vph at any entry point.
While the HCM procedures for other facilities produce
unique deterministic capacity results, the roundabout analysis procedures
only give a range of capacity values.
While it is acknowledged that more complex traffic
interactions exist, each entry point to the roundabout is treated as an
independent junction using a simple gap acceptance model.
The HCM also
advises that caution is necessary in the interpretation of the results
produced by these models because their internal assumptions and parameters
have not been well validated in the United States. It further suggests that
the procedures should be used with care until additional research is
conducted. Heeding that advice, we will limit our investigation to one
simple question: “Would a single lane roundabout be able to accommodate the
projected traffic volumes at this location?”
read the limitations of the HCM roundabout analysis procedure listed above.
Take a few minutes to consider these limitations. Click continue when you
are ready to proceed.
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