Sub-problem 6a: Feasibility of Conversion to a Roundabout
Step 1. Setup
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 issue.
The HCM section on roundabouts begins by acknowledging the limited scope of the procedures. The limitations include:
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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.
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The results of the computations are limited to capacity estimates. No delay or level of service values are produced.
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The procedure is not recommended for cases where the circulating flow exceeds 1,200 vph at any entry point.
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While the HCM procedures for other facilities produce unique deterministic capacity results, the roundabout analysis procedures only give a range of capacity values.
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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?”
Discussion:
Carefully
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.