Problem 6: Other Intersection Configurations for Okeechobee Road

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The previous two problems in this case study assumed that the intersection of Krome Avenue and Okeechobee Road would retain the form of a conventional at-grade intersection. This final problem examines two alternative intersection configurations:

Sub-problem 6a: Here we will consider the feasibility of conversion to a roundabout, using the analysis procedures suggested in HCM Chapter 17.
Sub-problem 6b: Here we will examine the effects of removing the high volume conflict points by the installation of a grade-separated flyover.

This problem differs from the previous five problems in the sense that the alternative configurations were not actually included in the study performed for the operating agency. The operating agency was able to rule out any interest in either of the alternative configurations based on criteria other than capacity considerations. So, Problem 6 is more or less hypothetical in nature. It is included in this case study to illustrate how the HCM procedures might be applied if there were an interest in constructing either a roundabout or a flyover at this location.

Discussion:
Consider the scenarios that will be analyzed in this problem. Take a few minutes to consider constraints that may have excluded these alternatives from further analysis by the operating agency. When you are ready to continue, click the link below to proceed.

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:

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.

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

Step 2: Results

The following steps are included in the HCM roundabout analysis procedure:

1. Define the layout, identify the junctions at which conflicts occur and determine the traffic volumes entering at each junction. Since this is a three legged intersection, the junctions will be configured as shown in the box at the right

2. Determine the conflicting (circulating) volume opposing the entry at each junction. If the circulating flow exceeds 1,200 vph, this procedure should not be used unless field data have been collected for the critical gap and follow-up time. 

3. Determine the capacity of the entry lanes using the same equation that was given in the chapter for TWSC potential capacity. The critical gaps and follow up times are adjusted downward to reflect the fact that a roundabout entrance is generally designed to facilitate the entry of traffic into the circulatory roadway.

4. Assess the general performance of the roundabout on the basis of the v/c ratio.

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.

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.

Sub-problem 6b: Consideration of a Grade-Separated Flyover

Step 1. Setup

It was demonstrated in Problem 5 that signalization would provide adequate capacity for the projected traffic volumes at this intersection, but v/c ratios would approach 90% and delays would be well into the LOS E range, especially for the minor movements. Further improvements are likely to require the elimination of conflicting movements by means of grade separation.

The HCM considers competing movements that are grade separated to be conflict-free. Therefore an HCM analysis of the benefits of a grade separation will usually involve the elimination of movements from the intersection.

Consider:

Discussion:
Take a few minutes to consider these questions. Click continue when you are ready to proceed.

Sub-problem 6b: Consideration of a Grade-Separated Flyover

What additional data should be gathered to determine whether grade separation is appropriate? In this limited review of the intersection we have only completed an analyses of questions related to capacity and performance for the peak hour. Justification for a grade-separated flyover must consider the traffic conditions during other hours of the day. Other data such as crash history should be gathered to determine the extent that safety considerations can justify the construction of the facility. Finally, other users such as pedestrians and cyclists should also be considered to determine the extent that a grade-separated flyover affects the connectivity and safety of these modes.

Sub-problem 6b: Consideration of a Grade-Separated Flyover

Step 2: Results

The flyover eliminates all of the critical conflict points and leaves us with only three conflict points as illustrated in Exhibit 3-44. We will consider these conflict points separately:

1. EB through vs. WB Left: This conflict was examined as a separate conflict point with TWSC operation in Sub-problem 4c (see Exhibit 3-31). It was concluded that the TWSC operation would accommodate the fairly low WB left turning volume (120 vph) with no difficulty. The v/c ratio was 0.56 and the estimated delay was 41.7 sec/veh, corresponding to LOS E.

2. EB through vs. NB right: This conflict point was examined as a freeway merge in Sub-problem 4d (see Exhibit 3-34), using the procedures from HCM Chapter 25. The resulting density (i.e. the service measure for freeway merging) was 17.7 pcpmpl, corresponding to LOS B.

3. NB left vs. WB through: This is a new merging conflict point created by the flyover as a substitution for the previous crossing conflict. If the flyover was designed for a freeway-type merge with an adequate acceleration lane, it is suggested that the HCM chapter 25 procedure would also be the best choice for analysis of this conflict point. Assuming the same operating parameters as the eastbound merge, with a 300 ft acceleration lane from the flyover, the resulting density is estimated at 8.5 pcpmpl, corresponding to LOS A.

So, we can conclude that the construction of a northbound flyover at this intersection would eliminate the critical conflicts and leave an acceptable operation at the remaining conflict points. The actual decision to construct a flyover would, of course, require consideration of many other factors that are beyond the scope of this analysis.