Because the Town is responsible for approving the project, the Proponent should provide an AM peak hour analysis of the roundabout operations. The Town should have assurances that the proposed mitigation will work in all peak conditions, not just those of the development. The AM peak hour frequently has a different directional distribution of traffic than the PM peak hour or Saturday peak hour. During the AM peak hour, it is more likely that the roundabout operation, if constrained, could impact school bus traffic. The traffic count data provided in the appendices that was collected by New York State Department of Transportation (NYSDOT) should not be considered a comprehensive data set. The proponent should collect new AM peak hour traffic data to perform the analysis.

The capacity analysis shows the level of service (LOS) and delay based upon the Percentile Delay Method, which is not consistent with the 2000 Highway Capacity Manual (HCM). The Proponent should provide the capacity analysis computer printouts of the HCM reports from Synchro, which provide LOS and delay based on Webster’s Method, and is consistent with the methodology of the HCM. This can be important because the Percentile Delay Method often shows results that are much better than those of Webster’s Delay Method. Comparing the Percentile Delay method results to the thresholds in the HCM is an apples-to-oranges comparison. The thresholds shown in the HCM are based upon Webster’s Method and should only be applied to results computed using that method.

NYSDOT’s standard software package for analyzing roundabouts is Rodel. No Rodel output data were provided. The proponent should analyze the roundabout using the standard software and provide the output data for review.

The traffic study and SDEIS prepared by the proponent indicate that the single-lane roundabout will operate under constrained conditions in the future. Based upon the analysis, we feel it would be appropriate for the Proponent to provide for a right-of-way reservation so that the roundabout could be expanded to a two-lane configuration in the future if necessary.

Since NYSDOT views the construction of the roundabout as a somewhat "experimental" mitigation method, we recommend that the applicant perform a post-construction study to evaluate operations at the roundabout. The study should be initiated two years after the construction is complete. The study should include new traffic counts, capacity analysis, and a comparison of the actual results to those of the original study. The post-implementation study should also include a future conditions analysis with volumes grown to a 20-year planning horizon, to determine the life span of the single-lane roundabout. The study should include accident analysis from the inception of the roundabout in order to determine whether the frequency and severity of accidents at the intersection have decreased.

1. The proponent should provide a sensitivity analysis of the capacity of the site driveway. If the driveway is at capacity, does this cause the roundabout to fail or does it shorten the life span of the single-lane roundabout?

2.  NYSDOT has developed guidelines for designing a roundabout that are based on the FHWA guidelines. The proponent should utilize these guidelines wherever possible in the design of the roundabout. If conditions dictate a departure from the guidelines, the proponent should explain the reason for the variation and the potential consequences of the variation, such as impacts to motorist and pedestrian safety, capacity, and delay.

R. Mark Dempf, PE