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FREEVAL Average Density Anomaly

Sudheer Dhulipala
Jacobs Engineering Group Inc
FREEVAL Average Density Anomaly was posted on December 2nd, 2011 at 1:44 PM EST
regarding Chapter 10: Freeway Facilities

I am trying to analyze a simple freeway off-ramp using FREEVAL under an undersaturated scenario for 4 time intervals. Everything is the same in these 4 time intervals. FREEVAL reports a density of 17.1 for each of the 4 time intervals, but the results summary shows a density of 12.5 pcpmpl. How can the summary differ from the individual time intervals if everything is same in all 4 time intervals? There is a footnote on the individual time interval sheets that says "* For Merge and Diverge Segments this Density is only for Ramp Influence Area!", but the results do not seem to change regardless of the Segment length entered.

Bastian Schroeder
ITRE
RE: FREEVAL Average Density Anomaly was posted on December 7th, 2011 at 11:14 AM EST

Very good question. The reason for the apparent difference in densities is in fact related to the footnote "For Merge and Diverge Segments this Density is only for Ramp Influence Area!". To elaborate, the methodology in Chapter 12, Merge and Diverge Sections, does not have a way to estimate the total average density across all lanes. The methodology is focused on the ramp-influence-area (RIA), which is defined as the two right-most lanes. The chapter therefore computes all performance measures, including density, for the RIA. In the implementation at the facility level, we report that same density in RIA in the individual time intervals, which is the 17.1 pcpmpl in your example. However, when averaging density across segments or across time intervals, we need the TOTAL average density in all lanes. In particular, the ramp chapter gives you the volume in the RIA and associated density. In FREEVAL, we then calculate the volume in the non-RIA lanes (which is lower than RIA), estimate a density based on segment length, and compute a weighted average density. This is the value reported in "results summary", which in your case is 12.5 pcpmpl. I hope this explanation makes sense. The densities may be a bit confusing in the way presented, but I believe the computations are correct.

Sudheer Dhulipala
Jacobs Engineering Group Inc
RE: FREEVAL Average Density Anomaly was posted on December 19th, 2011 at 5:27 PM EST

Bastian, Thanks for the clarification. It makes sense now. So, when reporting the facility LOS, is it recommended to report the average across all lanes even though the RIA has a higher density?

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