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BFFS for Multilane highways on different types of terrain

Eduardo Luís Festa
LabTrans/UFSC
BFFS for Multilane highways on different types of terrain was posted on May 9th, 2019 at 4:21 PM EDT
regarding Chapter 12: Basic Freeway and Multilane Highway Segments

In Chapter 12, the following guideline appears:

"Although speed limits are not always uniformly set, BFFS for multilane highways may be estimated, if necessary, as the posted or statutory speed limit plus 5 mi/h for speed limits 50 mi/h and higher and as the speed limit plus 7 mi/h for speed limits less than 50 mi/h."

However, conducting some tests between rolling segments and with specific grade, considering the same physical and operational characteristics, including BFFS, LOS was better for the segment with grade than the rolling. I realized that the results are becoming more realistic as the BFFS of the segment with rolling terrain is larger than the grade segment. But according to the guidance above, there is no distinction in the treatment of BFFS according to the different types of terrain.

I do not understand how using the same inputs the performance of the terrain with specific grade is higher than with rolling terrain.

How does this happen in the calculations? Can someone help me?

Thank you!!

(Edited May 9th, 2019 at 4:24 PM EDT)
Bill Sampson
University of Florida
RE: BFFS for Multilane highways on different types of terrain was posted on May 9th, 2019 at 8:01 PM EDT

While BFFS is not affected by terrain, the PCE values are lower than rolling (at 3.0) for almost all situations with 2% or less specific grades (except at 2% with very small truck percentages). This results in a lower flow rate and lower density for specific grades being modeled as have less effect on flow rate than those defined as rolling terrain. As for BFFS being defined only as a function of speed limit, field measuring is always an option to estimating if you feel the FFS being computed is not appropriate for your analysis. I hope this helps

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