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BFFS for Multilane highways on different types of terrain

Eduardo Luís Festa
BFFS for Multilane highways on different types of terrain was posted on May 9th, 2019 at 8:21 PM UTC
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 July 9th, 2020 at 5:31 PM UTC)
Bill Sampson
Sampson Engineering LLC
RE: BFFS for Multilane highways on different types of terrain was posted on May 10th, 2019 at 12:01 AM UTC

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

(Edited July 10th, 2020 at 5:32 PM UTC)
Lucas Zomignani Oliveira
Federal University of Santa Catarina
RE: BFFS for Multilane highways on different types of terrain was posted on July 31st, 2019 at 7:53 PM UTC

Thank you for your answer, Mr. Sampson. It was very helpful. However, there is something that still isn’t clear.

After performing some tests with HCS 7, I noticed that, in most cases, multilane segments analyzed as specific grades have a better LOS than if they were analyzed as rolling terrain segments.

As you said, this happens due to PCE values. For grades ranging between 3 to 5.5% and for a percentage of trucks higher than 25% (on the cases I analyzed it usually ranges around 65%), the PCE values from Exhibit 12-26 and Exhibit 12-28 (HCM 2016) are always lower than 3.0 (the value used for rolling terrain).

These results, however, don’t represent the Brazilian traffic reality. The performance of a highway segment worsens as the grade increases. It also worsens as the percentage of trucks within the traffic flow increases. How is it possible that the LOS of a segment analyzed as specific grade is better than the LOS of this same segment analyzed as rolling terrain?

Is it possible that the behavior of drivers in the USA justifies this performance improvement for specific grade segments?

Thank you!

(Edited July 31st, 2020 at 5:32 PM UTC)

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