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Free Flow Speed (for Freeways) Calculation Changes (2000 to 2010)

Tony Geara
Free Flow Speed (for Freeways) Calculation Changes (2000 to 2010) was posted on March 16th, 2013 at 3:27 PM EDT
regarding Chapter 11: Basic Freeway Segments

I realize there have been major Manual changes from 2000 to 2010 HCM. And it has been a challenge keeping up with what has changed and what has not. Especially for PE exam civil Transportation takers.

However, I have a specific question regarding the Free Flow Speed (FFS) equation. It seems it used to be covered in HCM2000 chapter-23, however in the HCM2010 the Freeway Facilities are covered in CH10, 11, 12 and 13. And the equation seems to have significantly changed.

Based on HCM 2000 : FFS = BFFS - flw - flc - fN - fID

Based on HCM2010: FSS= 75.4 - flw - flc - 3.22TRD0.84

HCM also now recommends starting with a speed of 75.4. Then they totally eliminated the lane number factor, and changed the equation governing the factor affecting the number of interchanges.

Can anyone confirm that these changes are correct?

I also noticed that if the BFFS if provided, that could be substituted into this equation instead of 75.4. But I am not sure if I am correct on that. Can anyone provide further clarification on the matter?

Any clarification would be appreciated.

Thanks Tony

Gustavo Riente de Andrade
TECTRAN - T├ęcnicos em Transportes ltda
RE: Free Flow Speed (for Freeways) Calculation Changes (2000 to 2010) was posted on March 21st, 2013 at 11:08 AM EDT

Yes, your understanding is correct. Those changes were made based in the results of recent research. The value of 75.4 is based on an average taken from American freeways. As you said, this value may be replaced by a locally calibrated BFFS.

Scott Washburn
University of Florida
RE: Free Flow Speed (for Freeways) Calculation Changes (2000 to 2010) was posted on March 23rd, 2013 at 11:27 AM EDT

This equation is recommended for use when field free-flow speed measurements are not possible or not available. If you have a field-measured free-flow speed for your analysis site, use this speed directly rather than using the FFS estimation equation.

Tony Geara
RE: Free Flow Speed (for Freeways) Calculation Changes (2000 to 2010) was posted on March 27th, 2013 at 6:25 PM EDT

Thank you both for your clarifications.

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