Star Sign up now to gain access to all the features of this website. NEW — access is now open to ALL USERS! — Already registered? Please log in.

Round FFS of weaving segments to multiple of 5 mph?

Alexandre Hering Coelho
Universidade Federal de Santa Catarina
Round FFS of weaving segments to multiple of 5 mph? was posted on June 29th, 2017 at 9:55 AM EDT
regarding Chapter 12: Freeway Weaving Segments

Hello! I’m applying the freeway model procedure to determine levels of service of a highway. I have some basic segments, some weavings and some on and off ramps. All FFS values ​​are measured in the field. For the basic segments I have to round the FFS values ​​to multiples of 5 mph (step 3 of the procedure). Should I do this also for waving segments and ramps??? Or should I use directly de values measured??? I searched all over Volume 2 of HCM and could not find a specific instruction about it. The examples on the chapters 12 and 13 all bring FFS values ​​multiple of 5 mph in the input data, which does not help to solve my doubt. Using the HCS 2010 software, for weaving, I am able to input any value between 35 and 80 mph for the FFS, and there is no “selected curve” there, what leads to the idea that any value is supported by the method. But it would be very good when I found this written in some official documentation. Thank you for any answer! Cheers, Alexandre.

Bill Sampson
University of Florida
RE: Round FFS of weaving segments to multiple of 5 mph? was posted on June 29th, 2017 at 10:05 AM EDT

The HCM 2010 prescribes computing travel speed from the rounded value for basic freeway segments only (not for weaving or ramps), and HCS 2010 implements that as well with the rounded value used shown in the report. The HCM 6th Edition no longer uses the rounded value in the basic segment analysis, but computes travel speed from the precise value and HCS7 implements that change as well.

Scott Washburn
University of Florida
RE: Round FFS of weaving segments to multiple of 5 mph? was posted on June 29th, 2017 at 11:02 AM EDT

Alexandre:

Page 12-22 of the HCM 2010 includes the statement "In situations that require the FFS to be estimated, the model described in Chapter 11, Basic Freeway Segments, is used." The premise behind the rounding recommendation in the HCM 2010 was because the inherent uncertainty of the underlying speed-flow curves really did not support using an estimated value of any greater precision. While the weaving and ramp segment methodologies do not use a 'speed-flow' curve as the basic segment methodology does, it is not reasonable to use any greater precision for estimated free-flow speeds with those methodologies than the basic segment methodology. In the case of measured free-flow speeds, the HCM 2010 basic segment methodology essentially "forces" you to still use rounded values (nearest 5 mi/h), whereas the weaving and ramp segment methodologies do not. Nonetheless, I think consistency is the prudent approach, and if you round one, I would round them all. The HCM 6th edition removes the rounding recommendation, not necessarily because of any changes to the accuracy of the underlying speed-flow curves, but primarily for convenience due to the implementation of a speed-flow curve that can be calibrated.

If you want to try out some alternative, and free, HCM software (for both the 5th and 6th editions), see:

http://swashware.com/HCMCalc

http://swashware.com/TTR_ATDM

(Edited June 29th, 2017 at 11:03 AM EDT)
Joseph Fazio
RE: Round FFS of weaving segments to multiple of 5 mph? was posted on June 29th, 2017 at 2:35 PM EDT

Since FFS was measured in the field, the 2010 HCM also states on p.12-22, "As with all analyses, FFS is best observed in the field, either on the subject facility or a similar facility. When measured, the FFS should be observed within the weaving segment."

If one wishes to use a weaving worksheet in Espanol, one may wish to try FAZWEAVE at http://www.fazioeware.com/ .

Alexandre Hering Coelho
Universidade Federal de Santa Catarina
RE: Round FFS of weaving segments to multiple of 5 mph? was posted on September 18th, 2017 at 11:43 AM EDT

Dear Mr. Sampson, Mr. Washburn and Mr. Fazio, I’m very grateful to you for helping me. Best regards.

Sign in to add a reply. Don't have an account? No problem, sign up for free.