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.

Freeway Weaving - LOS F in HCM but works in VISSIM

Kevin Mascarina
Freeway Weaving - LOS F in HCM but works in VISSIM was posted on January 25th, 2017 at 5:35 PM EST
regarding Chapter 12: Freeway Weaving Segments

It appears that the capacity due to weaving demand flows is highly sensitive to Nwl and VR such that even if you have volumes of VFF=0, VRF=0, VFR=2,400, VRR=0 the v/c ratio will be above 1.0 and will be assigned LOS F and terminates further calculations. If you assign Nwl=3 (just to get HCS to continue the calculations) you will find that the density is well below oversaturation and will have a good LOS.

A couple of weaving sections I'm analyzing appears as LOS F in HCS (due to capacity checks) but works in VISSIM. Sure freeway-ramp vehicles use the nearest exit from the freeway (consistent with HCM methodology) but the line is constantly moving. No queues are observed within the weaving segment nor upstream.

I would appreciate any rationale anyone might have with regards to the capacity checks in HCM and how reasonable it is specifically the one for weaving demand flows as the conditions I'm assessing works in VISSIM with no signs of congestion.

(Edited January 25th, 2017 at 6:09 PM EST)
Bill Sampson
University of Florida
RE: Freeway Weaving - LOS F in HCS but workable in VISSIM was posted on January 25th, 2017 at 5:44 PM EST

HCS 2010 and HCS7 are implementing the HCM 2010 and HCM 6th Edition procedures, respectively, to compute capacity based on both density and demand. The HCM dictates LOS F when the v/c ratio exceeds 1.0, so this is really an HCM (not HCS) question for the HCQS committee.

Kevin Mascarina
RE: Freeway Weaving - LOS F in HCM but works in VISSIM was posted on January 25th, 2017 at 5:50 PM EST

Thanks Bill. I will direct the query through the appropriate channel.

(Edited January 25th, 2017 at 6:07 PM EST)
Kevin Mascarina
RE: Freeway Weaving - LOS F in HCM but works in VISSIM was posted on January 26th, 2017 at 6:09 PM EST

With regards to the weaving section capacity, it is stated for Nwl values of 2 and 3 the capacity at which breakdown is expected is at 2,400 veh/h and 3,500 veh/h, respectively. However, I'm not sure if it was explicitly detailed how these values were reached. I'm fairly new to the HCM myself and would appreciate guidance on these values.

Also, on page 12-16 on the 3rd paragraph under Step 5, it is stated that "The existence of a third lane from which weaving maneuvers can be made with TWO or fewer lane changes in effect spreads the impacts of turbulence across segment lanes and allows for higher weaving flows". The definition of Nwl, however, is the number of lanes from which a weaving movement can be completed in ONE or no lane change. Is there a conflict?

(Edited January 26th, 2017 at 7:00 PM EST)
Paul Ryus
Kittelson & Associates, Inc.
RE: Freeway Weaving - LOS F in HCM but works in VISSIM was posted on January 27th, 2017 at 3:52 AM EST

Regarding the first part of your question, the source of the capacity values, the first paragraph of the methodology section (page 12-9) references NCHRP Project 3-75, Analysis of Freeway Weaving Sections, as the source of the methodology. The final report for that project (as well as the research reports supporting many other HCM methodologies) is available in the Technical Reference Library portion of Volume 4 (it's the first document listed in the Chapter 12 section). The discussion starting on page 67 of that report explains how the capacity values were determined. The short explanation for the 2,400 pc/h/ln value for Nwl=2 is that weaving vehicles occupy (block) portions of two lanes during their weaving maneuver; therefore, the capacity of the weave should not be greater than the capacity of either individual lane (2,400 pc/h/ln).

Paul Ryus
Kittelson & Associates, Inc.
RE: Freeway Weaving - LOS F in HCM but works in VISSIM was posted on January 27th, 2017 at 4:16 AM EST

Regarding the second part of the question, that paragraph is explaining why the capacity value is greater when Nwl=3, compared to Nwl=2 (because weaving activity occurs across portions of 3 lanes instead of 2). The definition of Nwl involves the minimum number of lane changes needed to make the weave, while the text here acknowledges that some vehicles do make 2 lane changes anyway when provided the opportunity (lane changes "can be made" vs. must be made), so I don't see any conflict.

Paul Ryus
Kittelson & Associates, Inc.
RE: Freeway Weaving - LOS F in HCM but works in VISSIM was posted on January 27th, 2017 at 4:16 AM EST

Regarding the second part of the question, that paragraph is explaining why the capacity value is greater when Nwl=3, compared to Nwl=2 (because weaving activity occurs across portions of 3 lanes instead of 2). The definition of Nwl involves the minimum number of lane changes needed to make the weave, while the text here acknowledges that some vehicles do make 2 lane changes anyway when provided the opportunity (lane changes "can be made" vs. must be made), so I don't see any conflict.

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