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Weave Segment Capacity Check

Joe Blasi
HNTB Corportation
Weave Segment Capacity Check was posted on April 24th, 2015 at 6:04 PM EDT
regarding Chapter 12: Freeway Weaving Segments

Does the weave segment methodology do a check of capacity for the outgoing legs of the weave segment?

I have a proposed stretch of freeway where a tapered entrance ramp joins three mainline lanes. 3000’ later the three lanes split to two freeway lanes and one ramp lane. Analyzing this as a weave yields LOS D. When we submitted to our local FHWA contact, they asked us to check the capacity of the exit ramp. I had thought the weave analysis checked that, but decided to double-check using the diverge segment ramp capacity check methodology. This fails the capacity check, resulting in LOS F.

So which is it? LOS D or LOS F? When analyzing a weave segment, should the analyst do a separate capacity check of the downstream ramp?

Joseph Fazio
RE: Weave Segment Capacity Check was posted on April 24th, 2015 at 7:22 PM EDT

In my opinion, since you are getting LOS F on a downstream segment/ramp, one must use the Freeway Facility Chapter to analyze upstream operations, i.e., FREEVAL that is freely available for download for all HCM 2010 registered users on the HCM website. If all freeway segments are better than LOS F, then one can use the individual freeway segment procedure for the appropriate freeway component.

Mark Sorrell
RE: Weave Segment Capacity Check was posted on April 27th, 2015 at 9:02 AM EDT

If you have an acceleration lane at the on ramp location that gets dropped at a taper, then I don't think the HCM allows this segment to be classified as a weave. A weaving segment shouldn't have any acceleration/deceleration lanes. It sounds like what you have is a merge segment, freeway segment, and then a major diverge (assuming there are no deceleration lanes).

The merge and freeway segments are analyzed normally. The HCM recommends analyzing all of the legs at major merge/diverge areas as freeway segments to check for LOS and capacity. In other words, provide an upstream freeway LOS as well as a freeway LOS for each of the downstream links. If everything checks out, you should be okay. If it doesn't, you may want to model the segment to see how a low capacity downstream segment affects the upstream segment.

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