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Merge/Diverge length

Scott Hissong
Merge/Diverge length was posted on December 7th, 2017 at 1:55 PM UTC
regarding Chapter 14: Freeway Merge and Diverge Segments

If you have a merge quickly followed by a diverge (say gore to gore = 1000') but not connected with an auxiliary lane, are the length of each section still 1500' (default influence area length) or should they be reduced to 1000' length?

I understand that you also have to analyze the overlap area but I was wondering if you reduce the length of these segments (essential the influence area).

(Edited July 7th, 2020 at 5:31 PM UTC)
Vishal Khanapure
University of Florida
RE: Merge/Diverge length was posted on December 7th, 2017 at 2:58 PM UTC

The HCM6 Chapter 10 section on Facility Segmentation Guidance provides some direction on this topic. I have included a couple of relevant bullet points below for quick reference:

• When the gore-to-gore length of two adjacent merge and diverge segments is less than 3,000 ft but longer than 1,500 ft and no auxiliary lane exists, the section should be coded as a series of three segments, with the middle segment being defined as an overlap segment (merge, overlap, diverge). In this case, the overlap segment length is the difference between 3,000 ft and the gore-to-gore spacing, and the merge and diverge segment lengths are equal to the gore-to-gore spacing minus 1,500 ft. • It is highly unusual to have ramp spacing (combinations of merge and diverge) less than 1,500 ft without the addition of an auxiliary lane to connect the two gore areas. However, when this occurs, the 1,500-ft merge or diverge segment length is truncated at the adjacent ramp gore point.

I hope this helps. Please email me at kvishal @ if you have further questions.

(Edited July 7th, 2020 at 5:32 PM UTC)
Scott Hissong
RE: Merge/Diverge length was posted on December 7th, 2017 at 3:19 PM UTC

Thanks. I missed the portion about truncating the merge/diverge segment lengths. I thought that was how it needed to be done. I just couldn't find it in writing.

(Edited July 7th, 2020 at 5:32 PM UTC)

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