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Turnouts in the new HCM two-lane highway analysis

Eric Royer
California Dept. of Transportation
Turnouts in the new HCM two-lane highway analysis was posted on October 29th, 2019 at 8:44 PM UTC
regarding Chapter 15: Two-Lane Highways

My question is about analyzing turnouts. Previously when two lane highway analysis was evaluated using PTSF and Average Travel Speed, there existed an estimation of performance for turnouts saying that they were equivalent to a short passing lane segment of 2/3 mile I believe. But with the implementation of segmented two lane highway analysis (HCS 7.8.5) you can't really insert or analyze a short segment in the middle of a detailed geometric breakdown of a two lane highway. Should we continue to use the previous methods, or is there guidance for analyzing turnouts in the new methodology? I've looked at what appears to be the source, "NCHRP Web-Only Document 255: Improved Analysis of Two-Lane Highway Capacity and Operational Performance" and turnout doesn't appear anywhere in the document!

(Edited July 29th, 2020 at 5:31 PM UTC)
Paul Ryus
Kittelson & Associates, Inc.
RE: Turnouts in the new HCM two-lane highway analysis was posted on November 25th, 2019 at 1:09 PM UTC

The new HCM two-lane method hasn't been officially released yet (it is still going through TRB's editorial process). The new method (v6.1) "does not explicitly address two-lane roadways with turnouts," because the NCHRP project that developed the method did not collect data from roads that use turnouts instead of, or in addition to, passing lanes. The current method (v6.0) offers the folllowing guidance: "A single well-designed and well-located turnout can be expected to accommodate 20% to 50% of the number of passes that would occur in a 1.0-mi passing lane in level terrain." The new method's Step 9 provides adjustments to follower density and average speed to account for the effects of upstream passing lanes that, in theory, could be modified to account for the effect of a turnout. However, it would be up to the analyst to decide whether/how to incorporate such an adjustment, and I don't know whether HCS accommodates user adjustments such as this.

(Edited July 25th, 2020 at 5:32 PM UTC)
Eric Royer
California Dept. of Transportation
RE: Turnouts in the new HCM two-lane highway analysis was posted on December 5th, 2019 at 10:12 PM UTC

Thank you for the detailed reply.

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

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