Improved Analysis of Two-Lane Highway Capacity and Operational Performance (2018) / Chapter Skim
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NCHRP 17-65 Improved Analysis of Two-Lane Highway Capacity and Operational Performance Final Report 1 1. Introduction 1.1.
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NCHRP 17-65 Improved Analysis of Two-Lane Highway Capacity and Operational Performance Final Report 2 1.2.1. Speed-Flow Relationship The current relationship between speed and flow given by the HCM is linear in form, as shown in Figure 1-1.
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NCHRP 17-65 Improved Analysis of Two-Lane Highway Capacity and Operational Performance Final Report 3 Furthermore, even if a linear or approximately linear speed-flow relationship were to be used, it is more logical that the slope of this linear relationship should be affected by free-flow speed (FFS) and the directional distribution of traffic, as illustrated in Figure 1-3.
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NCHRP 17-65 Improved Analysis of Two-Lane Highway Capacity and Operational Performance Final Report 4 performance measures to use for assessing two-lane highway performance. Both PTSF and ATS are used to evaluate Class I two-lane highways (high-speed, major inter-city routes and connectors of major traffic generators)
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NCHRP 17-65 Improved Analysis of Two-Lane Highway Capacity and Operational Performance Final Report 5 of a two-lane highway with a number of segments with varying characteristics and occasional intersections)
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NCHRP 17-65 Improved Analysis of Two-Lane Highway Capacity and Operational Performance Final Report 6 This issue is described further in the subsequent paragraphs and figure (1-4)
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NCHRP 17-65 Improved Analysis of Two-Lane Highway Capacity and Operational Performance Final Report 7 Figure 1-4. Divergence of passenger car and mixed flow speed-flow patterns on freeway upgrade Adapted from Exhibit 59, NCFRP Report 31 (Dowling et al., 2014)
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NCHRP 17-65 Improved Analysis of Two-Lane Highway Capacity and Operational Performance Final Report 8 This means a passing lane should not show major increases to the traffic-carrying capability of the highway section. Furthermore, the HCM two-lane analysis methodology chapter provides conflicting guidance on climbing lanes -- it indicates that adding a passing lane to a segment that is operating at LOS F will not improve LOS; thus, there is no need to perform a passing lane analysis.
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NCHRP 17-65 Improved Analysis of Two-Lane Highway Capacity and Operational Performance Final Report 9 Figure 1-5. Speed-flow values on Finnish arterial highway 4 (grade separated intersections and passing lanes)
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NCHRP 17-65 Improved Analysis of Two-Lane Highway Capacity and Operational Performance Final Report 10 using a software implementation of the methodology, experience by research team members in teaching this methodology has consistently shown this issue to be confusing to students. The current HCM analysis methodology chapter is heavy on the use of adjustment factor tables.
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