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From page 47... ... 47 Overview This chapter starts by presenting four research bundles prepared by the research team to address the knowledge gaps described in Chapter 1 and describing how they were prioritized to develop a Phase II work plan that could be accomplished within the time and budget allocated to NCHRP 07-26. The remainder of the chapter describes how this work plan was executed, starting with a summary of the team's site identification process and data quality control procedure applied by the team. Read the entire page →
From page 48... ... 48 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies ◾ Weaving methods for estimating freeway mainline capacity (HCM Chapter 13) do not converge to basic segment capacities at low ramp-volumes. Read the entire page →
From page 49... ... Research Approach 49 A turbulence factor (sensitive to geometry and flows) is applied to the basic segment speed to yield the predicted speed for a weave section. Read the entire page →
From page 50... ... 50 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies The research team further recommended that the following HCM weaknesses identified from the literature review not be addressed in the Phase 2 effort because the team believed they are unlikely to significantly improve the HCM method results, adequate work-arounds already exist for them, or there are no U.S. data for them: • Right-lane FFS and percentage of heavy vehicles: averages across lanes are not reflective of the right-hand lane. Read the entire page →
From page 51... ... Research Approach 51 Data Sources The research team first conducted an in-depth inventory of the available sensor data that were publicly available on data-sharing platforms. These platforms included the University of Maryland Center for Advanced Transportation Technology Laboratory's Regional Integrated Transportation Information System (RITIS) Read the entire page →
From page 52... ... Figure 10. Location and number of sites used for analysis by state. Read the entire page →
From page 53... ... Research Approach 53 Source: ©2020 Google Figure 11. Example analysis site in Tampa, Florida. Read the entire page →
From page 54... ... 54 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies where available, to obtain ramp volumes. However, no ramp sensor was available for this location. Read the entire page →
From page 55... ... Research Approach 55 1. Check temporal segmentation error a. Read the entire page →
From page 56... ... 56 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies 10. Check "volume/occupancy ratio over threshold" error a. Read the entire page →
From page 57... ... Research Approach 57 downstream of an on-ramp as shown below for a simple merge site and upstream of an o-ramp for simple diverge sites) Read the entire page →
From page 58... ... 58 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies That work developed a point-featured geodatabase with geographic coordinates and elevation data for freeway centerlines. The elevation product used by NCHRP 07-26 consisted of 10-ft elevation data -- that is, points are spaced at 10-ft intervals along highway centerlines. Read the entire page →
From page 59... ... Research Approach 59 StreetLight data processing for merge and diverge sites followed a similar process. For each site, a zone activity analysis was applied to get the StreetLight Index outputs for the two zones set on the mainline upstream and the on- or o-ramp. Read the entire page →
From page 60... ... 60 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies Metadata Collection The metadata were used to create structured and consistent information to enable the research team to describe and analyze site characteristics for each study site. Appendix A of the NCHRP Web-Only Document 343 provides a data dictionary with detailed information related to the variables used, along with a brief definition and applicable segment type (for example, weave only, all segments) Read the entire page →
From page 61... ... Research Approach 61 approach capacity across all lanes by aggregating approach speeds and flow rates. The approach speed for each 15-minute interval was calculated using a weighted average as follows: ( ) Read the entire page →
From page 62... ... 62 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies 5. Prebreakdown Volume a. Read the entire page →
From page 63... ... Research Approach 63 Weibull Parameter Estimation The average uncongested flow rate from each range and the breakdown probability for each range are needed to estimate the Weibull distribution parameters. Only breakdown flow rates that exceeded 1,000 vph/ln were considered, because breakdowns at lower volumes are typically due to work zones, incidents, or other nonrecurring events. Read the entire page →
From page 64... ... 64 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies Using the Speed–Flow Curve Instead of the Weibull Distribution to Estimate Capacity The previously described field-capacity estimation process using the Weibull distribution is generally a robust method for estimating freeway capacities and is consistent with the HCM's recommended approach. It accounts for the highly stochastic nature of breakdown events and provides a valid estimate of capacity under consideration of that stochasticity. Read the entire page →
From page 65... ... Research Approach 65 (a) Capacity Estimated from Weibull Distribution (approximately 2,400 vph/ln) Read the entire page →
From page 66... ... 66 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies Final Sites The team gathered data for a total of 121 sites, along with before-and-after data for four additional sites with ramp meters. Of the 121 sites, 17 were not used for capacity estimation due to concerns of downstream spillback. Read the entire page →
From page 67... ... Research Approach 67 where cIWL = per-lane capacity of the weaving segment under equivalent ideal conditions (pc/h/ln) ; cIFL = per-lane capacity of a basic freeway segment with the same FFS as the weaving segment under equivalent ideal conditions (pc/h/ln) Read the entire page →
From page 68... ... 68 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies The literature review and method assessment described in Chapter 1 identified a number of issues with the HCM weaving method. In addition to its complexity, its estimates of speeds and capacity do not always match well with field observations. Read the entire page →
From page 69... ... Research Approach 69 Figure 20 compares the HCM (Figure 20a) and STRIDE (Figure 20b) Read the entire page →
From page 70... ... 70 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies Merge and Diverge Analysis in the HCM 6th Edition The HCM's modeling concepts for merge and diverge segments have evolved over time. The 1985 HCM focused on estimating demand volumes in Lane 1 (rightmost lane) Read the entire page →
From page 71... ... Research Approach 71 Merge and diverge areas have widely been considered by system operators to be freeway bottle necks. HCM Chapter 14, however, assumes that there is no drop in capacity at those locations, which appears to conflict with recent research findings. Read the entire page →
From page 72... ... 72 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies estimates is 2 ln × 30 pc/mi/ln × 52 mph = 3,120 pc/h. This value is much lower than the actual flow in the RIA, which is 4,000 pc/h. Read the entire page →
From page 73... ... Research Approach 73 where VR = on-ramp flow rate, V = total flow rate on the segment = VF + VR, N = number of lanes, La = effective length of the acceleration lane, and α, β, γ, δ = model parameters to be optimized. The above model also incorporates the effect of FFS, which is included in the estimation of the basic segment speed SB. Read the entire page →
From page 74... ... 74 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies HCM Equation 13-5. However, this approach would require that density at capacity for weaving segments remain at a constant value of 43 pc/mi/ln as Chapter 13 stipulates (or 45 pc/mi/ln for merge and diverge segments) Read the entire page →

From page 47...

... 47 Overview This chapter starts by presenting four research bundles prepared by the research team to address the knowledge gaps described in Chapter 1 and describing how they were prioritized to develop a Phase II work plan that could be accomplished within the time and budget allocated to NCHRP 07-26. The remainder of the chapter describes how this work plan was executed, starting with a summary of the team's site identification process and data quality control procedure applied by the team.