Update of Highway Capacity Manual: Merge, Diverge, and Weaving Methodologies (2023) / Chapter Skim
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Pages 75-106
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From page 75... ...
75 Overview This chapter presents the results of the modeling performed on the data collected at the study sites. The first three subsections present results for speed estimation, density-at-capacity estimation, and capacity estimation, each of which include results for weaving, simple merge, and simple diverge sites.
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76 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies that were used to determine the 15-minute-interval speeds included in the model development. The selection criteria for including 15-minute-interval speeds were as follows: • Observed speed ≥ 0.7 × FFS, as speeds less than 70% of the FFS generally fall within the congested flow regime and should not be part of the speed–flow model development.
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Findings and Applications 77 STRIDE Model Results As discussed in Chapter 2, an alternative speed estimation model for simple weaving segments was developed by a STRIDE research project (Rouphail et al.
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78 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies The effect of the presence of a managed lane on weaving segment speed was found to be not significant. Two optimization algorithms were applied for the model testing and recalibration.
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Findings and Applications 79 forms were considered to explicitly consider the required lane changes needed to complete the weaving maneuvers. These additional models added multipliers to the Vrf and Vfr parameters to account for weaving segment configuration.
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80 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies NWrf = number of lanes from which a weaving maneuver from the on-ramp to the freeway can be made with one or no lane changes, NWfr = number of lanes from which a weaving maneuver from the freeway to the off-ramp can be made with one or no lane changes, Nl-GP = number of general-purpose lanes on the weaving segment (the impact of a managed lane on the weaving speed is not considered) , V/Nl = overall weaving segment volume (pc/h/ln)
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Findings and Applications 81 The research team ultimately converged on model Vproposed-1 as it provided the optimal results (despite a slight increase in RMSE)
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82 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies HCM Model Results Figure 26 compares the HCM's estimated speeds with the field-observed speeds. Unlike the simple weaving sites, the HCM merge model did a good job estimating field speeds with an overall RMSE of 3.81 mph.
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Findings and Applications 83 Model 4 500 1 (39)
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84 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies calculation process and input data requirements by dropping V12. In addition, the distribution of the Model 4 scatter plots is more evenly spread around the 45-degree line and can provide a greater range of speeds, while the HCM model speed estimates are almost always in the range of 50 to 65 mph, with limited variability in speed.
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Findings and Applications 85 were based on sensor data. This approach avoided additional errors related to V12 estimation using HCM Exhibit 14-9.
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86 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies speeds. Compared to the HCM model, the RMSE of the Model 4 is lower (6.12 mph for the HCM model versus 4.04 mph for Model 4)
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Findings and Applications 87 Unsurprisingly, at low-flow rates (<500 pc/h/ln) , the simple weave functions like a basic segment, as there is minimal turbulence from the weaving volume.
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88 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies The sensitivity results show that as the overall segment flow rate increases, speeds start to drop, and the reduction in speed is much greater compared to a basic segment. The speed impedance at merge sites is slightly higher than for equivalent simple diverge sites.
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From page 89... ...
Findings and Applications 89 However, this project's analysis found that density at capacity is considerably lower than 43 or 45 pc/mi/ln, and it varies substantially based on a site's geometric and operational site characteristics. Figure 32 shows average density at capacity for the NCHRP 07-26 sites (represented by the bars)
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90 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies 225,000200,000175,000150,000 Mainline AADT 125,000100,00075,000 PC - D en si ty a t C ap ac ity 20 25 30 35 40 PC - D en si ty a t C ap ac ity 20 25 30 35 40 PC - D en si ty a t C ap ac ity 20 25 30 35 40 45 PC - D en si ty a t C ap ac ity 20 25 15 30 35 40 45 5 Number of Mainline Lane (Weaving section) 4 Urban Area Type AADTArea Type Suburban 6,0005,0004,000 Short Length (Ls)
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Findings and Applications 91 200,000175,000150,000 Mainline AADT 125,000100,00050,000 75,000 PC - D en si ty a t C ap ac ity 27.5 30.0 32.5 35.0 37.5 40.0 42.5 PC - D en si ty a t C ap ac ity 27.5 30.0 32.5 35.0 37.5 40.0 42.5 PC - D en si ty a t C ap ac ity 30 32 34 36 38 44 42 40 PC - D en si ty a t C ap ac ity 27.5 30.0 32.5 35.0 37.5 40.0 42.5 Rolling Terrain Type Level Urban Area Type AADTArea Type SuburbanRural 900800 Length of acceleration lane Terrain Type Length of acceleration lane 700600500 PC = passenger cars. Figure 34.
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From page 92... ...
92 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies volume inputs for capacity estimation were the average speed and volume during the hour with the most observed prebreakdown intervals. The field-observed capacities were estimated based on the methodology described in Chapter 2.
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Site Observed HCM an PB Average Volume (pc/h/ln)
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94 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies speed and volume inputs for capacity estimation were the average speed and volume during the hour with the most observed prebreakdown intervals. The field-observed capacities were estimated based on the methodology described in Chapter 2.
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Findings and Applications 95 off-ramp sites, and one lane drop diverge site. No C-D weave site was included due to data limitations, as sensors were only available on the mainline and not on the C-D system.
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96 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies Site Location Highway Acceleration Lane Length (ft) Sample Size NCHRP 07-26 RMSE (mph)
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Findings and Applications 97 Close Merge_3 Close Merge_1 Two Lane on Ramp_1 Two Lane on Ramp_2 Figure 37. NCHRP 07-26 speed compared to observed speed for complex merge validation sites.
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98 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies were evenly spread around the 45-degree line, the model consistently overestimated speed for the complex merges and generally overestimated speed for the complex diverges. Capacity Model Complex Weaves Table 34 summarizes the capacity estimates from the HCM and proposed NCHRP 07-26 models for the complex weave sites and compares the estimates to the field-observed capacities.
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Findings and Applications 99 Table 37. Ramp metering before-and-after data.
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100 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies Pilot Implementation Testing The project included a pilot implementation testing phase of the recommended freeway merge, diverge, and weaving methods with two state DOTs. The objective of the testing was to obtain practitioner feedback on the recommended methods, using case study examples of actual facilities familiar to the participants.
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Findings and Applications 101 Session 1 • Welcome (15 minutes)
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102 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies – Special cases. ◾ Lane add/drop.
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Findings and Applications 103 – Overview of methodological steps. – Regression equations used to estimate lane-changing rate and speed.
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104 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies when applying the methods. The list below summarizes feedback on the new NCHRP 07-26 methods, as well as some constructive feedback for the team's consideration in refining the NCHRP 07-26 methods.
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Findings and Applications 105 segment, which would turn the location to a weaving segment. Participants appreciated that the new NCHRP 07-26 methods use the same computational approach for both segment types, providing greater consistency and more intuitive results when moving between segment types.
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106 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies As a result, the team focused additional efforts following the workshop on the weaving procedure. The team evaluated several additional model forms that are described in more detail in Chapter 3.
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