Update of Highway Capacity Manual: Merge, Diverge, and Weaving Methodologies (2023) / Chapter Skim
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Pages 6-46
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6 Overview This report documents the work conducted by NCHRP Project 07-26, "Update of Highway Capacity Manual: Merge, Diverge, and Weaving Methodologies." The report is organized as follows: • Chapter 1, "Background," describes the project background and objectives, synthesizes the results of a review of relevant literature, identifies key knowledge gaps, and summarizes the current (Version 6.0) Highway Capacity Manual (HCM)
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Background 7 enhancements to the HCM. NCHRP Web-Only Document 343 and products associated with this report can be found on the National Academies Press website (nap.nationalacademies.org)
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8 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies Task 2. Synthesize the literature review to identify the knowledge gaps and summarize information on the effectiveness of existing analytical methodologies for freeway merges and diverges, emphasizing any available information on their compatibility with basic freeway and weaving segment methodologies.
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Background 9 sizes and differences in traffic laws, traffic cultures, and road standards between countries. It then presents summaries of relevant literature on freeway merges, freeway diverges, and freeway weaving, respectively, and includes topics within each summary organized in generally decreasing order of previous research activity.
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10 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies – Zhao et al.
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Background 11 an even slower-moving truck)
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12 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies length of the acceleration lane. The smallest accepted gaps in the dataset were between 0.75 and 1.0 seconds, resulting in net headways between two cars of 0.25 seconds.
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Background 13 that on average, drivers used 41.5% of the acceleration lane length and that very few maneuvers occurred prior to the end of the solid white stripe. The average merging speed and the variation of speed were higher with a tapered merge compared with an acceleration lane.
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14 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies breakdown occurred because of mainline traffic changing lanes to the left in advance of the on-ramp; in this case, prebreakdown flow and queue discharge flow increased with an increasing proportion of ramp traffic due to poor mainline utilization of the right-hand lane. Mainline operations did not clearly deteriorate with increasing ramp volume, indicating the benefit of the ramp meters to manage the merging traffic.
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Background 15 Capacity Elefteriadou, Kondyli, and St. George (2014)
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16 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies the right and the left) and found that the merge section capacity decreased as the merging volume increased.
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Background 17 but ramp metering was twice as effective overall (in terms of percentage increase in throughput)
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18 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies Asgharzadeh and Kondyli (2018) compared four freeway merge capacity models -- van Aerde, Product Limit Method, Sustainable Flow Index, and HCM 6th Edition -- to data collected at six freeway merges in the Kansas City region.
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Background 19 the characteristics of a diverge bottleneck were like those of a merge bottleneck: the capacity drop increases as the diverging flow increases (most of the effect occurs with the first 100−200 vph) , the diverging length decreases, and the diverging speed decreases.
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20 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies (nine of which had cross sections of five or more lanes) , 13 balanced major weaves, and five unbalanced major weaves.
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Background 21 freeway-to-ramp weaves occurred within the first 630 ft of the physical weaving area, and 90% of ramp-to-freeway weaves occurred within the first 1,060 ft of physical weaving area (that is, just past the end of the short length)
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22 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies effect)
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Background 23 (2) continuing it to the next ramp.
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24 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies length has limited effect on the weaving section's capacity and operation. Speed and density homogenization caused the weaving section breakdown.
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Background 25 normal, moderately aggressive, aggressive) were applied to those segments.
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26 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies equally. The model was tested on a single three-lane weaving section in Grenoble, France, with reasonable results.
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Background 27 Sasahara, Elefteriadou, and Dong (2019) proposed a lane-by-lane analysis method for basic segments.
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28 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies Merge Segments Prebreakdown Capacity and Breakdown Probability Effect of Ramp Volume There is good international agreement in the literature that prebreakdown merge capacity decreases as ramp demand increases (Aksoy and Öğüt 2018; Bates et al. 2013; Dehman and Drakopoulos 2013, 2016; Elefteriadou et al.
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Background 29 flow per lane and the acceleration lane length per 10 mph of speed limit. Reina and Ahn (2014)
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30 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies Queue Discharge Flow Factors Affecting the Capacity Drop The developers of numerical models have suggested the following relations between various factors and the size of the capacity drop at a merge bottleneck: • Merging speed (Chen and Ahn 2018; Leclercq et al.
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Background 31 prebreakdown capacity and queue discharge flow values as for merge segments (see Table 4 and Table 5 earlier)
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32 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies • The method sometimes estimates that weaving speeds are greater than nonweaving speeds (Zhou et al.
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Background 33 sites found that prebreakdown capacity decreases as the weaving ratio increases. Rudjanakanoknad and Akaravorakulchai (2011)
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34 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies FFS of the weaving and nonweaving lanes may be substantially different. Sasahara, Elefteriadou, and Dong (2019)
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Capacity On-ramp flow rate, merge ratio, and ramp speed affect merge capacity. Merge capacity is adversely affected by higher ramp flows.
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36 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies Other Inputs Lane-by-lane volume and analysis can now be performed. Lane distribution is sensitive to volume-to-capacity ratio, percent heavy vehicles, upstream and downstream ramp ratios, weekend conditions, and nighttime conditions.
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Background 37 Performance Measure Basic Segment (HCM Chapter 12) Weaving Segment (HCM Chapter 13)
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38 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies Travel time reliability analysis requires the same data needed to estimate speeds, plus information on the variability of demand; the severity, frequency, and duration of incidents; the frequency of severe weather conditions; and the frequency of work zones by the number of lanes closed by duration. Geometric Con gurations Covered e HCM divides the freeway facility into uniform segments that are analyzed to determine capacity, performance, and LOS.
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Background 39 for the current HCM merge, diverge, and weaving methods are illustrated in Figure 6 through Figure 8 and include the following: 1. Close Merges – Close merges occur where two or more ramps fall within 1,500 of each other.
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40 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies Figure 6. Geometries not well covered by current HCM Chapters 13 and 14 (#1 through #5 in list in text)
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Background 41 5. Multiple Weaving Segment – Section 4 ("Extensions to the Methodology")
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42 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies 11. Diverge Off Five-Lane Freeway – Section 4 of HCM Chapter 14 provides a table to estimate the volume in the fifth lane.
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Background 43 • The method underestimates capacities at high weaving ratios (Skabardonis and Mauch 2014)
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44 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies Each of these freeway management strategies may employ one or more managed lane tactics. There is a wide variety of these tactics that can be classified by their salient characteristics: • Pricing.
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Background 45 After removing the real-time options, the following freeway management and managed lane tactics remain. It would be desirable for the HCM freeway methodologies to better address: • Ramp Management – Static time of day, local dynamic, system dynamic.
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46 Update of Highway Capacity Manual : Merge, Diverge, and Weaving Methodologies • The HCM's merge/diverge methods violate the fundamental relationship between flow, density, and speed. • The HCM's capacity guidance for merge and diverge sections is poor.
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