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41 Description These techniques are applied to interchange crossroads to adequately separate access points from interchange ramp terminals. Minimum distances are specified from an interchange ramp terminal to the first downstream and last upstream (1): driveway, unsignalized crossroad inter- section, median opening, and signalized intersection. Tables 27 through 29 follow. Quantitative Analysis Methods Motor Vehicle Operations No national research on arterial weaving operations has been completed at the time of writing, although NCHRP Project 15-66, âArterial Weaving on Conventional and Alternative Intersections,â will address the topic. The table from NCHRP Report 420 (3, 4) can be used to estimate the minimum distance required to make a weaving maneuver from the right lane to the left lane (e.g., from a free-flowing right-turn lane from an off-ramp to a downstream left- turn lane). The minimum separation distance should be greater than the sum of the weaving distance, the distance required to transition into a left-turn lane and come to a stop, and the 95th-percentile queue length in the left-turn lane (see Table 29). Methods in HCM6 (5) can be used to estimate the 95th percentile queue at a traffic signal by movement; the distance between a downstream signal and an off-ramp should be greater C H A P T E R 8 Interchange Areas Source: Photograph provided by the authors.
42 Guide for the Analysis of Multimodal Corridor Access Management Access Management Technique Performance Trends and Documented Performance Relationships Operations Safety Increase access separation distances in interchange areas. â â â Â Â â â â Â Â Â Â Â Note: See Chapter 7 for performance trends generally associated with increasing access separation distances. â = unchanged performance. Â Â Â Table 27. Multimodal operations and safety performance summary. Mode Operations Safety Provides more distance to make weaving maneuvers between free-flowing off-ramps and downstream left-turn lanes (1). Reduces the chance that queues will back up into the ramp terminal intersection, the ramp, or the freeway (1). Provides a better opportunity to time signals to progress traffic (1). See also Chapter 6. Improves safety by reducing the number of conflicts and decisions to be made in a potentially high-volume, complex, and unfamiliar environment. As a result, the potential for sudden decisions leading to erratic maneuvers is reduced (1, 2). No change in pedestrian LOS beyond that generally associated with increasing access spacing. See also Chapter 7. Improves safety by reducing the number of potential motorist distractions in advance of pedestrian crossings of on-ramps. Interchange design influences pedestrian safety (2). See also Chapters 7 and 12. No change in bicycle LOS beyond that generally associated with increasing access spacing. See also Chapter 7. Similar effects as for pedestrians. No documented effect beyond that observed for motor vehicle traffic generally. No documented effect beyond that generally observed for motor vehicle traffic. No documented effect beyond that observed for motor vehicle traffic generally. No documented effect beyond that generally observed for motor vehicle traffic. Table 28. General trends associated with increasing access separation distance at interchanges. Weaving Volume Speed (mph) (vehicles per hour) 25, 30 35 40 45 50 200 50 100 150 200 400 400 100 200 300 450 800 600 150 300 450 700 1,200 800 200 400 600 950 1,800 1,000 300 500 750 1,200 2,400 1,200 350 600 900 1,450 a 1,400 400 710 1,050 1,700 a 1,600 450 820 1,200 2,050 a 1,800 500 930 1,400 2,400 a 2,000 600 1,040 1,600 a a 2,200 700 1,150 1,800 a a 2,400 800 1,270 2,050 a a 2,600 900 1,400 2,300 a a Note: Use 400 feet for values above the solid line. aSpeeds are not attainable. Source: NCHRP Report 420, Table 85 (3), adapted from Leisch (4). Table 29. Minimum distance required to make a weaving maneuver from the right lane to the left.
Interchange Areas 43 than this length to avoid frequent queue spillback onto the ramp and, potentially, onto the freeway. The Access Management Manual, Second ed. (1) and Access Management Application Guide- lines (2) provide recommended access spacing distances at interchanges. Pedestrian and Bicycle Operations Quantitative methods for the pedestrian and bicycle modes near interchanges are the same as for arterials in general. See Chapter 7 for details. Pedestrian and Bicycle Safety Section 18.6 of the Access Management Manual, Second ed. (1) provides guidance on designing interchanges to safely accommodate pedestrians and bicyclists. Free-flow ramps are discouraged because they are difficult for pedestrians and bicyclists to cross safely, due to a combination of relatively high vehicular speeds, infrequent gaps in traffic, and the potentially unexpected nature of a pedestrian or bicycle crossing. Based on California research, the HSM states that âencouraging bicyclists to cross interchange ramps at right angles appears to increase driver sight distance and reduce the bicyclistsâ risk of a crashâ (6). Additional Information ⢠Chapters 6, 7, and 12 in this guide. ⢠Access Management Manual, Second ed.: Chapter 18, Interchange Area Access Management. ⢠Access Management Application Guidelines: Chapter 18, Access Management at Crossroads in the Vicinity of Interchanges. ⢠NCHRP Report 420: Chapter 9, Access Separation at Interchanges. References 1. Williams, K. M., V. G. Stover, K. K. Dixon, and P. Demosthenes. Access Management Manual, Second ed. Transportation Research Board of the National Academies, Washington, D.C., 2014. 2. Dixon, K. K., R. D. Layton, M. Butorac, P. Ryus, J. L. Gattis, L. Brown, and D. Huntington. Access Management Application Guidelines. Transportation Research Board, Washington, D.C., 2016. 3. Gluck, J., H. S. Levinson, and V. Stover. NCHRP Report 420: Impacts of Access Management Techniques. Transportation Research Board of the National Academies, Washington, D.C., 1999. 4. Leisch, J. E. Procedure for Analysis and Design of Weaving Sections. Project DT FHS 1-82-1-00050. Federal Highway Administration, Washington, D.C., 1982. 5. Highway Capacity Manual: A Guide for Multimodal Mobility Analysis, 6th ed. Transportation Research Board, Washington, D.C., 2016. 6. Highway Safety Manual, 1st ed. American Association of State Highway and Transportation Officials, Washington, D.C., 2010.
