Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges (2021) / Chapter Skim
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Pages 187-212
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9-1 9.1 Introduction The diverging diamond interchange (DDI) is an alternative to the conventional diamond interchange or other alternative interchange forms.
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9-2 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges 9.2.1 Motor Vehicles DDIs generally have a reduced number of signal phases relative to conventional diamond interchanges and provide greater vehicular capacity and throughput. The following general design features are commonplace at existing DDIs: • Two-phase signals that reduce lost time at the interchange, allowing for shorter cycle lengths and increasing capacity relative to conventional diamond interchanges; • Free-flowing left-turns (and right-turns)
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Diverging Diamond Interchanges (DDIs) 9-3 availability for pedestrian facilities.
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9-4 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges or signalized crossings and, in either case, can be configured to promote low vehicle speeds and good sight distance to the crosswalks. With outside walkways at DDIs, traversing pedestrians may cross four separate vehicle turning movements (the ramps)
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Exhibit 9-4. Inner walkway pedestrian safety and comfort.
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9-6 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges Where an outer walkway is provided, the right-turn and left-turn off-ramp movements will not run concurrently (Exhibit 9-3)
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Diverging Diamond Interchanges (DDIs) 9-7 9.2.3.1 Bicycle Lane Guidance Bicycle lanes at a DDI provide bicyclists with dedicated road space to travel across the interchange.
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9-8 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges left-turn and opposing through movements)
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Diverging Diamond Interchanges (DDIs) 9-9 Bicycle-vehicle conflict points for an inside bicycle path are presented in Exhibit 9-10.
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9-10 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges to an arterial through lane. The downstream merge point onto the limited access facility would be designed no differently than other interchange designs.
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Diverging Diamond Interchanges (DDIs) 9-11 Design Flag Description Mode/Travel Path Motor Vehicle RightTurns (Section 4.4.1)
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9-12 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges concerns with the channelized turn-lane design and should be treated as such for bicyclists in the design flag assessment. • Bicycle clearance time.
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Diverging Diamond Interchanges (DDIs) 9-13 the location of their discussion and applicable treatments (design flags and treatments whose discussion applies across alternative intersection types are in Chapter 4)
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9-14 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges • Indirect Path design flag: Pedestrians and bicyclists can cross the mainline by proceeding from one side of the roadway, onto the center island, and then to the opposing side of the roadway. This avoids the need for pedestrians to travel to an adjacent intersection to cross the mainline.
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Diverging Diamond Interchanges (DDIs) 9-15 Design Flag Description Mode/Travel Path Motor Vehicle RightTurns (Section 4.4.1)
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9-16 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges Exhibit 9-16. DDI on-street bike lane/outer walkway design concept.
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Diverging Diamond Interchanges (DDIs) 9-17 Exhibit 9-17.
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9-18 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges vehicles on either side of pedestrians in the inner walkway, pedestrians are further removed from motor vehicles. Care should be taken in the design to ensure proper space is provided for pedestrians to move past each other without encroaching on the bicycle lanes.
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Diverging Diamond Interchanges (DDIs) 9-19 Design Flag Description Mode/Travel Path Motor Vehicle RightTurns (Section 4.4.1)
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9-20 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges Exhibit 9-20. Undesirable use of single pole with two pedestrian pushbuttons, no APS, and insufficient separation of the two detectable warning surfaces.
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Diverging Diamond Interchanges (DDIs) 9-21 downstream side of the crossing to provide audible separation between the APS messages and oncoming traffic, as shown in Exhibit 9-21.
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9-22 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges If pedestrian facilities are placed on the outside, attention is needed regarding the placement, visibility, and vehicular speeds for the pedestrian crossing at the left-turn to the on-ramp. Exhibit 9-22 shows one example of such a left-turn crossing, viewed from the middle of the DDI.
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Diverging Diamond Interchanges (DDIs) 9-23 Whether or not pedestrian facilities are provided along the outside or in the center median, a unique issue with pedestrian crossings at DDIs is the propensity to look the wrong direction for gaps in traffic.
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9-24 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges for pedestrians to cross the arterial roadway at a DDI (Exhibit 9-24) –if a pedestrian needs to cross the arterial, they are expected to travel to the next intersection to do so.
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Abbreviations and acronyms used without definitions in TRB publications: A4A Airlines for America AAAE American Association of Airport Executives AASHO American Association of State Highway Officials AASHTO American Association of State Highway and Transportation Officials ACI–NA Airports Council International–North America ACRP Airport Cooperative Research Program ADA Americans with Disabilities Act APTA American Public Transportation Association ASCE American Society of Civil Engineers ASME American Society of Mechanical Engineers ASTM American Society for Testing and Materials ATA American Trucking Associations CTAA Community Transportation Association of America CTBSSP Commercial Truck and Bus Safety Synthesis Program DHS Department of Homeland Security DOE Department of Energy EPA Environmental Protection Agency FAA Federal Aviation Administration FAST Fixing America's Surface Transportation Act (2015) FHWA Federal Highway Administration FMCSA Federal Motor Carrier Safety Administration FRA Federal Railroad Administration FTA Federal Transit Administration HMCRP Hazardous Materials Cooperative Research Program IEEE Institute of Electrical and Electronics Engineers ISTEA Intermodal Surface Transportation Efficiency Act of 1991 ITE Institute of Transportation Engineers MAP-21 Moving Ahead for Progress in the 21st Century Act (2012)
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TRA N SPO RTATIO N RESEA RCH BO A RD 500 Fifth Street, N W W ashington, D C 20001 A D D RESS SERV ICE REQ U ESTED G uide for Pedestrian and Bicyclist Safety at A lternative and O ther Intersections and Interchanges TRB ISBN 978-0-309-67353-2 9 7 8 0 3 0 9 6 7 3 5 3 2 9 0 0 0 0
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