Diverging Diamond Interchange Informational Guide, Second Edition (2021) / Chapter Skim
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20 3.1 Introduction This chapter presents guidance when considering pedestrians, bicyclists, and transit vehicles at a diverging diamond interchange (DDI)
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Multimodal Considerations 21 pedestrian clearance may result in the pedestrian movement controlling the phase lengths, leading to longer cycle lengths and greater pedestrian delay. In contrast, vehicle movements typically control phase length at DDI signals resulting in sufficient time per phase to also serve pedestrians.
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22 Diverging Diamond Interchange Informational Guide 3.2 Facility Selection This section discusses the operational considerations at a DDI that have relevance to the pedestrian and bicyclist experience, so the reader can understand trade-offs associated with various facility types. As discussed in Chapter 1, identifying project outcomes should be conducted as early in the design process as possible.
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Multimodal Considerations 23 traffic to and from the freeway and naturally allow crossing the interchange in all directions (i.e., travel along the cross road and crossing the cross road from one side to the other)
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24 Diverging Diamond Interchange Informational Guide 3.2.1.1 Inner Walkways Exhibit 3-6 gives an example of a pedestrian facility located in the center of a DDI. While on- and off-ramp movements may be signalized or unsignalized, the exhibit shows movements that would time together if all ramp movements were signalized.
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Multimodal Considerations 25 Exhibit 3-6. Pedestrian movements given an inner walkway at a DDI.
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26 Diverging Diamond Interchange Informational Guide Benefits Challenges + Crossing of the cross road naturally provided at DDI for full pedestrian access – Possible crossing of free-flow rightturn movements to/from freeway + Crossing one direction of traffic at a time – Pedestrians may not know to look to the right when crossing from center + No exposure to left turns to freeway (typically free flowing) – Wait at center median dictated by length of signal phase for through traffic + Protected signalized crossing to walkway – Pedestrian signals can conflict with vehicle signals at crossovers + Pedestrian clearance time generally provided in crossover signal phasing – Out of direction travel for pedestrians not desiring to cross the cross road + Pedestrian delay to center minimized by short cycles at two-phase signals Exhibit 3-8.
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Multimodal Considerations 27 Exhibit 3-10. Outer pedestrian facilities at a DDI.
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28 Diverging Diamond Interchange Informational Guide 3.2.2 Bicycles Three basic facility options exist for bicyclists at a DDI. These options include providing: • Shared use of the travel lane, • A marked bicycle lane through the DDI, or • A separated bicycle path or shared-use path.
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Multimodal Considerations 29 Selection Guide (2) provides facility selection guidance with respect to motor vehicle volume and operating speed, as shown in Exhibit 3-14.
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30 Diverging Diamond Interchange Informational Guide the signalized crossovers. The bicycle lane should continue through the off-ramps from freeway, where motorized traffic would generally be required to yield to cross road traffic, including bicyclists.
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Multimodal Considerations 31 Exhibit 3-16. Bicycle lane extending through inbound crossover (3)
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32 Diverging Diamond Interchange Informational Guide 3.3 Pedestrian & Bicycle Assessment Research has documented vehicular safety benefits for DDIs. The reduction in vehicular crashes relative to a traditional diamond interchange is attributable to the separation of conflict points and the elimination of high-risk and high-severity crash patterns (e.g., angle crashes between left-turn and opposing through movements)
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Multimodal Considerations 33 3.3.2 Pedestrians -- Key Safety Challenges DDIs present a reduced number of conflict points for pedestrians relative to a conventional intersection or interchange. At the same time, there are design factors inherent to a DDI that should be flagged as the design is developed.
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34 Diverging Diamond Interchange Informational Guide • On-ramp movements -- In general, the capacity benefits provided by unsignalized on-ramps at a DDI degrade the crossing environment for pedestrians and bicyclists on separated paths. While left turns are typically free flowing, the right turn is either free flowing with an acceleration lane (Exhibit 3-21)
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Multimodal Considerations 35 • On-ramp movements -- If free-flow movements are provided for motorists to access freeway on-ramps, then an uncontrolled diverging conflict point is introduced between bicyclists and motorists (essentially a "right hook" opportunity)
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36 Diverging Diamond Interchange Informational Guide Exhibit 3-23. Motor vehicle right turns design flag.
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Exhibit 3-25. Nonintuitive motor vehicle movements design flag.
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38 Diverging Diamond Interchange Informational Guide Exhibit 3-28. Executing unusual movements design flag.
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Multimodal Considerations 39 Exhibit 3-30. Motor vehicle left turns design flag.
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40 Diverging Diamond Interchange Informational Guide Exhibit 3-32. Grade change design flag.
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Multimodal Considerations 41 Exhibit 3-34. Undefined crossing at intersection design flag.
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42 Diverging Diamond Interchange Informational Guide Exhibit 3-36. Bicycle clearance time design flag.
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Multimodal Considerations 43 Riding between motor vehicle lanes creates safety concerns. Exhibit 3-38.
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44 Diverging Diamond Interchange Informational Guide 3.4 Intersection-Level Concepts Three design concepts are presented in this section to offer techniques for improving pedestrian and bicycle safety and operational performance of DDIs. These concepts are not suggested as designs to be replicated as-is; rather, they illustrate the DDI options that are possible in various contexts.
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Multimodal Considerations 45 • Undefined crossings at intersections design flag -- All locations where pedestrians and bicyclists cross motor vehicles are marked. This will reduce the likelihood that vehicles will encroach on the crossing areas when stopped for a signal.
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46 Diverging Diamond Interchange Informational Guide Design Flag Description Motor Vehicle Right Turns DDIs tend to have high volumes of turns to and from the freeway. This design still requires pedestrians to cross the right turn from the off-ramp.
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Design Flag Description Motor Vehicle Right Turns DDIs tend to have high volumes of turns to and from the freeway. This design still requires pedestrians to cross the right turn from the off-ramp.
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48 Diverging Diamond Interchange Informational Guide Benefits -- This design addresses the following key elements with respect to safety and comfort: • Uncomfortable/tight walking environment design flag -- This design provides an exclusive path for pedestrians separate from both bicyclists and motor vehicles. By placing motor vehicles on either side of pedestrians in the inner walkway, pedestrians are further removed from motor vehicles.
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Multimodal Considerations 49 Design Flag Description Motor Vehicle Right Turns DDIs tend to have high volumes of turns to and from the freeway. This design still requires pedestrians to cross the right turn from the offramp.
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50 Diverging Diamond Interchange Informational Guide Exhibit 3-46. Back-to-back off-ramp crossings during the same signal phase.
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Multimodal Considerations 51 Exhibit 3-47. Channelization toward center crosswalk (4)
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52 Diverging Diamond Interchange Informational Guide Exhibit 3-49. Example of pedestrian wayfinding provision at DDI via curbing (4)
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Multimodal Considerations 53 • Provide audible speech messages to communicate directionality of traffic (from left or from right) at all crossing points.
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54 Diverging Diamond Interchange Informational Guide accessibility installations may need to be consulted for specialized applications and signal installations at a DDI to ensure that the crossings are accessible to and usable by all pedestrians as required by ADA. 3.5.4 Turning Movements at Ramps Channelization of all turns to and from the freeway is used to discourage wrong-way maneuvers and to move ramp terminal intersections away from the crossover intersection.
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Multimodal Considerations 55 Exhibit 3-53. Example of pedestrian crossing at free-flow left onto freeway (8)
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56 Diverging Diamond Interchange Informational Guide 3.5.6 Indirect Paths DDI design and signal phasing provide a natural time and location for pedestrian access across the mainline. Yet, some existing DDIs with outer walkways do not include provisions for pedestrians to cross the cross road at a DDI (Exhibit 3-55)
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Multimodal Considerations 57 3.6.2 Transit on Freeway If rail transit or a busway is located within the freeway right-of-way and a station is provided at the interchange, a wider median on the cross road would allow buses on the cross road to stop directly above or below the station. This would facilitate passenger transfers, with passengers boarding and alighting directly from a median walkway with an elevator and stairs connecting the walkway to the freeway station platform.
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58 Diverging Diamond Interchange Informational Guide • Outer pedestrian facilities or two inner walkways (with the light-rail tracks in the middle) need to be provided.
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