Diverging Diamond Interchange Informational Guide, Second Edition (2021) / Chapter Skim
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Pages 67-120
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67 This chapter summarizes considerations and analysis required to develop and verify the lane configuration and phasing scheme for a diverging diamond interchange (DDI) as depicted in the flowchart in Exhibit 51.
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68 Diverging Diamond Interchange Informational Guide SEC 7.6 Exhibit 5-1. DDI operations and design flowchart.
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Conceptual Operations 69 3. Bridge Zone -- the area between the two crossovers where vehicles will drive over or under the bridge.
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70 Diverging Diamond Interchange Informational Guide Exhibit 5-4. Key operational characteristics of a DDI.
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Conceptual Operations 71 different geometric designs or signal phasing. Movements are different from both lanes and from phases.
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72 Diverging Diamond Interchange Informational Guide • Rightturn movements from the offramps are assigned numbers 4 and 8. • Leftturn movements from the offramps are assigned numbers 3 and 7.
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Conceptual Operations 73 5.2.3 Phases Versus Intervals There are multiple ways that movements at an intersection can be conveyed and represented visually. Two common ways are ringandbarrier diagrams and interval diagrams.
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74 Diverging Diamond Interchange Informational Guide NCHRP Report 812: Signal Timing Manual, 2nd Edition (1)
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Conceptual Operations 75 5.2.5 Time-Space Diagram In addition to the phaseassignment and ringandbarrier diagrams, each phasing scheme described in the next section has a timespace diagram that combines these two elements. For more information about reading a timespace diagram, refer to NCHRP Report 812: Signal Timing Manual, 2nd Edition (1)
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76 Diverging Diamond Interchange Informational Guide programmed phase clearance times. While the twocriticalmovement phasing scheme example only shows bandwidth for the cross street, other phasing schemes in Section 5.3 will depict the available bandwidth for the ramps as well.
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Conceptual Operations 77 Exhibit 511 depicts the total clearance distance for each through movement (approaching and departing) at the crossovers.
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Note: The rings have been drawn to focus on clearance considerations for the northbound through movement. Not all phases are shown in the ring-and-barrier diagrams.
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Conceptual Operations 79 5.2.6.3 Cycle Length The travel time from one crossover to the other is also an important factor to consider when selecting the cycle length. Conceptually, if the split for each direction could equal the travel time between crossovers and the cycle length could equal twice that value, progression could be achieved for both directions with an appropriate offset.
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80 Diverging Diamond Interchange Informational Guide 5.2.7 Phasing Scheme Layout Three phasing schemes will be discussed in this guidebook. Exhibit 515 depicts the three pieces of information provided for each phasing scheme, which were described in the previous sections -- (1)
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Conceptual Operations 81 these schemes and a comparison of their operational performance is provided in Section 5.5. Splits in the following phasing scheme examples are for illustrative purposes only; splits should be adjusted to reflect demand.
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82 Diverging Diamond Interchange Informational Guide Exhibit 5-16. Two-critical-movement phasing scheme (cross-street progression)
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Conceptual Operations 83 Exhibit 5-17. Two-critical-movement phasing scheme (ramp progression)
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84 Diverging Diamond Interchange Informational Guide Exhibit 5-18. Three-critical-movement phasing scheme.
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Conceptual Operations 85 • Because each crossover operates in an independent ring, no barriers are required. Coordina tion between the crossovers can be achieved through ring offset.
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86 Diverging Diamond Interchange Informational Guide Exhibit 5-19. Four-critical-movement phasing scheme Option A
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Conceptual Operations 87 Exhibit 5-20. Four-critical-movement phasing scheme Option B
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88 Diverging Diamond Interchange Informational Guide • Both options use fixedtime traveltime phases for efficiency (unrelated to clearance for the ramps)
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Conceptual Operations 89 With actuatedcoordinated control, minor movements can gap out or be skipped if demand is low. The extra time is often given to the coordinated movements.
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90 Diverging Diamond Interchange Informational Guide 5.3.4.4 Dynamic Overlap Phasing As discussed in Section 5.2.6, additional clearance time is needed at DDIs to ensure that crossstreet traffic can clear conflict points with left turns and right turns from the freeway. Those clearance times are estimated based on vehicular travel times.
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Conceptual Operations 91 capacity of the downstream signal. The queue that was spilling back from the downstream adjacent intersection into the outbound crossover would then be transferred to the upstream adjacent intersection.
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92 Diverging Diamond Interchange Informational Guide the default DDI phasing scheme, especially for DDIs with high volumetocapacity ratios and multiple dominant movements. 5.3.5.3 Four-Critical-Movement Phasing Scheme Benefits and Challenges Exhibit 524 summarizes the benefits and challenges of the fourcriticalmovement phasing scheme.
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Conceptual Operations 93 points may be closely spaced, resulting in significant interaction between the interchange and adjacent intersections. Adjacent intersections may also be roundabouts or have alternative intersection forms.
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94 Diverging Diamond Interchange Informational Guide Exhibit 5-25. Two-critical-movement phasing in corridor context.
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Conceptual Operations 95 Exhibit 5-26. Three-critical-movement phasing in corridor context.
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96 Diverging Diamond Interchange Informational Guide Exhibit 5-27. Four-critical-movement phasing (Option A)
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Conceptual Operations 97 Exhibit 5-28. Four-critical-movement phasing (Option B)
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98 Diverging Diamond Interchange Informational Guide downstream adjacent intersections. The use of timespace diagrams is strongly encouraged to visualize corridor bandwidth and properly evaluate the selected phasing scheme in a corridor context.
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Conceptual Operations 99 0 5 10 15 20 25 30 35 40 45 50 Sp ee d (M ile s Pe r H ou r) Speed Limit Speed between Crossovers Crossover Speed Left-Turn Speed from Freeway Exhibit 5-29.
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100 Diverging Diamond Interchange Informational Guide Exhibit 5-31. Queue spillback at a DDI.
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Conceptual Operations 101 lane utilization can be predicted by the leftturndemand ratio (2, 3, 4)
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102 Diverging Diamond Interchange Informational Guide a practitioner must determine which dominantmovement patterns are likely to occur at the DDI in question, likely through a planninglevel demand assessment. The dominantmovement schemes may vary based on time of day.
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Conceptual Operations 103 long queues. For example, queues for Movements 1 and 5 are considered to be significant if they extend beyond the crossover spacing (300 feet, 750 feet, and 1,200 feet, respectively)
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104 Diverging Diamond Interchange Informational Guide Heavy Demand Path(s) (Reference Exhibit 5-36)
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Conceptual Operations 105 delay impacts of the three phasing schemes (two, three, and four critical movements)
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106 Diverging Diamond Interchange Informational Guide Other strategies can be applied at the adjacent signalized intersections in order to influence operations at the DDI: • Optimize timing and/or meter traffic at upstream adjacent signalized intersection, • Alternate sidestreet phases at downstream adjacent signalized intersection, • Lead/lag phasing for outbound left turns at downstream adjacent signalized intersection, and • Eliminate phases at adjacent signalized intersection. Additionally, one strategy can be applied at the corridor level (i.e., at the DDI crossovers and adjacent signalized intersections)
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Conceptual Operations 107 phase or a beaconing device such as a pedestrian hybrid beacon (PHB) or flashing yellow beacon.
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108 Diverging Diamond Interchange Informational Guide 5.4.4.3 Dedicated Phase for Concurrent Off-Ramp Left and Right Turns For continuously high demand at the exit ramp during peak hours, an additional phase can be added to the timeofday plan, which serves the left and right turns from the exit ramp. This strategy uses a similar approach as was discussed in Section 5.3.4.3, but instead of a true "ramp flush," it would use a phase to serve the ramp movements concurrently.
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Conceptual Operations 109 Exhibit 542 suggest that this strategy is not appropriate for any of the highvolume scenarios because it results in significant increases in delay over the corresponding base case. However, for lowvolume periods this strategy can more readily adapt to changing traffic patterns and has been applied successfully at several DDIs in the United States during offpeak periods.
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110 Diverging Diamond Interchange Informational Guide a DDI analysis does not differ fundamentally from any other interchange evaluation, but some DDIspecific considerations for tool selection are described below.
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Conceptual Operations 111 The intersections and interchanges are evaluated using criticallanevolume summation. Implemented in a spreadsheet workbook, CAPX is a simple and costeffective sketchplanning tool that can help users focus on the most appropriate intersection/interchange forms for a particular location, prior to conducting a more demanding traffic simulation.
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112 Diverging Diamond Interchange Informational Guide offramp movements are signalized; freeflow movements should be treated differently. For more information on applying CMA, refer to NCHRP Report 812: Signal Timing Manual, 2nd Edition (1)
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Conceptual Operations 113 Exhibit 5-45. CMA volumes.
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114 Diverging Diamond Interchange Informational Guide Exhibit 5-47. CMA volumes with revised lane configuration.
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Conceptual Operations 115 Exhibit 5-49. CMA calculations.
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116 Diverging Diamond Interchange Informational Guide Using Exhibit 551, a practitioner can identify that the critical volume in the first example (1,400 vehicles) requires a cycle length greater than 120 seconds.
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Conceptual Operations 117 • Calculation of internal DDI queue lengths. • Analysis of unsignalized turning movements at DDIs.
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118 Diverging Diamond Interchange Informational Guide The greatest disadvantage of simulation is the increased resource requirements, as every DDI model needs to be built and configured. Specifically, proper calibration and validation of the simulation model is needed to assure that the predicted operations adequately reflect conditions at the DDI.
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Conceptual Operations 119 Exhibit 5-52. Simulation look-back distance.
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120 Diverging Diamond Interchange Informational Guide should be followed as well. Three key validation parameters recommended in the literature for accurately modeling DDIs include (8)
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