Diverging Diamond Interchange Informational Guide, Second Edition (2021) / Chapter Skim
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121 6.1 Overview Chapter 6 introduces the objectives, principles, and performance checks that guide the geometric design process to evaluate and design a diverging diamond interchange (DDI)
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122 Diverging Diamond Interchange Informational Guide Addressing geometric design fundamentals (such as including tangent segments between reverse curves) and considering DDI-specific design features help guide project decision making.
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Geometric Design 123 Similarly, the full range of motorized users should be integrated into design configurations, as anticipated for a given interchange location. A DDI configuration may emphasize and serve pedestrians and bicyclists by creating lower speed environments.
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124 Diverging Diamond Interchange Informational Guide between crossovers. This could put the bicyclist between the travel lane and raised barrier in some DDI configurations.
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Geometric Design 125 New facilities often have the most opportunity to include geometric design values and dimensions with the fewest restrictions. Planning and designing a new DDI facility provide the means of integrating the DDI within the adjacent roadway network and establishing adjacent access points.
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126 Diverging Diamond Interchange Informational Guide As noted previously, a DDI is a diamond interchange and generally includes the same design considerations as those for other high-capacity forms such as the single point and tight diamond. Traditional interchange forms have relatively simple cross street horizontal alignments, commonly tangents or flat curves.
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Geometric Design 127 and from the entrance and exit ramps. The crossover function and design affect and are affected by the ramp terminal intersection turning movements.
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128 Diverging Diamond Interchange Informational Guide 6.3 Project Constraints When considering a DDI, there are many project constraints that can influence the geometric design of this type of interchange. Understanding the unique constraints, how those may influence design decisions, and identifying solutions to minimize overall impacts to adjacent areas can help a practitioner prioritize decisions and trade-offs.
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Geometric Design 129 and limited speed differences between successive geometric elements. The crossover proper takes up more longitudinal and cross-sectional space than a conventional signalized intersection.
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130 Diverging Diamond Interchange Informational Guide 6.3.3 Adapting to Site Constraints Compared to traditional diamond interchanges, DDIs have special considerations in the roadway approaches and in developing optimal crossover geometry for a given location. Understanding and considering constraints begins in early concept development and evaluation.
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Geometric Design 131 ramp terminal intersection elements or to integrating the existing overcrossing during construction while building new bridges. Integrating an existing highway overcrossing creates a fixed location to which crossover planning and design must match.
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132 Diverging Diamond Interchange Informational Guide Four-span bridges create the opportunity to place pedestrian walkways under the first and fourth span; however, in some four-span configurations there may not be enough room to place pedestrian facilities. Bridge configurations without space for the sidewalks because of abutment fill slopes may require cutting back and retaining the sloped abutment fill between the columns and the approach bent.
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Geometric Design 133 6.3.5.2 Matching to an Existing Overcrossing DDIs are often considered because they add capacity at the ramp terminal intersections and potentially allow narrower roadways that retain an existing bridge. Similar to matching an undercrossing, key considerations include assessing the width of the existing bridge and the ability to serve motorized and nonmotorized users and locating the crossover and ramp terminal intersections a sufficient distance away from the existing bridge to attain target geometry.
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134 Diverging Diamond Interchange Informational Guide Exhibit 6-13. Options for bridges at existing highway crossings.
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Geometric Design 135 6.3.6 Existing Ramp Locations Existing ramp locations and vertical and horizontal geometry can influence DDI planning and design. It is often desirable to maintain the existing entrance and exit ramp terminal locations and to use substantial portions of existing ramp alignments.
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136 Diverging Diamond Interchange Informational Guide and geometric performance while accommodating the design vehicle and nonmotorized users. For new construction, it may be relatively easy to meet target objectives.
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Geometric Design 137 an existing interchange at a given location may require design compromises to adapt to that site. Considering intended project outcomes and assessing project performance metrics can help optimize and select design features at a constrained location.
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138 Diverging Diamond Interchange Informational Guide Counterclockwise skew creates operational effects on the entrance ramp by: • Reducing the left-turning minimum radii, • Increasing right-turn speeds, and • Increasing the downstream convergence angle and speed shear. Counterclockwise skew increases the propensity for a violation of cross street route continuity (turning to stay on the designated route)
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Geometric Design 139 terminal intersections, drivers at entrance ramps have an expectation of acceleration to the higher type roadway. As with any intersection, skew creates acute (less than 90 degrees)
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140 Diverging Diamond Interchange Informational Guide skew on counterclockwise configurations often requires right-turning drivers to travel past the crossover before initiating their turn. From a visual perspective upstream, the "straight" road leads to the right turn, while through drivers must turn to the left (to access the crossover)
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Geometric Design 141 As the configurations are customized for the specific site conditions, the lane numbers and arrangements might change as more detailed traffic volumes and traffic signal timing schemes are considered in more robust evaluations than in early concept development. As project alternatives are screened and refined in later ICE or other engineering evaluations, the traffic operations evaluations become more refined to help select a preferred interchange concept that is revised and finalized, leading to final design.
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142 Diverging Diamond Interchange Informational Guide the median between the contraflow lanes and affects the pedestrian quality of experience. When pedestrian facilities and design vehicle swept paths are considered, the limitations of the initial configuration become clear.
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Geometric Design 143 Exhibit 6-26 depicts pedestrian features that increase difficulties for users with low vision that involve locating the pedestrian push button, orienting themselves to the edge of the crossing, and then determining crossing path to reach the receiving pedestrian ramp. Exhibit 6-27 presents design features that integrate quality pedestrian treatments.
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144 Diverging Diamond Interchange Informational Guide 6.4.5.1 Design Vehicle DDI configurations are affected by the need to accommodate the largest vehicle likely to use the interchange. Turning path requirements for this vehicle, referred to as the design vehicle, will dictate many of the dimensions of the DDI.
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Geometric Design 145 In some cases, it may be appropriate to consider different design vehicles for different approaches. For example, there may be oversized/overweight vehicles traveling certain routes through the interchange.
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146 Diverging Diamond Interchange Informational Guide and crossovers. There is an integral relationship between crossroad alignment and transition to and through DDI crossovers.
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Geometric Design 147 and from the ramps are served by ramp terminal intersections. As noted previously, the ramp terminal intersections and crossover proper should be consolidated as much as possible.
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148 Diverging Diamond Interchange Informational Guide an angle point. Whether a curve or angle point is used, the curve radii developed should support the speed transition relationship.
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Geometric Design 149 • Driver discomfort -- Greater crossing angles require corresponding reverse curves, unless a wide median is present. Overall speed profiles approaching, navigating crossovers, and departing the interchange ideally result in speed reductions between successive movements of less than 15 to 20 mph.
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150 Diverging Diamond Interchange Informational Guide In addition to meeting fundamental horizontal geometric design principles, crossover tangents also encourage path alignment. This is the same natural path objective of multilane roundabout entry design and avoiding path overlap.
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Geometric Design 151 components within the crossover and after the crossover. Basic geometry and trigonometry that consider approach widths and crossover angles can be used to compute tangent element values.
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152 Diverging Diamond Interchange Informational Guide Examples for using this equation are shown below. • Example 1: DDI with a 45 degree crossover angle and two 14-foot lanes in both directions (assumes no shoulder or bike lane)
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Geometric Design 153 As noted in the prior section, crossover width (footprint) is affected by the cross street cross section and the median width.
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154 Diverging Diamond Interchange Informational Guide 6.4.8.1 Intersection Design Fundamentals DDI crossovers and ramp terminal intersections (left- and right-turning movement to and from the ramps) should follow fundamental intersection design principles including appropriate channelization that supports and reinforces intended or restricts nonintended turning movements.
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Geometric Design 155 The degree to which this issue exists on a site-by-site basis should be considered when selecting the control for off-ramp turning movements and traffic control. If signal control is selected, having an adequate view angle or not may determine whether to allow RTOR.
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156 Diverging Diamond Interchange Informational Guide Adjacent intersection considerations are common when constructing or reconstructing existing interchange configurations and should be assessed in conjunction with the DDI. This means considering traffic flows to and from the DDI and the effects each adjacent intersection has on the DDI and the DDI effects on adjacent intersections.
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Geometric Design 157 and appropriate curbing and guidance to support users with low vision. This may also include ensuring the median crossing locations have adequate room for traffic furniture, such as pedestrian call buttons, crosswalk signals, and illumination.
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158 Diverging Diamond Interchange Informational Guide Exhibit 6-40 compares the influence of median width on crossover location and influence area. Tangent alignments through the crossover promote desired vehicle tracking and reduce driver workload by separating driving tasks at each curve.
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Geometric Design 159 Exhibit 6-41. Crossover intersection as the basis for locating the ramp terminal intersections.
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160 Diverging Diamond Interchange Informational Guide configuration of possible downstream lane drops or lane additions for high-occupancy vehicle (HOV) bypass lanes or for ramp metering queue storage.
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Geometric Design 161 road typical section. This includes accounting for developing right-turn lanes to the entrance ramps to have definitive tapers to distinguish the right turn from a through movement and right-turn treatments from the exit ramps.
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162 Diverging Diamond Interchange Informational Guide to aim pedestrians to the receiving landing, should be integral to early concept designs. These features at the ends of the central crossing area are sometimes an afterthought leading to undesirable crossing treatments.
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Geometric Design 163 A) Center Walkway, Single Structure (MO-13, Springfield, MO)
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164 Diverging Diamond Interchange Informational Guide Each project has a unique context in design vehicles and if some off-tracking is allowed, it should be a deliberate design decision. 6.5.2.1 Travel Lanes Lane widths along the crossroad tangents typically range from 12 to 15 feet wide, depending on local design practice.
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Geometric Design 165 A) No Walkway, Columns Outside (SR-92, American Fork, UT)
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166 Diverging Diamond Interchange Informational Guide such as curve radius, crossover angle, and tangent segments at the crossover can influence lane width dimensions. Design vehicle swept paths using templates or software are useful for determining the necessary lane widths through different radius curves.
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Geometric Design 167 Assessing design options for pedestrians and bicyclists is key, given the variety of pedestrian and bicycle facilities that have been used at DDIs to date. Due to the considerable amount of information on this topic, an entire chapter of this guide is devoted to it.
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168 Diverging Diamond Interchange Informational Guide Exhibit 6-51 presents the pedestrian crossing location at the DDI at Pioneer Crossing in American Fork, Utah. Exhibit 6-52 presents the upstream sight line from the pedestrian crossing and the influence of the parapet wall at the DDI at Pioneer Crossing in American Fork, Utah.
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Geometric Design 169 Similarly, a DDI is a high-capacity diamond interchange form. Maximizing capacity with large radii and free flow or yield control could contribute to the overall capacity.
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170 Diverging Diamond Interchange Informational Guide DDI planning and design is an iterative process to optimize roadway and intersection features for each project location. Optimizing the DDI configuration is based on considering a variety of general and DDI-specific performance categories associated with the crossover design, ramp terminal intersection, and unique driver expectation issues associated with contraflow roadway.
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Geometric Design 171 separated pathways are increasingly common and should be accounted for in allocated cross section width dimensions. • Providing well-defined and well-located crosswalks that include appropriate supporting features such as tactile warning strips and pedestrian call buttons.
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172 Diverging Diamond Interchange Informational Guide 6.7.1.2 Design Vehicle The choice of design vehicle will vary depending upon the approaching roadway types and the surrounding land use characteristics. The local or state agency with jurisdiction of the associated roadways should usually be consulted to identify the design vehicle at each site.
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Geometric Design 173 to be out of view. Where traffic signals are not visible to oncoming drivers, advance signals can provide the necessary information to drivers regarding who has the ROW (13)
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174 Diverging Diamond Interchange Informational Guide movements. As drivers cannot cross from the exit ramp to the entrance ramp (i.e., crossing the major road)
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Geometric Design 175 RTOR or LTOR is an available traffic control strategy at a DDI ramp terminal intersection. Because of the curvilinear alignment to develop the crossovers, designers must work deliberately to configure left- and right-turning movements so that drivers have a view angle to upstream conflicting traffic.
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176 Diverging Diamond Interchange Informational Guide traffic. Exhibit 6-60 demonstrates that consolidating the ramp terminal intersections so the left and right turns are approximately centered on the crossover intersection proper improves DDI performance.
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Geometric Design 177 , 0.02 , 0.02 for e for e 3.4415 3.4614 0.3861 0.3673 V = R V = R = + = − where V = predicted speed, mph; R = radius of curve, ft; and e = superelevation, ft/ft. Using the equations, the speed-radius relationships can be plotted to estimate speeds for a given radius and horizontal curve orientation (left or right)
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178 Diverging Diamond Interchange Informational Guide 6.7.2.2 Speed Profile and Speed Checks Designers can use speed curvature relationships to create initial DDI concepts and then subsequently assess and refine the configurations. • Speed Profile: The speed profile represents a holistic view of intended speed performance to and through a DDI.
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Geometric Design 179 The design goal is a smooth and flowing alignment with proportionate curve lengths to avoid the appearance of kinks in the alignments.
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180 Diverging Diamond Interchange Informational Guide be no more than approximately 10 to 15 mph (15 to 25 km/h)
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Geometric Design 181 Exhibit 6-66 presents three examples of terminal vistas where the terminal vista is fully blocked, partially blocked, or not blocked. Exhibit 6-67 presents an example of an approach vista at a DDI.
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182 Diverging Diamond Interchange Informational Guide 1. Barrier Installation, Right Side, Entry Approach (Pioneer Crossing, UT)
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Geometric Design 183 ahead versus the path they must navigate to the right. The exhibit shows the motorist sight line focuses on the oncoming traffic stream while the actual path needed to navigate the crossover is out of sight to the far right.
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184 Diverging Diamond Interchange Informational Guide a perpendicular line extending from the center of the stop bar to determine the line representing ideal path alignment. This represents the direction to which a driver is oriented when stopped at or proceeding through the intersection.
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Geometric Design 185 overlap. A similar issue can exist at multilane roundabout entries, and research at roundabouts found that entries with path overlap experienced a higher rate of sideswipe crashes than entries with proper path alignment (12)
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186 Diverging Diamond Interchange Informational Guide Exhibit 6-76. Median effects of vista through crossover.
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Geometric Design 187 8. Cunningham, C., B
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