Diverging Diamond Interchange Informational Guide, Second Edition (2021)

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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). This chapter emphasizes context-sensitive and cost-sensitive design as well as the iterative nature of performance checks to refine horizontal and vertical designs. The performance checks support Intersection Control Evaluation (ICE) activities and guide practitioners in evaluating and optimizing DDIs for a given project context. 6.1.1 Integrating with ICE Activities ICE is often evaluated in two to three stages; at a minimum, the general geometric design considerations for DDIs should begin at the earliest stages. ICE activities can occur during broad system planning efforts, corridor evaluations, and interchange planning and design studies. During this time, constrained locations or other design implementation issues could screen a DDI in favor of other interchange forms more adaptive to the environment. Staged ICE activities can parallel the general project development stages with evaluations increasing in levels of detail until an alternative is selected for final design. Geometric design details and evaluations should increase commensurate with the level of detail of staged ICE activities. The principles and concepts noted in this chapter can support any stage of ICE activities. 6.1.2 Optimizing for Project Context and User Type DDI configurations should be based on conducting applicable geometric performance evalua- tions on an absolute basis while comparing and assessing how that performance relates to each user. Performance evaluations are described in Section 6.7. Project context is set by land use types and dictates the associated uses for a given environment. DDI configurations should be based on each user and vehicle type, providing design configurations that optimize the quality of service for each user. Users are described in Section 6.4.5. 6.1.3 Considering Each Project Development Stage As described in Chapter 2, interchange evaluations may vary depending on the stage of the project development process with geometric performance checks for a DDI being neces- sary at each project development stage. However, the level of detail and analysis conducted at each stage will vary. Conducting planning-level geometric design checks, commensurate with planning-level traffic operations evaluations, can support early ICE activities. In general, geometric evaluations and details should correspond with ICE stages, as geometric design is advanced with increasing detail at later levels of project development. C H A P T E R 6 Geometric Design

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. Understanding DDI-specific features, such as crossover fundamentals for path alignment and speed management, results in better assessment of footprint impacts. This supports planning and design decisions when comparing interchange forms and ramp terminal intersection control. Considering design vehicles and generalized treatments to establish the alignments approaching the crossover at initial stages leads to better and optimized interchange configura- tions at later project development stages. 6.2 Principles and Objectives A DDI is fundamentally a diamond interchange with uniquely designed ramp terminal inter- sections that create crossover intersections. Ramp terminal intersection treatments for left and right turns are based on intersection design principles that are generally like other diamond interchange forms. Ramp terminal intersection planning and design should follow the same intersection design and channelization principles that consider lane configurations, design users, design vehicles, oversized/overweight trucks, speed management, path alignment, and sight distance values. In the past, it was common to design DDIs to include highly curvilinear left- and right-turn movements to and from the entrance and exit ramps at the ramp terminal intersections. In some cases, these movements were established as yield control or free-flow. These design features have sometimes been copied and applied in locations inconsistent with a new project’s context. However, in contemporary DDI designs, the land use context and user types should dictate whether an interchange should include rural or urban characteristics to appropriately serve each anticipated user. As with any interchange form, there are design considerations and configuration trade-offs to develop interchange forms and features consistent with the project context. Performance-based evaluations support flexible design applications to meet project needs for a given location and project condition. Contemporary roadway geometric planning and design considers and appropriately integrates nonmotorized and motorized users. This is also true for DDIs. Vulnerable users and persons with vision or mobility disabilities require special design considerations including providing space and guidance for nonmotorized users and appropriate features for pedestrians with vision or mobility disabilities. Similarly, the full range of motorized users should be integrated into design configurations, as anticipated for a given interchange location. Information about designing for each user and vehicle type is presented in subsequent sections, and additional detail is presented in Chapter 3. Exhibit 6-1 presents prominent features of the DDI. 6.2.1 DDI Planning and Design User Considerations A DDI should be customized to serve the range of intended users at a given location. DDI configurations should be based first on specifically considering each user and vehicle type and providing design configurations that optimize the quality of service for each user. Vulnerable users and persons with vision or mobility disabilities require special design considerations, and these users should be considered and integrated at the earliest planning and design stages. Providing space and guidance for nonmotorized users and appropriate features for pedestrians with vision or mobility disabilities affects design details for other users and should be included as integral to the DDI configuration versus being added later in design.

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. Oversized/overweight trucks may dictate specific design configurations, while overall higher speeds may be applicable in rural or undeveloped environments. DDI design, refinement, and evaluations is a performance- based process based on first identifying the range of users and user types and then conducting applicable geometric performance evaluations—comparing and assessing how that performance relates to each user. 6.2.1.1 Pedestrians Pedestrians have a range of abilities such as walking speed, stamina, and judgment of vehicular approach speed and distance. Young and older pedestrians may have more difficulty judging vehicle speed and crossing distances. This can affect ramp terminal intersection design and traffic control. As a high-capacity diamond form (akin to tight or single point diamonds), DDIs are commonly used in high-traffic volume environments. Whether in low- or high-volume locations, DDI design features should specifically optimize pedestrian quality of service (i.e., minimize crossing distances and reduce motor vehicle speeds) if pedestrian facilities are expected at the opening year or in the future. DDIs can include pedestrian facilities and navigational needs that are different than those for other diamond interchange forms. There should be special consideration in helping individuals with vision disabilities and other special users navigate the features leading to and through a DDI. This can include wayfinding through the interchange, locating crossing loca- tions, aligning with the crosswalk direction, and directional guidance on islands. Principles from NCHRP Research Report 834: Crossing Solutions at Roundabouts and Channelized Turn Lanes for Pedestrians with Vision Disabilities: A Guidebook are applicable to DDIs at channelized right- and left-turn locations (1). 6.2.1.2 Bicyclists Bicyclists have a range of abilities and comfort levels. DDI crossovers transpose the roadway such that a bicycle lane on the outside approaching the crossover becomes on the inside Exhibit 6-1. Prominent DDI features.

124 Diverging Diamond Interchange Informational Guide between crossovers. This could put the bicyclist between the travel lane and raised barrier in some DDI configurations. This could affect the quality of experience, and the needs of various users should be integrated into early DDI planning. Similar to considering pedestrians, design features should specifically optimize bicyclists’ quality of service. 6.2.1.3 Buses Bus (school and transit) accommodation is not expected to have significant influences on DDI planning and design. Buses are variable in size (i.e., length overall, wheelbase, turning radius, and tail swing). The variability of school and transit vehicles in practice may lead to swept paths and tracking needs that differ from those for standard vehicles in vehicle path modeling software. Transit stops at DDIs may be affected by left- and right-turning movement design and proximity to the crossover compared to conventional diamond forms. There may be special considerations of near- and far-side stop locations and how placement and form (e.g., curbside or pull outs) could influence or be influenced by pedestrian and bicycle facility design in the DDI. Additional considerations are presented in Chapter 3. 6.2.1.4 Passenger Cars Like other users, automobile drivers have a range of skill and confidence levels. As with any interchange form, there should be special consideration of human factors including the needs of older drivers. Fundamental principles for interchange and intersection design apply to DDIs. These topics include entry view angles, navigation and signing, and other considerations such as increased complexity of DDIs associated with curvilinear roadways and other horizontal alignment transition needs for approaching and departing the DDI. 6.2.1.5 Trucks Interchanges commonly serve large numbers of trucks, and the range of vehicle types can vary significantly. DDIs have special truck service needs associated with vehicle tracking and the impacts of speeds associated with the crossover design. There could be special implications of speed management for approaching a DDI to transition drivers from higher speed approaches to relatively lower speeds through the crossovers. The design configuration established has implica- tions and resulting impacts to operating speeds, safety performance, and other user needs. Standard trucks are those that are typically allowed access to the roadway system without special permits. Standard trucks could include single-unit vehicles or other tractor trailer units that could be as large as WB-67 or similar. Oversized/overweight trucks are those that typically require permits for travel on the roadway system. Planners and designers should identify the number of expected vehicles by volume and type and consider the trade-off considerations in serving these vehicles. Special attention should be placed on accessing the specific vehicle specifications to accurately model the turning paths through the curvilinear roadway alignments. DDI interchanges do not typically allow through off-ramp to on-ramp movements for directing an over-height vehicle to bypass an undercrossing. This would be a planning and design consideration unique to DDIs that should be addressed in ICE activities. 6.2.2 Project Type The two following project types should be considered for DDIs, and within each type of project, the project land use context may impact the limitations and design constraints. 1. New facilities 2. Reconstructing existing interchanges

Geometric Design 125 New facilities often have the most opportunity to include geometric design values and dimen- sions 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. Overall project value and benefit (versus “cost”) are common evaluation considerations. There can be more flexibility in better serving each anticipated design user through appropriate features and design choices when constructing new facilities. Reconstructing existing interchanges increases the difficulty in DDI planning and design. In rural or suburban areas with limited adjacent land development, there can be more flexibility in locating the crossover and ramp terminal intersections to match existing entrance and exit ramp alignments. The relative lack of constraints could provide more flexibility to develop configurations that use more of the existing roadways and bridges. However, in constrained locations (commonly, developed urban conditions) with adjacent public and private accesses and developed land or other constraints in proximity, there could be any number of design trade-offs to optimize a DDI at that location. In some cases, the physical chal- lenges at the DDI to attain proper ramp terminal intersections and roadway approaches may mean the DDI is not a preferred solution at that location. 6.2.3 Project Context Land use context influences the intended project outcomes and associated project perfor- mance. For example, a DDI in an urban location may necessitate slower design configurations and emphasize pedestrians and bicyclists, while a rural highway application may emphasize oversized/overweight trucks. Reconstructing an existing interchange to a DDI may create significant challenges to opti- mize a configuration at a given location. In these locations, a less than ideal DDI could lead to increased safety and operational performance over a “no-build” alternative or other inter- change forms. For rural locations, right-of-way (ROW) is likely less constrained by adjacent land uses. Compared to urban locations, the presence of pedestrians and/or bicycles could be lower, and the roadway speeds approaching the crossover may be relatively higher. The turning move- ments at the ramp terminal intersections may more commonly be free or yield controlled. Finally, the design objectives for any interchange can be different and depend upon the surrounding environment and project context. At urban locations, ROW footprint, access management in the vicinity of the interchange, and pedestrian and bicycle considerations will likely influence design configurations. Fundamental design principles and configurations that support target performance apply to DDIs as they would to any other interchange form. Exhibit 6-2 summarizes potentially differing considerations between urban and rural locations. The descriptors are generalized and not rigid. Land use context is not binary, and DDI planning and design should consider the potential continuum of land use context and customize the design features based on serving anticipated users. 6.2.4 DDI Performance Considerations DDI design and refinement is an iterative process. In addition to the information shared in this section, the configuration may be influenced by efforts to attain operational performance targets. DDIs have unique attributes compared to other diamond forms. Evaluating and assessing DDI performance includes specific performance considerations of the unique features. These are described in more detail in Section 6.7.

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 align- ments, commonly tangents or flat curves. Because of this relatively simple alignment, the emphasis is on the ramp terminal intersection details. A DDI requires a change in mindset in order to recognize the unique features that impact the design approach, such as: • Cross street horizontal geometry, • Alignment elements (tangents and curves), and • Surrounding environments and project context. The DDI cross street horizontal geometry has two (sometimes unique and different) horizontal alignments for each direction. Attaining DDI design configurations that meet target operations and safety performance often results in greater distances between ramps/crossovers compared to single point and tight diamond forms. There can be a tendency among practitioners to try and adapt a DDI to a location by compromising the horizontal alignment features. There must be special consideration of DDI alignment elements (tangents and curves) that create a smooth transition entering, traveling through, and exiting the DDI. Horizontal alignment fundamentals of avoiding back-to-back reverse curves and considering curve radii that reflect desired speed transitions to and from the interchange continue to apply to DDIs. 6.2.5 Crossover and Ramp Terminal Intersection Definitions Interchange ramps have two “terminals”: the entrance and exit ramp terminal on the highway and the ramp terminal intersections on the crossroad. A DDI ramp terminal intersection is distinct from other forms in that attaining the contraflow requires a “crossover.” This is separate from the ramp terminal intersection function of serving left- and right-turning vehicles to Urban and Rural Considerations Design Considerations Urban Rural Crossroad Speed Low to moderate 25 mph to 35 mph Moderate to high 35 mph to 55 mph Crossover Speed 20 mph to 30 mph 30 mph to 35 mph Design Vehicle Lower percentage of total traffic. Limited oversized/overweight. Higher percentage of total traffic. Some oversized/overweight. Pedestrians and Bicycles High likelihood for each user. Could influence traffic control. Fewer of each user. Less influence on traffic control. Crossover and Ramp Terminal Intersection Design Consolidated with smaller radii. Lower speed features. Less right-turn-on-red. Consolidated with larger radii. More opportunities for yield or right-turn-on-red. Traffic Volume High volumes. Multiple peaks. Omni-directional patterns. Moderate volumes. Fewer peaks. Directional patterns. Exhibit 6-2. Urban and rural considerations.

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. For the purpose of this chapter, the DDI ramp terminal intersection describes the features separately. The DDI crossover is the intersection proper that supports the crossroad contraflow configu- ration that distinguishes the DDI from other diamond forms. The ramp terminal intersection represents the intersection features that support turning movements to and from the exit or entrance ramp proper. As with any intersection, DDI ramp terminal intersections (left- and right-turning) and crossover design should be as consolidated as possible to avoid a series of isolated turning and crossing locations. Exhibit 6-3 displays the key features of the ramp, the crossover, and the ramp terminal intersection. DDI crossover and ramp terminal intersection design follows the principles of intersection design to keep the intersection compact while separating conflicting movements. A DDI differs from other diamond forms in that the crossover location and design set the foundation for the ramp terminal intersection configuration. Contrary to some oversimplified characteriza- tions, a DDI is not just “two, two-phased signals.” Optimizing traffic flow at a DDI is directly affected by the geometric attributes of the crossover and ramp terminal intersection designs. In constrained DDI locations or DDIs with a skew between the highway and cross street, it can be challenging to keep the ramp terminal intersection elements (left and right turns to and from the cross street) consolidated with the crossover location. Spread intersections result in ineffi- cient signal timing and increased signing to address the various movements. To the extent possible, the DDI should result in ramp terminal treatments in the vicinity of the crossover proper. Exhibit 6-4 depicts the concept of consolidating the ramp terminal intersection elements. Exhibit 6-3. Key DDI features: ramp, crossover and ramp terminal intersections.

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. The following sections provide guidance and consideration for the following items: • Overall footprint, • Indirect impacts, • Adapting to site constraints, • Constraints at the crossover and ramp terminal intersections, • Matching to an existing highway crossing, and • Existing ramp locations. 6.3.1 Overall Footprint Project constraints influence most projects. A common metric in conducting ICE is the rela- tive footprint between alternatives. A DDI will generally occupy a larger footprint than other high-capacity diamond forms (single point and tight diamond). Like a roundabout where capacity is added at the intersection allowing for narrower roadways, a DDI crossover creates a high-capacity configuration that can reduce the overall number of lanes. Like roundabout ramp terminal intersections, this creates the opportunity to retain existing overcrossing bridges (in retrofits) or less overall construction than other interchange forms. The design features that may increase the overall footprint of a DDI are: • Crossover proper, • Ramp terminal intersection/crossover spacing widths, and • Curvature required to transition to and from the crossover. However, DDI crossovers require special attention, and the transition to, through, and departing them requires thoughtful horizontal geometry to attain desired absolute speeds Exhibit 6-4. Consolidating the ramp terminal intersection.

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 inter- section. Avoiding horizontal curves on cross street overpasses or underpasses means the cross- overs will be spaced farther apart than in single point or tight diamond ramps, contributing to a larger footprint. The horizontal geometry of the DDI leads to ramp/crossover spacing widths that are typi- cally greater than single point and tight diamond forms. In total, the overall cross-sectional and longitudinal impact of the DDI can lead to larger than expected footprints compared to other high-capacity diamond interchanges. Exhibit 6-5 conceptually presents three high-capacity diamond interchange forms (tight, single point, and DDI), allowing for a comparison of the generalized forms. 6.3.2 Indirect Impacts As with other interchange forms, the constraints can be influenced beyond the physical footprint. This might include how access management needs associated with the crossover and ramp terminal intersections affect or are affected by adjacent access. A DDI crossover and ramp terminal intersection could be deemed “too close” to an adjacent public or private drive- way access. A DDI may have more “capacity” than the first signalized intersections adjacent to the interchange. This means the DDI could potentially serve more interchanging traffic than a downstream intersection might serve, resulting in queuing beyond adjacent accesses or even back to the DDI. Exhibit 6-5. High-capacity diamond interchange forms.

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. Under- standing and considering constraints begins in early concept development and evaluation. If a DDI has been determined to be a feasible alternative in early evaluations, there should be few surprises as subsequent preliminary engineering activities identify ROW and environmental permitting requirements. ROW impacts are a common constraint with DDIs in relation to developing appropriate cross- over design and the roadway approaches to the crossovers. ROW can be affected by the location of the crossover and by the associated width of the horizontal geometry of the crossover proper. ROW footprints can be longitudinal and narrow or more compact and wider. Longitudinal ROW impacts might be associated with narrower ROW and associated curvature needed for appropriate transitions to and from the crossover. Wider ROW on roadway approaches can simplify transi- tions to and from the crossover and reduce the longitudinal distance of the affected area. Site constraints can include: • Constraints at the crossover and ramp terminal intersections, • Matching to an existing highway crossing, and • Existing ramp locations. 6.3.4 Constraints at the Crossover and Ramp Terminal Intersections A common project type for DDIs is reconstructing an existing interchange. Constraints at these locations play a key role in determining the horizontal layout of the DDI to minimize cost and other impacts. Available ROW and other constraints may preclude certain DDI alignments. Even in new construction situations, adjacent constraints may influence the crossover and ramp terminal intersection location, and that location and transition to and from the crossover may influence the overcrossing design. Exhibit 6-6 depicts how constraints at crossover and ramp terminal intersection can influence planning and design decisions. 6.3.5 Matching to an Existing Highway Crossing When reconstructing an existing interchange, the highway bridge (overcrossing or under- crossing) is a fundamental constraint. This can be related to matching the DDI crossover and Exhibit 6-6. Constraints at crossover and ramp terminal intersections.

Geometric Design 131 ramp terminal intersection elements or to integrating the existing overcrossing during construc- tion while building new bridges. Integrating an existing highway overcrossing creates a fixed location to which crossover planning and design must match. An overcrossing and undercrossing have different effects on how pedestrians and bicyclists are served, and those constraints could influence planning and design decisions. Because the crossover is a special consideration in DDI configurations, locating the crossover is a key consideration of matching to an existing highway crossing. Later sections of this chapter will present a variety of factors that influence where and how to locate the crossover. Exhibit 6-7 depicts how matching to an existing highway crossing can influence where the crossover and ramp terminal intersection are located. 6.3.5.1 Matching to an Existing Undercrossing In a location where a crossroad passes under an existing highway, one of the most critical considerations is to assess the available space between columns, walls, or other constraints to verify if there is adequate space to serve motorized and nonmotorized users. Clear span bridges (with no center columns) increase the flexibility of developing the road- ways and pedestrian paths compared to multispan bridges that provide separate portals in which to pass the roadways and pedestrian facilities. Exhibit 6-8 depicts a cross section of a single-span bridge and contraflow traffic patterns at the highway crossing. Two-span configurations (with center columns) must be able to provide space to develop the roadway median but may not have enough space to include a pedestrian walkway because of the center columns. In these cases, pedestrian walkways will need to be on the outside of each roadway. Bridge configurations without existing sidewalks or sufficient room for the road- way and the pedestrian walkways may require cutting back, retaining the sloped abutment fill between the roadways and the approach bent and relocating the pedestrian walkways to this recovered space. Exhibit 6-9 depicts the cross section of two-span configurations. Exhibit 6-10 shows a wall built to retain a sloped abutment fill and install a sidewalk at a DDI in Maryland Heights, Missouri. Exhibit 6-7. Matching the existing highway crossing. Exhibit 6-8. Single-span bridge.

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. Exhibit 6-11 depicts a four-span bridge and how the sloped abutment fill might be modified to serve pedestrians and bicyclists. Matching to an existing undercrossing typically means the crossover and the ramp terminal intersections must be located a sufficient distance away from the existing bridge constraints to be able to match the available cross section width under the highway. Attaining acceptable crossover and associated ramp terminal intersection designs guide where those intersections will be located. Locating the crossover and ramp terminal intersection could influence the alignment of the ramp proper. Exhibit 6-12 depicts how realigning a ramp to achieve a desired crossover and ramp terminal intersection could require relocating the highway exit ramp terminal to attain an appropriate ramp configuration. Exhibit 6-9. Two-span bridge configurations with modification for multiuse path. Exhibit 6-10. Example vertical wall and sidewalk (Maryland Heights, Missouri) (2).

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. If there is insufficient bridge width, the existing bridge may either be widened, if the bridge type allows, or an adjacent bridge might be constructed next to the existing bridge. Depending on the location, a new bridge might be constructed on either side of the existing bridge. One benefit of this approach is that traffic will be maintained on the existing bridge while the new bridge is constructed. Exhibit 6-13 depicts various methods in which bridges at an existing highway crossing might be modified to develop the DDI. An existing bridge could be widened or a separate bridge constructed with a wider separation between bridges. This same approach might be used to stage construction even if the existing bridge is to be replaced. Another option is to maintain the existing bridge while new bridges are constructed on either side of the existing. This approach allows much of the DDI to be constructed on the “outside” while maintaining traffic on the existing roadway on the “inside.” Another advantage of this approach is that the new bridges create a relatively wide median. As presented in the section on crossover width, the wider median creates the opportunity to use fewer horizontal curves to attain appropriate crossover geometry and could allow shorter distances between DDI crossovers and a reduced footprint. Exhibit 6-14 depicts various ways in which bridges at an existing highway crossing might be modified to develop the DDI. An existing bridge could be replaced by symmetrical widening to two separate bridges, or the widening could occur by constructing a new bridge north or south of the existing bridge. Exhibit 6-11. Four-span bridge configuration with modification for multiuse path. Exhibit 6-12. Realigning the ramp.

134 Diverging Diamond Interchange Informational Guide Exhibit 6-13. Options for bridges at existing highway crossings. Exhibit 6-14. Options for bridges at existing highway crossings. Exhibit 6-15 provides an example of a DDI design at Pioneer Crossing in American Fork, Utah. The general alignment of the original overcrossing was maintained, and a new alignment shifted south was used to take advantage of the available ROW. The DDI constructed on the Glenn Highway at Muldoon Road outside Anchorage, Alaska, applied the symmetrical widening technique and constructed new bridges on either side of the existing bridge. The wide median simplified the crossover design by reducing the crossover spacing. This also allowed large portions of the roadway approaches to be constructed while maintaining traffic on the existing bridge and cross street. Exhibit 6-16 depicts this concept.

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. Ramp geometry and footprint dictate how much opportunity there is to adapt to possible crossover locations and the ramp terminal intersection configuration. Depending on the ramp locations, adjacent constraints, and the desire to integrate an existing highway overcrossing, it may not be possible to create a DDI configuration that meets target operational and design performance without modifying the ramp. This could potentially result in significant ramp modifications that lead to moving the entrance or exit ramp terminal on the highway. Shifting a highway ramp terminal could increase the extents and magnitude of construction. This could lead to screening the DDI for other interchange forms. 6.4 Horizontal Alignment The geometric design of a DDI requires balancing competing objectives. Most geometric parameters are governed by the design vehicle requirements, speed control needs, and other per- formance objectives. Therefore, designing a DDI requires carefully considering safety, operations, Exhibit 6-15. Shifted alignment south of centerline (red) (Pioneer Crossing, American Fork, Utah) (3). Exhibit 6-16. Symmetrical widening technique (Anchorage, Alaska) (4).

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. In reconstruction projects, there may be an increased focus on optimizing the design configuration and considering trade-offs for a given project location and context. The design objectives for any interchange can be different and depend upon the surrounding environment and project land use context. At urban locations, ROW footprint, access manage- ment in the vicinity of the interchange, and pedestrian and bicycle considerations will likely influence many of the project design decisions. This includes managing speeds by reducing horizontal curve and turn radii, and operating vehicle turning movements through signal control versus free or yield movements. 6.4.1 Alignment Fundamentals A DDI is simply a diamond interchange form with special geometric attributes that use a crossover to create contraflow conditions in the interior of the interchange. DDI horizontal alignment follows fundamental design principles that create geometric design configurations that support the target user operations. The following alignment fundamentals should be considered when beginning the geometric design of a DDI: • Horizontal curve radii—Horizontal curve radii should be commensurate to the anticipated or target speeds. This means curves approaching the DDI should account for the potential higher crossroad speeds compared to relatively slower speeds between the crossovers. It is not necessary to maintain the design speed of the crossroad through the crossover intersections. • Tangents between reverse curves—From a functional level, tangents between reverse curves allow for appropriate superelevation transitions from one curve to the next. From a motorist perspective, tangents between curves allow drivers to read and perceive their navi- gation and track tasks from one geometric element to the next. As in any other horizontal alignment, tangents should be used between reverse curves unless no other option is available. • Tangents approaching and through the DDI—Tangents approaching and through DDI crossovers allow the roadway geometry to be the primary guide for motorists to track to the receiving lane. A self-describing roadway reduces driver navigation workload and errors. At a DDI crossover, positive guidance can help reduce the potential for drivers to make wrong-way movements in the contraflow section. Some early DDIs in the United States had crossovers with back-to-back reverse curves. To the extent possible, this practice should be avoided. Crossover designs without tangents or with short tangents can create conditions where vehicles at the stop bar are not aimed to their target receiving lane across the crossover. This is like “path overlap” in multilane roundabout design, in which an entry lane does not align with the proper receiving lane in the roundabout. Lane markings and lane extension lines work complementary to target geometry and are a mitigation, but not a replacement, for tangent sections that support crossover navigation. 6.4.2 Developing a DDI Layout New construction or a site with limited constraints will create more design flexibility to optimize the configuration to meet user needs. This might include using symmetrical crossovers and crossroad alignments and a wider median on the cross street approaching the interchange area, although it is not necessary for a DDI to be symmetrical. A wider median allows for a shorter distance between crossovers. A DDI being considered as part of reconstructing an existing interchange will create different opportunities and challenges compared to designing a new DDI interchange. Reconstructing

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. Even in these locations, a less than ideal DDI could lead to increased safety and operational performance over a no-build alternative or other interchange forms. 6.4.3 Effect of Skew on Crossovers and Ramp Terminal Intersections Skew between the crossroad and highway affects tight, single point, and diverging diamond interchanges. Tight and single point forms are founded on a narrow footprint with ramps tucked in close to the highway mainline. Skew and associated turning vehicle turning paths affect the operational effectiveness of the relatively tight ramp spacing. However, the DDI has wider ramp spacing, and the skew can result in separating the crossover location from the left- and right-turning movements at the ramp terminal intersections. This can result in the intersection elements being spread out to individual elements and degrading intended traffic operations. This effect should be minimized as much as is possible to keep the ramp terminal intersections and crossover consolidated. In new or reconstruction proj- ects, reducing or eliminating skew simplifies the crossover and ramp terminal intersection configuration. 6.4.3.1 Types of Skew There are two kinds of skew between the highway and the cross street. Clockwise skew is where the highway is angled clockwise from the cross street. This skew affects all ramps and intersections and creates noteworthy operational effects at the exit ramps. Counterclockwise skew is where the highway is angled counterclockwise to the cross street. This skew also affects all ramps and intersections and creates noteworthy operational effects at the entrance ramps. Exhibit 6-17 presents the concepts of clockwise and counterclockwise skew. Clockwise skew creates operational effects on the exit ramp by: • Reducing the left-turn minimum radii and • Decreasing the right-turn viewing angle. Exhibit 6-17. Clockwise and counterclockwise skew concepts.

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). 6.4.3.2 Addressing Skew at Exit Ramps To address exit ramp operational effects associated with clockwise skew, the ramp terminal intersection could be realigned to a more perpendicular configuration. This helps facilitate left turns and increases the right-turn view angle. Realigning the ramp terminal intersection consolidates the crossover and ramp terminal intersection features and could increase the footprint. Exhibit 6-18 shows how reducing the effects of skew can require modifying the ramp. Attaining target ramp terminal intersection geometry and developing appropriate queue storage and deceleration to the back of the queue could increase the exit ramp length and affect the exit ramp terminal location on the highway. If the intent is to match an existing exit ramp, attaining target ramp terminal intersection geometry and queue storage could require recon- structing some or all of the existing exit ramp. Practitioners will need to assess trade-offs of maintaining the existing ramp location and attaining target ramp terminal intersection geometry. In some cases, reconstructing the ramp could be a “fatal flaw,” and the DDI is screened in favor of other interchange forms. Exhibit 6-19 shows how reducing the effects of skew could require lengthening the ramp. 6.4.3.3 Addressing Skew at Entrance Ramps Counterclockwise skew creates operational effects to the entrance ramp that are unique compared to those for an exit ramp. At an exit ramp, drivers slow their vehicles for their turns or to come to a stop. At an entrance ramp, drivers focus on leaving the crossroad to access the higher type roadway. Opposite of exit ramps, where drivers expect to slow or stop at ramp Exhibit 6-18. Modifying the ramp to reduce skew.

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) and obtuse (greater than 90 degrees) angles for turning traffic. At an entrance ramp, the minimum left-turn radius issues are the same as at an exit ramp: they create a turn radius that facilitates design vehicle move- ments. Because of the reduced radii, these movements can be some of the slowest movements at an intersection. This slow movement at an entrance ramp is where drivers wish to accelerate. The obtuse angle for right turns to an entrance ramp creates conditions that promote higher right-turn speeds when compared to 90-degree intersections. The propensity for higher speed is amplified by driver expectation to accelerate to the higher order facility. In combination, the slower left-turning and faster right-turning create speed shear between these two movements where they converge on the entrance ramp. It is not uncommon for designers to match the forward bearing of the left- and right-turning movements to create the alignment of the entrance ramp. The small left-turn radius creates a high convergence angle, meaning that the vehicles are aimed at each other with little separation as the paths converge to the ramp alignment. Off-setting those converging curves by a minimum of 3 to 5 feet allows the two converging movements to attain the same forward bearing before tapering to merge or lane addition. Exhibit 6-20 shows how separating converging ramps can reduce the speed shear and convergence angle. The minimum radius of the off- or on-ramp left turn will often be the controlling factor for the impacts to the approach alignments. Assessing the controlling minimum radii to serve design vehicles should be an early step in evaluating a DDI with a skew. 6.4.3.4 Skew and Route Continuity Counterclockwise skew increases the propensity for a violation of cross street route continuity (turning left or right to stay on your designated route). While this can occur at nonskewed DDIs, Exhibit 6-19. Lengthening the ramp to counter skew.

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) to stay on their designated route. To counter this effect, initiating the right-turn lane in advance of the crossover makes the accessing of the right-turn movement a deliberate deviation from the through route on the cross street. Exhibit 6-21 shows how definitively adding the right-turn lane can maintain route continuity on the crossroad. 6.4.3.5 Examples of Skew The DDI at Pioneer Crossing in American Fork, Utah, has a counterclockwise skew and includes some of the previously described characteristics: the left turn (with a relatively small radius) to the entrance ramp, the right turn on a tangent well beyond the right turn, a high convergence angle, and an upstream alignment of the right turn that remains essentially on the same bearing as the original cross street. The net result is that the through route curves away from the right-turn lane that traps to the entrance ramp. These features are presented to demonstrate the described geometric attributes and are not presented as a critique of the Pioneer Crossing DDI. Exhibit 6-22 shows the effect of skew at the Pioneer Crossing interchange. 6.4.4 Lane Numbers and Arrangements Determining lane numbers and arrangement can be an iterative approach. During early interchange form assessments, such as early screening as part of intersection control evaluations, planning-level reviews might be used to screen and advance various forms for more detailed evaluations. Exhibit 6-20. Separating converging ramps. Exhibit 6-21. Maintaining route continuity.

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 alterna- tives 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. Early traffic operations considerations should include assessing if left and right turns are controlled (stop, yield, or signalized) or free-flow movements. This could include replacing an exit ramp single free-flow right turn with signalized, dual right turns to promote pedestrian crossings or to reduce downstream weaving and traffic operations impacts. Early implemented DDIs often included free-flow or highly curvilinear yield control left or right turns that did not fully account for yielding driver view angles and sight lines to upstream traffic. If free-flow move- ments are not appropriate for a site or view angles and sight lines cannot be attained—requiring changing the ramp terminal intersection configuration—the traffic operations must be revised to match the proposed lane numbers and arrangements and geometrics. Geometric configurations and traffic operations testing is an iterative process. Assumed traffic operations should guide corresponding ramp terminal intersection geometry, and later revisions of attainable geometrics may necessitate revised traffic operations. For example, if free right turns or right-turn-on-red (RTOR) was assumed and the appropriate geometry for either cannot be attained, the traffic operations evaluations should be revisited to ensure target traffic opera- tions performance is attainable. Lane numbers must not always be symmetric and can vary by direction as shown in Exhibit 6-23. 6.4.5 Design User and Type Motorized and nonmotorized users must be evaluated in the earliest concept develop- ment. Design vehicle swept paths will affect lane widths and traffic island location and shape. Pedestrian and bicycle facilities must be considered early and as a key influence on the DDI configuration. It is not uncommon to develop potential interchange concepts early in alter- natives development and evaluations; the concepts are often being generated quickly and with the pretense of not expending too many resources. At first glance, a practitioner may believe the concept is viable, but when design users and type are eventually integrated, the shortcomings of the original concept become apparent. Exhibit 6-24 shows a horizontal functional plan that was generated quickly without considering pedestrian treatments and the influence of truck tracking. Consideration of trucks reduces Exhibit 6-22. Effects of skew at Pioneer Crossing interchange (American Fork, Utah) (5).

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. The changes to the median treatment affect pedestrian treatments that consider design vehicles. The revised crossing creates skewed pedestrian crossings, challenges in integrating detectable warning strips, and greatly decreased positive guidance for users with low vision. Exhibit 6-25 shows an example of highly skewed pedestrian crossings that create a challenging experience for users with low vision at I-85 and Pleasant Hill Road in Duluth, Georgia. Exhibit 6-23. Example asymmetric lane numbers (Maryland Heights, Missouri) (5). Exhibit 6-24. Example median treatments and pedestrian treatments.

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. The median width and length support pedestrian crossings by including easy-to-locate push buttons, detectable warning strips at each crosswalk approach, and shorter, 90-degree crossings. These crossings help users with low vision to find the receiving ramp. These examples emphasize how critical it is to incorporate design users in the early project concepts. Attempting to add features later for these users could result in less effective solutions than had they been considered from the start. Exhibit 6-25. Example of highly skewed pedestrian crossing (Duluth, Georgia) (5). Exhibit 6-26. Example of pedestrian features that create difficulties for users with low vision (Duluth, Georgia) (6).

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. Selecting the design vehicle and determining the corresponding swept paths using turning templates or a computer assisted design and drafting (CADD) -based vehicle turning path program will establish lane widths and chan- nelization configurations. Four key areas of the DDI are directly affected by the design vehicle: 1) through movements at the crossover, 2) left turns at the exit ramp, 3) left turns at the entrance ramp, and 4) right turns at the exit ramp. The first three areas are unique to the DDI with respect to interchange design; however, they build on concepts used for designing other street facilities such as roundabout and one-way street designs. The fourth area, right-turning vehicle paths, is not unique to DDIs. The left turn from the crossroad to the entrance ramp is unique in a DDI. While other right- turning movements occur in other intersection forms, the same movement in a DDI does not cross an opposing lane. This makes the left-turn movement hug the left edge of traveled way and truck drivers sometimes swing wide toward the channelizing island to allow tracking as can sometimes happen with traditional right turns. Designers may consider adding additional buffer beyond the traditional vehicle swept path to account for driver error with this relatively new maneuver. The separation of turn lanes with a vane island can allow trucks to complete turning move- ments without encroaching into adjacent lanes, as shown in Exhibit 6-28. Lane separation is typically provided to prevent off-tracking by heavy vehicles in turns, and the additional pave- ment creates the potential for emergency vehicles to pass other vehicles. The choice of design vehicle will vary depending on the crossroad facility type and surrounding environment. Most often, state or local agencies and project stakeholders help determine the appropriate design vehicle for each site. The most common design vehicle used at DDI facilities is the WB-67. In more urbanized areas, it may be more appropriate to use smaller design vehicles, such as the WB-62 or WB-40. At a minimum, fire trucks, transit vehicles, and single- unit delivery trucks should be considered in urban areas. In rural areas, farming or other larger vehicle types may govern design vehicle needs. The need for design vehicles to travel side- by-side should be considered on a case-by-case basis and given higher priority as heavy vehicle volumes increase. Exhibit 6-27. Example of quality pedestrian treatments (Minnesota) (6).

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. These larger vehicles would need to be accounted for in the design of certain movements, with the balance of the movements designed to serve a smaller design vehicle. Oversized vehicles may need to be considered at a DDI just like at any other interchange form. These vehicles often require a special permit to travel on the street. However, if they are expected to use the DDI, special consideration should be given to geometrics; signal height, placement, and installation; and most importantly, to the structural soundness of the facility. Dimensions for special vehicles can be established by working with local haulers or the industry or agricultural enterprises served by those vehicles. CADD-based turning path programs allow the designer to customize the electronic template for these vehicles. An example of an over- sized load making a left turn onto I-44 from MO-13 in Springfield, Missouri, is provided in Exhibit 6-29. 6.4.6 Crossroad Alignment Design The crossroad alignment considerations for DDIs are generally the same as those for other diamond forms. However, the impacts of a DDI to the crossroad alignment include transi- tioning vehicle speeds prior to the interchange to prepare for the lower speeds at the DDI Exhibit 6-28. Vane island lane separation for heavy vehicle accommodation (3). Exhibit 6-29. An oversized load making a left turn onto I-44 at MO-13 (Springfield, Missouri) (7).

146 Diverging Diamond Interchange Informational Guide and crossovers. There is an integral relationship between crossroad alignment and transition to and through DDI crossovers. DDI crossover-specific design considerations are presented in subsequent sections. Unlike other interchange forms that have tangent crossroad alignments, a DDI crossroad alignment must be customized for travel direction to create a horizontal alignment that provides the transition to and from the crossover proper. This is similar to considerations of inter- changes that use roundabout ramp terminal intersections in which the crossroad alignment must be created to explicitly transition to the roundabout approaches and entries. Like at roundabout ramp terminal intersections, the DDI crossroad alignment may need to be specifically tailored to attain desired performance at the crossover proper. In addition, to appropriately transition to the crossover proper, crossroad alignments must include specific horizontal geometry to match the existing typical section while transitioning to and from the DDI crossover. Horizontal alignments supporting crossover design should be developed to consider the crossroad approach speed; this means considering horizontal curve radii commensurate to anticipated operating speeds and curve radii configurations in balance with the desired speed profile approaching and navigating the interchange. If initial curves are to be used in advance of the crossover, the curves should have radii that support the speed transition from the higher speed approaching the interchange to the lower speed between crossovers. This means the first curve that a driver navigates in advance of the crossover (higher relative speed) would be at least as large as or larger than the curvature leading to the crossover. Smaller curve radii may be used on the crossroad between the crossovers (lower relative speed). The transition curvature into the DDI reduces operating speeds compared to oper- ating speeds from the crossroad approaching the DDI and smaller radii are consistent with operating speeds. Exhibit 6-30 presents a conceptual speed profile approaching a DDI and the change in anticipated operating speed relationship associated with the crossovers. 6.4.7 Crossover Design DDI crossovers create the crossroad contraflow through-movement operation over or under the highway. The crossover serves crossroad through movements while left and right turns to Exhibit 6-30. Conceptual speed profile.

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. Consolidating intersection functions and features combines signing messaging to support navi- gation tasks and meets driver expectations by eliminating random, isolated turning movements. 6.4.7.1 Crossover Curvature Crossover target speeds typically range from 20 mph to 35 mph and are influenced by hori- zontal curves with commensurate radii (100 feet to 400 feet). Slower crossover speeds allow designs to minimize the spacing between crossovers. Field observations at five DDI sites documented average free-flow speeds through the crossovers for inbound and outbound movements ranging from 22.3 to 31.1 mph (8). This corresponds to curves with radii between 180 to 350 feet. Providing curve radii values corresponding to design speeds below intended crossover operating speeds can lead to vehicles off-tracking intended travel paths and encroaching into adjacent lanes. DDI crossover intersections are unique in that they introduce curvature for a through move- ment. This curvature may not meet the design speed designation of the cross street. If this is the case, it is necessary to lower the design speed through the interchange area. This is primarily a regulatory issue and not an operational or safety issue affecting the driving public. States have not reported issues with the decreased design speed through DDIs. The curve radii approaching a crossover from the cross street (the first curve a motorist must navigate) should be at least as large as the curve radii departing the crossover (the interior curves between the crossover), so that the first curve is consistent with the transition effects into the DDI and the second curve is consistent with slower speeds in between crossovers. If different radii are used to create the crossover, the smaller radii should generally be used on the interior to reflect the lower speeds between the crossovers. In rural or other high-speed environments approaching the DDI, speed transitions are a focus. The first curve (R1) a motorist must navigate typically may require a speed transition compared to the crossroad approach speeds in rural locations. The speed transition could involve changes in the cross section including adding raised medians and curb and gutter to reinforce the change in driving environment. Exhibit 6-31 shows two horizontal configurations between the existing crossroad cross section and the crossover. The first configuration includes a first horizontal curve, and the second uses Exhibit 6-31. Horizontal configurations between existing crossroad cross section and the crossover.

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. It has been common in DDI design to use a horizontal curve on the cross street to initi- ate the crossover configuration. However, angle points are acceptable and can help reduce over- all footprint. Angle points are used at highway ramp terminal exits in high-speed environments. Highway exit ramp diverge angles typically range from 1 to 5 degrees. This corresponds to taper rates of approximately 60:1 to 12:1. Since angle points have been used successfully in high-speed environments, they may be used on the slower speed crossroad. 6.4.7.2 Crossover Spacing Crossover spacing is affected by the width between cross street roadway centerlines and the number of reverse curves needed to attain the desired crossover. As with any horizontal align- ment, tangents should be included between reverse curves. Crossover spacing (centerline to centerline) can range from 400 feet to 800 feet. The spacing range assumes 90-degree cross street crossings at the highway, and the spacing values can increase if the cross street over the highway has significant skew. The larger value is attributed to roadways with limited median widths (narrow) between cross street roadways. • Wider Distance—With a wider distance between cross street roadways, as few as four reverse curves can be used. This is because the wider distance allows a single left-hand curve on the approach to the first crossover while still providing a tangent between the subsequent right- hand curve in the contraflow section between crossovers. • Narrower Distance—A narrower distance between cross street roadways can result in up to eight reverse curves (each) approaching and departing the interchange. The first right- hand curve directs drivers to the right to set up the subsequent tangent and left-hand curve to the crossover. This is needed in narrow cross section areas to discourage wrong-way movements into the contraflow section. Exhibit 6-32 presents crossover spacing with narrower and wider medians. A narrow highway crossing and narrow roadway conceptually has the longest influence area compared to a wider highway crossing and wider crossroad median. 6.4.7.3 Crossover Angle It is desirable to provide the largest crossing angle while adapting to each site’s unique conditions. Early DDI design practice targeted crossover angles of 45 degrees or greater to facilitate proper vehicle path alignment through the crossover and reduce the likelihood of wrong- way movements. Research findings from a study of seven of the first DDIs built identified a corre- lation between lower crossover angles and wrong-way maneuvers into opposing lanes (8). Since that time, there has been less emphasis on crossover angle in DDI design—it is often difficult to achieve 45 degrees in practice—and a greater focus on a range of operational and design features that are presented in Section 6.7. This includes general and DDI-specific performance categories. Several factors influence cross angle selection: • Wrong-way maneuvers—Minimizing the likelihood of a wrong-way maneuver into opposing traffic is a key consideration in DDI design. The greater the crossing angle, the more the intersection will appear conventional and support use of the crossover to attain contraflow versus a right turn to maintain typical diamond flow. • ROW constraints—The surrounding environment will influence a DDI configuration. For instance, a retrofit design may be constrained by bridge abutments and built-out develop- ments on either side of the crossover. These constraints can make it difficult for designers to maximize crossover angles.

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. • Exposure—As with any skewed intersection, larger crossing angles decrease the amount of time that a vehicle is exposed to conflicting traffic and reduce the potential for angle collisions if drivers disobey traffic signals. • Heavy vehicles—Greater crossing angles could increase the potential for overturning unless appropriate horizontal curve radii can be provided to manage lateral acceleration. Minimizing speed reduction differences between successive geometric elements can mitigate this. DDI horizontal alignment should be based on the crossroad alignment approaching the DDI and the transition curves used to develop contraflow through the interchange. Smooth and consistent speed transitions will better serve all motor vehicles. Commonly used treatments to supplement the crossover angle as a means of discouraging wrong-way movement at the crossovers include signing at the gore, pavement markings, and signal heads with arrows. These treatments are summarized in more detail in Chapter 7. 6.4.7.4 Using Tangents As in any other horizontal alignment, tangents should be used between reverse curves unless no other option is available. It is ideal to have the DDI crossover proper to be two crossing tangent alignments (versus horizontal curves through the crossover). Considering each alignment separately allows the DDI to be customized to each environment. This can include adapting to different lane numbers and arrangements in each direction, accounting for various pedestrian and bicycle treatments or other site-specific conditions. Exhibit 6-32. Crossover spacing with narrower and wider medians.

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. The tangents direct the drivers at the stop bar approaching the crossover intersection toward the intended receiving lane departing the crossover intersection. Creating tangents approaching and departing the crossover inter section is reinforced by adjacent curb and gutter. In total, this creates self-describing roadway features through the intersection versus relying on striping to maintain lane discipline. Exhibit 6-33 depicts a crossover intersection and demonstrates how effectively the tangent section clarifies the intended geometry with corresponding features of curb and barrier reinforcing the alignment. Striping complements these strong geometrics. DDIs should include tangent sections if at all possible. If none are provided, curves should be of sufficient radii to match intended operating speeds. Curve radii values corresponding to design speeds below intended operating speeds can lead to vehicles off-tracking intended travel paths. If DDIs are being considered with no tangent section throughout the intersection for all lanes, other guidance such as signing, striping, and signal head placement should be considered to aid drivers to the proper lane and provide adequate visibility to the signal heads. If back- to-back reverse curves are to be used, the horizontal curve radii provided should be commen- surate with the anticipated operating speed. This means not using 100-foot radii (20 mph) if the anticipated speed profile indicates 35 mph crossover speeds (400-foot radii). 6.4.7.5 Determining Crossover Tangent Lengths Tangents at the crossover support driver navigation and path alignment. The driver lane orientation at the stop bar is typically the location where roadway lane striping ends. Lane extension stripes are then used to provide path guidance. Striping and overhead signing are supplements to horizontal geometric alignment; roadway geometrics that promote path alignment and lane discipline should be a design objective. The tangent length can be calculated by accounting for crossover angle, considering the amount of tangent in the vicinity of the stop bars approaching the crossover, and tangent length Exhibit 6-33. Tangent crossover intersection at I-580 and Moana Lane (Reno, Nevada) (9).

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. In total, these elements support developing the crossover tangent lengths. Exhibit 6-34 depicts how elements from an example DDI crossover can be used to compute the tangent length. Tangent Length A B C D= + + +“ ” “ ” “ ” “ ” where “A” = 15′ – 25′ considers the length from the point of tangent (POT) and the intersection point as shown in Exhibit 6-34. This distance considers the stop bars placed no closer than 5 feet to the edge of travel way. The 15 to 25 feet also creates the opportunity to have a vehicle at the stop bar to be aiming to the receiving lanes beyond the crossover proper. “ ” sin sin 90 Law of Sines sin sin 90 Law of Sines “ ” Pythagorean Theorem “ ” 10 , , 2 2 B W crossover angle crossover angle b W crossover angle crossover angle C W b D minimum W total width motor vehicle lanes bicycle lanes shoulders ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) = × − = × − = + = ′ = Exhibit 6-34. DDI elements and tangent length.

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) 14 14 28 sin sin 90 28 sin 45 sin 90 45 28 28 28 28 40 “ ” “ ’ “ ” “ ” 10 40 28 25 2 2 2 2 W B W crossover angle crossover angle B B b C W b C L Tangent Length A B C D Tangent Length Tangent Length = 103 ( ) ( ) ( ) ( ) = ′ + ′ = ′ = × − = ′ × − = ′ = ′ = + = + ≈ ′ = + + + = ′ + ′ + ′ + ′ ′ • Example 2: DDI with a 35 degree crossover angle and two 14-foot lanes in both directions (assumes no shoulder or bike lane) W B C 124 28 , 40 , 49 Tangent Length = = ′ = ′ ≈ ′ ′ • Example 3: DDI with a 35 degree crossover angle and three 14-foot lanes and two 4-foot shoulders in both directions W B C 194 50 , 71.4 , 87.2 Tangent Length = = ′ = ′ ≈ ′ ′ The shallower the crossover angle, the larger the tangent length becomes. This is demonstrated between Examples 1 and 2. The greater number of travel lanes and shoulder or bicycle lanes in each direction, the larger the crossover intersection becomes. This is presented in Example 3. The crossover tangent length and horizontal curves are needed to transition the roadway approaches to the crossroad alignment. The horizontal alignment needs can also influence crossover width, which could influence ROW impacts. If target lengths cannot be attained, the tangent length should be maximized within the project context and should not be less than 100 feet between back-to-back reverse curves. A method to maximize the tangent length could be to reduce the back-to-back curve radii. A small reduction in horizontal curve radii has a limited reduction in anticipated driving speeds. However, for a given set of curves with an existing tangent, simply reducing each curve radius by 50 feet results in a relatively large increase in tangent length.

Geometric Design 153 As noted in the prior section, crossover width (footprint) is affected by the cross street cross section and the median width. The wider the crossover, the more simplified the horizontal align- ment can be. In some cases of wider cross street medians, the crossover width can be developed within the roadway cross section width. In roadways with narrow medians, the crossover width may represent a bulge in the configuration between the approach roadway and the cross street (between crossovers). Larger median width negates the need for reverse curves (just the reverse curve of the crossover itself) on the closest side to the overcrossing bridge. Larger median widths typically increase the vista through crossovers as described in Section 6.7.2.5. Exhibit 6-35 presents the geometric influence of narrower and wider medians. 6.4.8 Ramp Terminal Intersection The ramp terminal intersection represents the intersection features that support turning movements to and from the exit or entrance ramp proper. The right- and left-turn lanes on DDI off-ramps tie into the crossroad to the left and right of the crossover intersection proper. While the DDI configuration is unique because of its contraflow arrangement, the ramp terminal intersection design follows intersection design principles for type of control, channelization, and adapting to the specific land use context. Each left- or right-turning movement to or from the crossroad should be configured to best serve each user and consider the vulnerability of pedestrians and bicyclists. The first DDIs in Missouri were constructed with ramp terminal radii in the range of 100 to 175 feet, and many other states followed suit with similar dimensions. The corresponding design speeds are in the range of 20 to 25 mph. In addition, those left- and right-turn movements commonly intersected at flat angles, implying a free-flow or yield condition. Those design features were copied and included in subsequent DDI configurations regardless of the land use context. In many cases, those features create challenges for pedestrians, particularly for unsignalized movements. In addition, these conditions sometimes created weaving conditions on the crossroad from right-turning vehicles or from the left turn to the downstream left-turn lane at the adjacent ramp terminal intersection serving the entrance ramp. Exhibit 6-36 compares two exit ramp terminal intersection configurations. The curvilinear example on the left represents legacy features of early DDIs. The more squared up version on the right reduces turning vehicle speed while maximizing the driver view angle to opposing upstream traffic. Exhibit 6-35. Geometric influence of narrower and wider medians.

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 appro- priate channelization that supports and reinforces intended or restricts nonintended turning movements. Intersection sight distance fundamentals (i.e., stopping and intersection sight lines) apply at each DDI movement. Other principles such as “view angle” also apply. Intersection sight lines, stopping sight distance (SSD), or view angles should be evaluated, and intersection geometrics should be configured to meet appropriate values. If those values are not attainable, then the traffic control should be adapted accordingly. For example, if a right turn from an exit ramp does not have target sight distance, this movement might not be appro- priate for a yield condition or RTOR. In this case, the geometry provided may be counter to an assumed traffic capacity need, and the DDI may not operate as intended. There are numerous intersection planning and design considerations for any interchange form that also apply to DDIs. The following are just some of the general considerations that represent fundamental features or topics that should be included in DDI planning and design: • Land use (urban or rural) and site context, • Ramp terminal control (stop, yield, signal control), • Locations and types of crosswalks and bicycle facilities, • Distance of adjacent signals on cross street, and • Design vehicle accommodation. 6.4.8.2 DDI-Specific Intersection Design Considerations DDI crossovers and the contraflow operations lead to fundamental changes in driver expecta- tions when navigating the ramp terminal intersections turning left or right from the exit ramp. The contraflow operation creates a condition in which drivers must look to the contraflow approach compared to their expected views in a traditional diamond form. Because the cross- overs create contraflow, drivers may inadvertently look at the departing traffic that has been transposed by the crossover and not see an oncoming vehicle on the other side of the crossroad. This means that there should be special attention to the approach angles at the ramp terminal intersection to maximize the ability for drivers to see conflicting traffic flows. This is shown in Exhibit 6-37. Exhibit 6-36. Comparing exit ramp terminal intersection configurations.

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. For sites with limited available ROW and capacity limitations for the left or right exit ramp terminal intersection movements, glare screens have been used to assist drivers looking down the correct approach. The angles of visibility should follow the same guidelines as conventional intersections. The intersection angle for the off-ramp right turn may be measured as the angle between the entry angle at the stop bar and the line drawn from an oncoming vehicle located at a given inter- section sight distance. Guidance for designing for older drivers and pedestrians recommends using 75 degrees as a minimum intersection angle. Intersection view angle constraints should be taken into consideration when assessing whether to allow RTOR. 6.4.9 Adjacent Driveways and Intersections DDIs have similar access management considerations compared to conventional diamonds and other service interchange forms. Raised medians automatically restrict nearby driveways to right-in/right-out. Minimizing the negative effects of adjacent access in the vicinity of the ramp terminal intersections also can lead to potential restrictions on adjacent driveways and public access points. Like other interchange ramp terminal intersections, the type of traffic control serving right turns from the exit ramps can influence adjacent access spacing and considerations. Free-flow right turns can create downstream weaving. Signal-controlled right turns generally have less capacity, but they eliminate downstream weaving concerns while creating opportunities for signalized pedestrian crossings. Minimum dimensions to driveways should be established based on local policy and guidance on a project-specific basis based upon traffic volumes, the number of lanes, ramp terminal intersection control, and local access management practices and standards. These operational conditions are like those of a conventional diamond interchange form. In new construction conditions, adjacent access spacing may trigger supplemental traffic operations analysis beyond the ramp terminal intersection analyses. These analysis results may help inform access manage- ment decisions and intersection spacing. Exhibit 6-37. Alignment of exit ramp right- and left-turn movements.

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 inter- section has on the DDI and the DDI effects on adjacent intersections. From an operational perspective, DDIs often provide higher throughput than can be provided from or served by adjacent signals. Downstream operational considerations include providing appropriate lane numbers and arrangements departing the DDI to minimize queue spillback into the DDI. The crossover intersections prohibit both directions of the arterial from receiving a green indication at the same time, which creates challenges for coordination with adjacent signals. Similarly, adjacent signals with more than two phases may, in some conditions, not have the efficiency to serve the available capacity of the DDI. To the motoring public, the DDI design may appear to be ineffective when, in fact, the DDI is operating as intended. Transportation agencies considering the DDI at locations with nearby signalized intersections and congested crossroads have made geometric and signal design modifications to nearby inter- sections. Some potential geometric treatments that may improve operations include: • Closing the closest signalized intersection or converting it to unsignalized right-in/right-out control. These treatments were used at Dorsett Road in Maryland Heights, Missouri. • Using grade separation to eliminate one or more movements at the adjacent intersection. This treatment was used at National Avenue in Springfield, Missouri, where a left turn into a hospital was modified to take a right, followed by another immediate right turn, leading to an undercrossing passing under the crossroad and accessing the hospital. • Alternative intersection designs could potentially be used to reduce the number of necessary signal phases at adjacent intersections along the corridor. This treatment was used on Poplar Tent Road in Charlotte, North Carolina, and at several DDIs in Utah. 6.4.10 Developing a Design from the Inside Out As noted above, a DDI follows fundamental horizontal alignment and intersection design principles. Unlike a typical diamond form, the DDI crossroad and intersection geometry is more complex. Developing an initial DDI configuration requires developing and combining many geometric elements. Designing the DDI from the “inside out” can help address seemingly minor details that form the base of the interchange configuration. The following is a suggested sequence for initially configuring a DDI: 1) Establish median width between the crossovers, 2) Locate the crossover and ramp terminal intersections, 3) Develop ramp geometry, 4) Create crossroad alignment, and 5) Performance check. 1. Establish median width between the crossovers. From the center of the cross street between the crossovers, work from the inside out to establish the median width between the contraflow lanes. If pedestrians are going to be directed to the median between crossovers, a quality pedestrian environment resulting in a high quality of service is as important as any other user type. This means providing adequate width for pedestrians and bicyclists who may be walking their bicycles through the walkway. Widths should consider volume and the ability of users to pass each other along the way. Exhibit 6-38 and Exhibit 6-39 depict two different pedestrian paths. The configuration in Exhibit 6-38 shows a relatively narrow section. If the site is to serve high pedestrian and bicycle volumes, one might consider widths that account for higher two-way flows of the two user types. The configuration in Exhibit 6-39 is at an underpass and provides a greater width for users. The extra width counters the closed-in feel between the contraflow traffic. The pedestrian access from the median walkway to the outside is a critical area, and enough space should be provided for appropriate refuge areas, detectable warning strips,

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. This equipment should not conflict with the defined pedestrian areas. 2. Locate the crossover and ramp terminal intersections. From the center of the highway, work outward to locate the crossover center and ramp terminal intersections. This means considering the horizontal geometry approaching and departing the overcrossing so that appropriate alignments can be provided. The crossover location should, ideally, allow a consolidation of the crossover and the ramp terminal intersections so they are combined versus a series of separate intersection elements. The number of curves and providing needed tangents between reverse curves increases the overall spacing requirements between crossovers. Eliminating some reverse curves reduces driver workload and allows shorter spacing. With a wider median, the number of reverse curves between the crossovers can be minimized. Reducing the number of reverse curves to develop the crossover geometry will reduce the distance from the highway centerline to the crossovers and therefore reduce the overall distance between the crossovers. Exhibit 6-38. Example cross section for pedestrian path (Gwinnett County, Georgia) (6). Exhibit 6-39. Pedestrian path at an undercrossing with greater width at the MN-101 and 140th Street interchange (Rogers, Minnesota) (6).

158 Diverging Diamond Interchange Informational Guide Exhibit 6-40 compares the influence of median width on crossover location and influ- ence area. Tangent alignments through the crossover promote desired vehicle tracking and reduce driver workload by separating driving tasks at each curve. The tangent section between reverse curves is consistent with fundamental highway design principles. The tangent creates a self-describing alignment and allows drivers to see and prepare for the subsequent reverse curve. These principles apply to any succession of reverse curves approaching, traveling through, and departing a DDI. With the crossover in place, the ramp terminal intersections can be located by using the crossover intersection as the basis for locating the entrance and exit ramps and the associated ramp terminal intersection. In new construction, there may be more flexibility in locating ramp terminal intersections and designing the ramp configurations based on needed lane numbers and arrangements. When adapting a DDI to existing ramps, it can become more difficult to consolidate the ramp terminal intersection. Exhibit 6-41 shows how to use the crossover intersection location as the basis for laying out the ramp terminal intersection elements. Exhibit 6-42 presents a crossover configuration with crossover stop bars located at the beginning of the tangent section and the subsequent reverse curve located beyond the crossover. This provides excellent path alignment and enhances positive guidance through the crossover. The entrance ramp terminal intersection aligns well with the crossover inter- section. However, the exit ramp terminal intersection is offset to the left towards I-69. Skew affects these intersection configurations, and if the crossing is skewed, special attention may be required at the ramp terminal intersections. This could be to consolidate the crossover and ramp terminal intersections and to do so while providing appropriate view angles and minimizing speed differentials between turning movements as described in Section 6.4.3. Exhibit 6-40. Influence of median width on crossover location and influence area.

Geometric Design 159 Exhibit 6-41. Crossover intersection as the basis for locating the ramp terminal intersections. Exhibit 6-42. Tangent crossover configuration at I-69 and Campus Parkway interchange (Fishers, Indiana) (5). 3. Develop ramp geometry. From the crossover and ramp terminal intersections, work outward to develop ramp geometry. On exit ramps, this means designing the ramp terminal left and right turns with appropriate lane numbers and horizontal alignment features to provide intersection sight distance and view angles. This could also mean working upstream on the ramp proper to develop the left- and right-turning lanes consistent with traffic volumes and patterns to be served. Exhibit 6-43 shows how ramp design must be coordinated with and integrated with lane numbers and arrangements at the ramp terminal intersection. DDI design may focus on the crossroad, but ramp geometrics that create a smooth transition from the ramp proper to the ramp terminal intersection must be established. On entrance ramps, this would be considering the convergence of the left- and right- turning vehicles from the cross street to provide the appropriate number of lanes and the

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. On skewed interchanges where the left-turning movement from the cross street is on an acute angle, the controlling radii may result in relatively low speeds compared to a larger radius or more acceleration distance for the right-turning vehicles from the cross street. This speed differential could be significant and may require added on-ramp length before merging the two travel streams. Exhibit 6-44 depicts a separation of the converging left and right turns and the desirable transition to the ramp. 4. Create crossroad alignment. Work from the crossover and ramp terminal intersection to create transition geometry that sets up the alignment to the crossover and matches to the cross- Exhibit 6-43. Ramp design and lane numbers and arrangements. 3-5’ separation at common forward bearing Painted taper Reduced convergence and speed shear Exhibit 6-44. Separating converging left and right turns.

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. Those right-turn treatments will vary based on the type of traffic control provided. Reverse curves beyond the crossover intersections are often needed unless a wide median is present to provide the transition to and from the cross street typical section. Approach curves create the necessary approach angle through the crossover, and horizontal curve geometry should be established to create a smooth speed profile to, in, and through the interchange. 5. Performance checks. Like roundabouts, DDIs are performance based. As early concepts are developed, checking their performance should begin with the earliest draft versions. This includes general categories typical of other interchange forms. In addition, there are special considerations based on the unique attributes of the DDI. These performance checks are described in more detail in Section 6.7. 6.4.11 Performance Checks and Considerations Like designing roundabouts, DDI 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. These performance checks are described in more detail in Section 6.7. 6.5 Cross Section Cross section elements at a DDI are consistent with other service interchange forms. How- ever, there are some features unique to the DDI that affect cross section considerations and values such as: • Median width—The central walkway (noted below), the overall width between contraflow lanes, and the median width of the cross street have the most influential effects on DDI configurations. In short, the narrower the median on the crossroad or at the DDI, the more extensive the crossroad horizontal alignment to and from the crossovers becomes. The wider the separation, the more simplified the DDI design becomes. • Crossover design—Because of the crossover design, through drivers must travel a curvi- linear alignment to and through the crossover. The alignment is a turning roadway, and vehicle lane discipline needs to be considered. Lane widening will typically be needed from the crossroad typical section through the reverse curves of the crossover to maintain vehicle lane discipline. • Central walkway for pedestrians—A central walkway between the contraflow lanes places pedestrians between two opposing travel streams. This is a unique configuration compared to other interchange forms, and pedestrian comfort and quality of service should be a focus. • Bicycle lanes—Some DDIs provide bicycle lanes through the interchange. The relatively small horizontal curves increase the potential for encroachment by motor vehicles. Extra width for a painted buffer could be included in the cross section. Even with bicycle lanes, some bicyclists may not use them and will cross in the center walkway with pedestrians. Extra width might be appropriate to create ample space for these two user types and to account for being confined by a barrier and contraflow traffic. • Pedestrian crossing treatments—Pedestrian crossing treatments, such as push buttons, tactile warning strips and crossing channelization to support wayfinding to the crossing and

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 lead- ing to undesirable crossing treatments. This cross section element should be integral to DDI design. Additional details and considerations for these unique elements of a DDI are described further in the remaining sections. 6.5.1 Overpass/Underpass Cross Section Section 6.3.5 described considerations of matching to an existing highway crossing. Whether matching an existing cross section or developing a new DDI cross section, the cross section features influence the design configuration. DDI planning and design should consider lane widths early in concept development to fully assess possible impacts to adjacent bicycle facilities or other off-tracking considerations. Each project has a unique context in design vehicle choices, and if some off-tracking is allowed, it should be a deliberate design decision. The interchange design will be directly affected by whether the arterial passes over or under the freeway. In most cases, DDIs with a crossroad designed as an overpass offer the most design flexibility, particularly in serving pedestrians. With a crossroad passing under the freeway, bridge abutments and columns can impede sight distances and limit roadway alignment options, particularly for retrofit projects where bridge elements are already in place. 6.5.1.1 Overpass Examples Exhibit 6-45 provides several cross sections of DDIs with crossroad overpasses. Overpass designs can use a single, dual, or even four-span bridge structure. Overpass DDIs often use a single bridge design, as seen in Examples A through D. A single overpass is frequently the least costly overpass design option, especially for retrofit projects where there is typically an existing single-span bridge. The center crosswalk is common with an overpass as there are rarely obstruc- tions between the two directions of travel on the cross street. Several DDIs in Utah (including Example E) were retrofits and added lanes to the cross street by adding a parallel overpass beside an existing one. The additional lanes were needed to accommodate land use changes and increased traffic volume. 6.5.1.2 Underpass Examples Underpass facilities tend to have less flexibility in their design. At an underpass facility, the available space between the two crossover intersections is directly affected by any components of the bridge substructure. At retrofit DDIs, this sometimes limits the locations for pedestrian facilities, and in the case where columns are in the median, limit the placement of lanes if unbalanced designs are under consideration. Underpasses can sometimes have a center pedestrian path as presented in Exhibit 6-46. Several illustrations of crossroad underpass designs are provided in Exhibit 6-47. These show varying examples of column designs directly affecting the available space for vehicle lanes and pedestrian and bicycle facility considerations. Examples A and B provide limited space for vehicular traffic lanes. Pedestrian facilities were limited or nonexistent. Examples C and D provide enough space for vehicular traffic and pedestrian facilities. Pedes- trians were routed through the center as no columns were present in the median. Example D also provides some accommodation for bicycle lanes within the space. Examples E, F, and G each show cases of pedestrian facilities along the outside. Note the varying use of barrier

Geometric Design 163 A) Center Walkway, Single Structure (MO-13, Springfield, MO) B) Center Walkway, Single Structure (Botts Road, Branson, MO) C) Center Walkway, Single Structure (Ashford Dunwoody Road, Dunwoody, GA) D) Center Walkway, Single Structure (MO-248, Branson, MO) E) Outside Walkways, Dual Structure (S 500 E St., American Fork, UT) F) Outside Walkways, Dual Structure (Roe Avenue, Overland Park, KS) Exhibit 6-45. Examples of various overpass cross sections (3). protection between vehicles and pedestrians ranging from no protection or a landscaping strip in Example E to fencing and concrete barriers used in Examples F and G, respectively. 6.5.2 Lane Width DDI planning and design should consider lane widths early in concept development to fully assess possible impacts to adjacent bicycle facilities or other off-tracking considerations.

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. The lane width is most critical in the transitions to and from the cross over. Lane widths generally should not vary unnecessarily, and widths developed in the curves approaching the crossover should be maintained for a consistent cross section through the alignment. Where necessary, lane widths should achieve their crossover lane width prior to the first curve approaching the crossover and at the end of the last curve departing the crossover. The intent would be to increase (on approach) and decrease (on departure) cross- over lane widths through bridge abutments. The abutment may be a critical cross section constraint. Lane widths in longer tangent sections can be narrower if needed to reduce the overall cross section. 6.5.2.2 Turning Roadway Considerations A crossover represents a “turning roadway,” and the lane widths on the turning roadway are often widened to address issues associated with handling vehicles on curvilinear alignments. This is consistent with widening at interchange ramps or approaching and in roundabouts. Adding width to a turning roadway supports truck and large vehicle off-tracking through a curved alignment section. This applies to the geometry providing transitioning to and through the crossover, in addition to the ramp terminal intersection treatments. There is various design guidance provided by AASHTO and other transportation agencies associated with turning roadway widths. Widths can be based on the turning radii provided and delta, the angle turned by a roadway curve. For example, considering a WB-67 design vehicle and a two-lane roadway, roadway widths on curves with a 150-foot radius curve or a 300-foot radius range from 41 feet and 32 feet, respectively. As the delta angle of curve increases beyond 15 degrees to 90 degrees, the roadway widths increase to the values that approach the higher width values. The lane widths of the crossover and ramp movement are determined based on the design vehicle and the likelihood of multiple design vehicles being side by side. Horizontal geometrics Exhibit 6-46. Undercrossing at the MN-101 and 140th Street interchange (Minnesota) (6).

Geometric Design 165 A) No Walkway, Columns Outside (SR-92, American Fork, UT) B) Single Walkway Outside, Columns in Median (Thurbers Ave., Providence, RI) C) Center Walkway, Columns Outside (Front St., Kansas City, MO) D) Center Walkway, Bicycle Lane, Columns Outside (Moana Ln., Reno, NV) E) Outside Walkways w/ No Barrier, No Columns (Winton Rd., Rochester, NY) F) Outside Walkways w/ Fencing, Columns in Median (Dorsett Rd., Maryland Heights, MO) G) Outside Walkways w/ Barrier, Columns in Median and Outside (Harrodsburg Rd., Lexington, KY) Exhibit 6-47. Examples of various underpass cross sections (3).

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. Lane widths for left- and right-turn movements at entrance and exit ramp terminal inter- sections may be designed using similar methods for right-turn ramp designs (entry and exit) employed at conventional interchanges. Left-turn movements entering and exiting the crossroad function in the same fashion as right-turning movements at conventional diamond interchanges. Lane widths can be increased to accommodate design vehicles at entry or exit ramps, depending on the project context. 6.5.3 Shoulder Width DDIs are typically built with shoulders less than the width of a vehicle. Near the crossover, narrow shoulders (no more than 4 feet wide) are recommended to discourage wrong-way movements and effectively channelize vehicles. Between crossovers, shoulder width typically has considerable cost implications because shoulders are on or beneath a bridge. If a wider shoulder is desired between crossovers for snow storage or as a refuge for disabled vehicles, it should be placed to the right side of travel lanes (i.e., on the inside). Shoulders will need to be wider than 4 feet if they are designated as bicycle lanes to provide some minimal level of comfort next to the barrier. Shoulders will also need to be wider than 4 feet if it is desired for an emergency vehicle to be able to pass queued traffic. 6.5.4 Bicycle Lanes The needs of all modes should be considered in the earliest stages of DDI design so that design decisions include, or do not preclude, high quality facilities for all users. Buffers between bicycle lanes and the travel lanes increase the quality of service for the bicyclist. However, bicycle lanes and buffers add cross section width. If separate bicycle lanes are not provided, bike and pedestrian multiuse paths must consider these two users and the cross section width should be established to reduce conflicts between these user types. Exhibit 6-48 depicts a bicycle lane approaching the DDI crossover. Exhibit 6-48. Bicycle lane approaching the DDI crossover at I-580 and Moana Lane (Reno, Nevada) (9).

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. Refer to Chapter 3 for pedestrian and bicycle design considerations. 6.5.5 Sidewalk Sidewalk widths have a direct influence on the quality of service for pedestrians. In some cases, there may be no bike lane, and a multiuse path must be provided if the DDI is serving bicycles. This path should be designated at a minimum width of 10 feet. Pedestrians can be accommodated on the outside or within the median. However, with an underpass with multiple bridge spans, the median treatment may not be feasible since there may be insufficient median space considering the columns. Consider cross section needs for sidewalks or multiuse paths early in the concept development activities. More guidance on these facilities is provided in Chapter 3. 6.5.6 Crosswalks Crosswalk width is commonly dictated by local or state design policies and standards. Crosswalk location to median pedestrian paths should connect in such a way that tactile warning strips and other aids to support wayfinding can be provided. 6.6 Vertical Alignment Vertical considerations at DDIs are similar to those considered for other interchange forms. However, because of the curvilinear alignment, DDIs have special considerations that need to be reviewed and coordinated closely with the horizontal alignment. Primary considerations for vertical alignment at DDIs include: • Crest vertical curve on the overcrossing—A crest vertical curve on the overcrossing could potentially mask the turning roadway alignment to the crossover. Therefore, horizontal and vertical alignments need to be coordinated closely. • Skew in an overcrossing—In a typical diamond form, a left-turn movement from the crossroad to the entrance ramp occurs near the location of the companion right-turning movement to the same entrance ramp. In a DDI, the left-turn movement occurs upstream of the right-turn movement and at a location where the crossroad roadway profile is the highest. This means the left turn has a more pronounced downgrade to match the profile grade of the right-turning ramp movement. In some conditions, it may be beneficial to raise the right- turning ramp profile to better meet the left-turning movement. • Crosswalk visibility—A second byproduct of the crest vertical alignment and skew angle is visibility to the crosswalk across the left-turning roadway. The relatively sharp horizontal curve to the left and bridge parapet can obscure views to a user waiting at the pedestrian crossing. As the left-turning roadway is dropping away, the crosswalk is not visible until the turn is initiated. Exhibit 6-49 calls out some of the vertical and horizontal alignment attributes of the DDI at Pioneer Crossing in American Fork, Utah. Exhibit 6-50 presents the influence of vertical alignment and visibility considerations at the DDI at Pioneer Crossing in American Fork, Utah. The horizontal curvature and vertical alignment coordination results in the parapet wall impeding the driver sight line to the pedestrian crossing location.

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. 6.7 Performance Checks DDI concept design involves balancing and optimizing trade-offs associated with user performance, capacity, costs, maintenance, and construction staging, among other items. For instance, considering large design vehicles at the crossover may lead designers to contemplate larger design radii or wider lanes; however, this could promote higher speeds through the crossover for other vehicle types. Instead, to provide adequate facilities for the design vehicle while maintaining safe speeds for other motorists, designers may want to consider designs resulting in off-setting one or more of the approaches to the DDI. This method may increase street alignment radii resulting in comparatively narrower lanes to serve design vehicle off-tracking. Exhibit 6-49. Vertical and horizontal alignment attributes at the Pioneer Crossing interchange (American Fork, Utah) (5). Exhibit 6-50. Vertical alignment and visibility considerations at the Pioneer Crossing interchange (American Fork, Utah) (5).

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. However, those movements can increase vehicle speeds and degrade the quality of service experience for pedestrians at crossing locations. Selecting multilane left or right turns controlled by signals could lead to less vehicular capacity; however, it could increase the quality of service for pedestrians by improving pedestrian crossing safety performance by reducing turning speeds and allocating signalized pedestrian crossing time. While the previous sections provided general geometric parameters and principles related to the DDI, this section provides more specific guidance to assist designers in verifying the quality of their designs. The primary source for geometric design guidance is the AASHTO Green Book, A Policy on Geometric Design of Highways and Streets (11). This chapter augments AASHTO Green Book guidance and includes traditional roadway and intersection design fundamentals to support decisions regarding DDIs. Exhibit 6-51. Pedestrian crossing location at the Pioneer Crossing interchange (American Fork, Utah) (10). Exhibit 6-52. Pedestrian crossing sight line at the Pioneer Crossing interchange (American Fork, Utah) (10).

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. These performance checks are described and summarized as follows: • General performance categories typical of other interchange and intersection forms: – Verifying design users are integrated into the configuration. This specifically means accounting for pedestrians, bicyclists, and other design vehicles. – Stopping sight distance (SSD). – Intersection sight distance (ISD) (sight triangles). – View angle considerations: Supporting left or right turns to the cross street from the ramp terminal intersection. • DDI-specific performance considerations and categories include: – Speed profile: Considering the roadway approach speeds on either side of the interchange and accounting for appropriate transition features (horizontal and cross section) to and from the interchange. – Approach vista: Discouraging wrong-way movements to opposing contraflow traffic by limiting sight lines on the approach to the contraflow roadway. – Path alignment: Assessing if the configurations appropriately direct vehicles to the receiving lanes. – Vista through crossover: Supporting a self-describing roadway that emphasizes contra- flow through the interchange. 6.7.1 General Performance Categories As our industry moves to performance-based evaluations, it is becoming increasingly common to conduct evaluations to help assess, evaluate, and compare concepts. Performance metrics have been integral parts of roundabout planning and design guidance since the publication of Roundabouts: An Informational Guide, 1st Edition (12). For roundabouts, this included specific categories directly associated with the specific characteristics of roundabouts. General performance measures are described in the following sections. 6.7.1.1 Pedestrians and Bicycles DDI planning and design begins with understanding the anticipated specific users and providing appropriate geometric features and elements that serve nonmotorized and motor- ized users. In urban and suburban areas where pedestrians are expected, the configuration should be checked to assess how pedestrians are served. In cases where pedestrian use may be limited, to the extent possible, configurations should be developed to not preclude future treatments. For example, this could include ensuring that there is adequate cross section width for side- walks and paths or that channelizing islands could be readily converted to include pedestrian features and wayfinding elements. The following represents checks to assess how nonmotorized users are integrated into design configurations: • Minimizing the number of travel lanes to improve the simplicity and safety performance of DDI pedestrian facilities, including walkways and crossings. • Designing for slow vehicle speeds. Wherever possible, using the smaller range of curve radii in pedestrian areas while still meeting motorized user needs. Higher speeds increase severe and fatal crashes for pedestrians. • Providing sidewalks that are set back from roadways where possible and creating sidewalks that maximize the available width where possible to increase the quality of service. Barrier-

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. • Considering general navigation features with a focus on visually impaired users. This includes wayfinding treatments to and through the interchange area and providing crosswalk and pedestrian ramp design details that support direction to the crossing and clear alignments to the receiving pedestrian ramps. • Designing for integral bicycle facilities such as bicycle lanes and multiuse paths. Target configurations could include buffers from motor vehicle travel. Exhibit 6-53 depicts the median pedestrian crossing and pedestrian treatments provided at the I-580 and Moana Lane DDI in Reno, Nevada. Exhibit 6-54 depicts the median pedestrian crossing and pedestrian treatments provided at the MN-101 and 140th Street interchange in Rogers, Minnesota. Exhibit 6-53. Median pedestrian crossing and pedestrian treatments at the I-580 and Moana Lane interchange (Reno, Nevada) (9). Exhibit 6-54. Median pedestrian crossing and pedestrian treatments (Rogers, Minnesota) (6).

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. Appropriate design vehicle consideration will depend on road classification, input from jurisdictions and/or road authorities, and the surrounding environment. These users have a significant effect on interchange geometrics and should be established in the early planning and concept design stages. Performance checks begin with documenting the underlying design vehicle assumptions and assessing if, fundamentally, the selected vehicles are appropriate for the site conditions. A second assessment is understanding the potential range of vehicle types (e.g., oversize/overweight) or special vehicles (e.g., farm implements) and the risk of those nonstandard vehicles navigating the DDI. While some vehicles will be designed for (the agency may purposefully choose to serve trucks or other large vehicles with limited or no lane off-tracking), others may be assessed on how well they could be accommodated. “Accommodated” means assessing how the occasional vehicle could navigate the DDI and considering modifications that allow nondestructive off- tracking or lane encroachment. Vehicle swept paths can readily be assessed using off-the-shelf software packages. 6.7.1.3 Stopping Sight Distance SSD is the distance required by a driver to perceive and react to an object in the road and come to a complete stop without colliding with the object. SSD is a fundamental performance metric for any roadway, interchange, and intersection geometric design. As with any other intersection, DDI intersections should provide SSD on roadways and intersections. Drivers on the roadway must have the visibility to see, perceive, and react to an event and avoid a collision. SSD should be provided at every point in the DDI roadways and intersections (11). The AASHTO Green Book provides an equation for calculating SSD (11). SSD V t V a 1.47 1.075 2 ( )( )( )= + where SSD = stopping sight distance, ft; t = perception-brake reaction time, assumed to be 2.5 s; V = initial speed, mph; and a = driver deceleration, assumed to be 11.2 ft/s2. SSD values are based on an assumed driver’s eye height of 3.5 feet and an object height of 2 feet (11). At DDIs, SSD constraints will be most prevalent at the following locations where yield control or pedestrian crossings are provided: • Exit ramp left and right turns—Drivers must be able to stop at conflicts with approaching vehicles or crossing pedestrians at the exit ramp terminal intersection. • Entry ramp left and right turns—Drivers must be able to stop at pedestrian conflicts at free- flow left turns and yield-controlled right turns onto the entrance ramp. This topic is discussed in Chapter 3. While not an SSD issue, a crest vertical curve combined with horizontal curves from the contraflow lanes approaching the crossover may result in the impending crossover intersection

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). Exhibit 6-55 and Exhibit 6-56 provide examples in which the advance signal is provided to the left of the crossroad. 6.7.1.4 Intersection Sight Distance Intersection sight distance (ISD) is a fundamental geometric design performance metric for any intersection. Drivers on any portion of the intersection must have the visibility to see, perceive, and react to an event and avoid a collision. ISD is a contributing factor in street crashes and near collisions. As with any other intersection, DDI intersections should provide ISD. Drivers approaching or departing an intersection should have an unobstructed view of traffic control devices and sufficient length along the crossroad to safely navigate the intersection. ISD, sometimes called “sight triangles,” is the distance along a clear line of sight allowing a stopped driver from a minor approach to accept an appropriate gap in traffic when entering or crossing the major road. For a DDI, the minor approaches are the movements to and from the entrance and exit ramp terminals. ISD constraints will apply where stop- or yield-controlled movements are provided. ISD also applies to RTOR or left-turn-on-red (LTOR) at signalized Exhibit 6-55. Example of supplemental signals. Exhibit 6-56. Example of supplemental signal (14).

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), only traffic on one direction of the major road is considered (11). The AASHTO Green Book equation for calculating ISD is provided below (11). 1.47ISD V tmajor( )( )( )= g where ISD = intersection sight distance, ft; Vmajor = design speed of major road (mph); and tg = time gap for minor road vehicle to enter the major road(s). The AASHTO Green Book Section 9.5.3 describes a variety of procedures to determine sight distances according to different types of traffic control (11). The exit ramp maneuvers represent a Case C (yield control on the minor road), maneuver 2 (left and right turns from the minor road). The left and right turns from the minor road are considered using different gap time requirements for each movement type; however, the exit ramp left turn only that conflicts with one direction of major street traffic is essentially the same as the right turn maneuver. Therefore, the requirements are the same as at a DDI. The time gap requirements based on the design vehicle are provided in Exhibit 6-57 (11). Exhibit 6-58 presents ISD distance derived from the ISD equation using the time gap values for the three design vehicles. Calculations of ISD and SSD for each design vehicle are provided in Exhibit 6-59. Design Vehicle Time Gap, tg (s) Passenger Car (PC) 8 Single-unit truck (SU) 10 Combination (Comb) truck 12 Exhibit 6-57. Time gap, Case C2, left and right turn from minor approach (11). Design Speed (mph) Stopping Sight Distance, SSD (ft) Intersection Sight Distance, ISD (ft) Passenger Cars (PC) Single-Unit (SU) Truck Combination (Comb) Truck 10 46 118 147 29 15 77 176 221 265 20 112 235 294 353 25 152 294 368 441 30 197 353 441 529 35 246 412 515 617 40 301 470 588 706 45 360 529 662 794 Exhibit 6-58. SSD and ISD calculations (Case C2) (11).

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 delib- erately to configure left- and right-turning movements so that drivers have a view angle to upstream conflicting traffic. The view angle must be provided to the contraflow condition of the DDI, and this is different than typical driver expectations at a conventional interchange. Target view angles should be 90 degrees and no less than 75 degrees to account for older drivers and pedestrians (15). The view angle can be defined as the angle between the alignment of the right-turning vehicle and the tangent of the crossover. The performance of the targeted view angle should also measure the angle between the alignment of the right-turning vehicle and the line drawn from the right-turning vehicle to the oncoming vehicle at the ISD. If the values are not acceptable, the ramp terminal intersection should be modified and optimized with other crossover features. Exhibit 6-60 demonstrates how right-turn view angles can affect ISD. In this example, left- and right-turn movements have been modified to increase visibility to upstream conflicting 0 5 10 15 20 25 30 35 40 45 50 0 100 200 300 400 500 600 700 800 900 De si gn S pe ed (m ph ) Length of Sight Triangle Leg (ft) Stopping Sight Distance ISD - Passenger Car ISD - Single Unit Truck ISD - Combination Truck Exhibit 6-59. SSD and ISD for yield-controlled left and right turns (11). Exhibit 6-60. View angles and ISD.

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. The following discussion supports performance checks for left- and right- turning vehicles. RTOR or LTOR should not be allowed if ISD values to the correct direction of travel (oncoming contraflow) cannot be attained. The view angle influences sight line dimensions, and design checks are based on considering the view angle to the upstream conflicting traffic. Designers must evaluate left- and right-turn view angles, and LTOR or RTOR should only be allowed if ISD values are attained. Since ISD values are smaller as design speed is reduced on the major roadway, smaller radii and slower speeds from the crossovers are a way to attain sight distance. This is an example of how to use an iterative, performance-based approach to modify interchange geometrics to meet operational and safety performance objectives. Median barrier between contraflow lanes between the crossovers can block driver sight lines. The barrier can truncate the needed line of sight even if the ramp terminal intersections have acceptable view angles. If median barriers are used, a performance check would be to verify the barrier does not impede ISD. To attain ISD, the end of the median barrier wall could be truncated or the wall height reduced to provide enough sight distance. If ISD values cannot be attained and speed on the crossroad cannot be reduced to meet ISD needs, RTOR should not be allowed. Exhibit 6-61 demonstrates the lack of sight distance caused by median barrier walls along crossroads under and over a limited access facility. 6.7.2 DDI-Specific Performance Categories DDIs include unique qualities and characteristics that can be evaluated to assess, evaluate, and compare alternatives. The following sections describe DDI-specific performance categories. 6.7.2.1 Speed-Radius Relationship The relationship between horizontal curvature and travel speed is documented in the AASHTO Green Book. Typically, the equation is used to determine a curve radius given a specific superelevation rate. By using the equation, assuming a superelevation rate of ±0.02 and solving for speed, the predicted speed associated with minimum radii can be determined using the equations provided below (11). Exhibit 6-61. Median barriers blocking ISD for exit ramp right turns.

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) considering ±0.02 superelevation. This allows rapid assessments of predicted speed as various curve radii are being evaluated to transition to and through the DDI. The DDI geometric influence area is a low speed environment, and superelevation beyond the normal or reverse crown is not necessary. Superelevation in speed transitions from the crossroad to the initial DDI transition alignment should be considered on a site-specific basis. The overall intent is to create a low speed environment, and superelevation should reflect transition needs approaching the DDI. Speed reduction and smooth transition are goals in approaching a DDI. Considering cross- road speed (e.g., 45 mph) and approximate speed for the slowest horizontal curve (e.g., 25 mph), one can select an intermediate speed in the transition (e.g., 30 mph). In this example, the first curve radius in the transition could be approximately 300 feet. This approach can be used to assess the performance of the transition speeds and radii approaching the DDI. Similarly, the speed-curve equation and graph can support performance evaluations to assess the speed transition needs between horizontal curves transitioning through the crossover. Exhibit 6-62 provides a quick reference for the speed-curve relationship for the superelevation rate (12). 0 5 10 15 20 25 30 35 40 0 50 100 150 200 250 300 350 400 Radius (ft) S p ee d ( m ph ) e=+0.02 e=-0.02 Exhibit 6-62. Speed-curve relationship for superelevation rate (12).

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. This concept can be used in at least three ways: 1. To estimate speed relationships to and through the DDI and establish corresponding horizontal curve radii for the transition between the crossover and crossroad speed. 2. To estimate the transition length to attain speed reductions between the crossroad and the crossover proper. 3. To assess the specific horizontal curve radii and speed relationships of the crossover curves to meet the speed profile concepts. Exhibit 6-63 presents a conceptual speed profile approaching a DDI and the change in anticipated operating speed relationship associated with the crossovers. The transition distance between the crossroad speed and the crossover can be estimated using freeway exit ramp deceleration as a model. The AASHTO Green Book presents various decel- eration distances between the exit speed and the ramp controlling curve. These distances can approximate the interchange’s geometric influence area. In rural or urban areas where speed differentials may be substantial, the distances could guide the location of potential speed transition treatments (11). Exhibit 6-64 presents deceleration/transition length considerations based on various cross- road and first crossover curve speeds. • Crossover Speed Checks: Considering the speed profile in more detail, the lowest speeds should be between and through the crossover proper. Crossover target speeds typically range from 20 mph to 35 mph that correspond to horizontal curves with commensurate radii of 100 feet to 400 feet, respectively. This concept can be used at least two ways: 1. To develop the initial horizontal alignment by selecting the horizontal curve radii that will result in the target speed profile relationship. This means selecting specific radii that will result in target speeds consistent with the transition from higher to lower speeds. Tangents should be provided between all horizontal curves unless there are no other design options. Exhibit 6-63. Conceptual speed profile approaching a DDI.

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. 2. To check the performance of the alignments as the DDI is refined and finalized. This could include reducing crossover speeds to attain ISD values as described previously or adapting the horizontal configurations to meet site constraints. Exhibit 6-65 conceptually presents target design speed and horizontal curve radii relationships. Speed consistency between the various movements and geometric elements at specific loca- tions within the DDI is a primary DDI design goal. This means low speed differential between turning and through movements or converging travel streams. This also means low speed differentials between successive geometric design elements (i.e., curve 1 to curve 2 to curve 3 to curve 4, etc.) regardless of whether the profile speeds are decreasing or increasing. Relative speeds between conflicting traffic streams and between consecutive geometric elements should be minimized such that the maximum speed differential between movements should Deceleration/Transition Lengths (ft) Crossroad Speed (mph) First Crossover Curve Speed 65 55 45 35 45 490 340 -- -- 35 580 440 285 -- 30 620 470 316 185 25 645 500 355 235 Exit ramp deceleration values adapted from AASHTO Green Book (11) Exhibit 6-64. Deceleration/transition length considerations (11). Exhibit 6-65. Target design speed and horizontal curve radii relationships.

180 Diverging Diamond Interchange Informational Guide be no more than approximately 10 to 15 mph (15 to 25 km/h) (12). As with other design elements, speed consistency should be balanced with other objectives in establishing a design. Conducting performance checks allows the user to optimize design and performance objectives for a given project context. 6.7.2.3 Approach Vista Approach vista describes how much of the conflicting movement is within a direct path for a driver approaching the crossover. The crossover geometry and islands can help discourage wrong-way movements into the conflicting approach. Exhibit 6-66 shows example geometry and corresponding approach vistas for the left through lane side. The goal would be to create a terminal vista to opposing, contraflow lanes. The entrance ramp terminal intersection may include features that fully block, partially block, or do not block direct views to the conflicting movement. Some transportation agencies use barriers at the ramp terminal intersection islands to obstruct views to the contraflow lanes. This application is called an “eyebrow” by some agencies. Creating a terminal vista allows drivers approaching the approach vista to focus on the immediate roadway and transition alignment to the crossover. Minimizing a direct view of the contraflow lane should begin in early concept development. A terminal vista can generally be achieved as an outcome of designing an adequate tangent length and crossover angle for the crossover intersection. As configurations are refined and advanced, the approach vista should be revisited along with these other performance categories. Exhibit 6-66. Three examples of terminal vistas.

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. The traffic island at the ramp terminal intersection provides full blockage to the contraflow travel lanes. However, with no vertical features, there are unimpeded sight lines through the traffic island. A traffic barrier can be used to specifically block lines of sight through the traffic island. Exhibit 6-68 depicts the application of a barrier placed on a traffic island to impede driver sight lines. Exhibit 6-69 presents a range of examples of barriers and traffic islands to reduce sight lines. Approach vista is most critical approaching a DDI since driver approach speeds are typically higher and drivers are adapting to the change of workload and driving navigation tasks compared to the crossroad. However, approach vista can apply between crossovers. While entry sight lines are most critical at the transition into the DDI, sight line considerations apply between crossovers from the contraflow lanes. Exhibit 6-70 presents the approach vista from the contraflow lanes to the crossroad approach departing the DDI. Visually, there is a large disparity between the driver’s forward view directly Exhibit 6-67. Approach vista at a DDI (Reno, Nevada) (9). Exhibit 6-68. Barrier application to impede driver sight lines (9).

182 Diverging Diamond Interchange Informational Guide 1. Barrier Installation, Right Side, Entry Approach (Pioneer Crossing, UT) 2. Barrier Installation, Left Side, Exit Approach (Pioneer Crossing, UT) 3. Traffic Island, Right Side, Entry Approach (MO-13 - Springfield, MO) 4. Traffic Island, Left Side, Exit Approach (MO-13 - Springfield, MO) Exhibit 6-69. Examples of barriers and traffic islands to reduce sight lines (3). Motorist intended path Motorist sight line Exhibit 6-70. Approach vista from the contraflow lanes to the crossroad approach (Atlanta, Georgia) (6).

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. 6.7.2.4 Path Alignment Desired path alignment at crossovers is based on geometric alignments that direct vehicles to the receiving lanes. It is the geometrics that should provide the most dominant guidance, while striping and lane extension markings in the crossover intersection proper complement the foundational horizontal alignment feature. If used, reverse curves between crossovers should include enough tangent length between curves to provide a direct alignment. A lack of tangents between reverse curves or indirect path alignments can lead to vehicle path overlap, or even worse, inadvertently guide motorists into opposing traffic. Path alignment supports lane discipline for tractor trailer vehicles, and the trailer can track behind the cab in the tangent section. Through drivers do not expect to make alignment changes when passing through an inter- section. Curve radii in the middle of the crossover movements place the point of curvature or tangent in the intersection where drivers do not typically turn. Back-to-back reverse curves with no tangent compound the violation of driver expectation. This is especially true for vehicles at rest behind the stop bar waiting for a green light when drivers need to continue to the left before the impending right-hand curve to maintain lane discipline. Exhibit 6-71 shows the orientation of vehicles at a stop bar waiting for a green indica- tion. Each vehicle is not directed to the receiving lane beyond the crossover and must navigate back-to-back reverse curves through the intersection. This creates the potential for lane depar- tures or deceleration in the through movement (on green) as drivers slow to maintain lane discipline. Path alignment issues are easy to observe in the earliest concept development. This exhibit demonstrates the value of applying tangent sections at crossovers and between reverse curves. Path alignment can be checked by assessing the forward alignment and vehicle path at the stop bar. Path alignment should be integral to early concepts, and the alignment should be evaluated as concepts are refined and advanced. Path alignment can be measured by drawing Motorist natural paths Motorist actual paths Exhibit 6-71. Orientation of vehicles at the stop bar (6).

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. This may be different than the curvilinear alignment provided to navigate the crossover; that line can be projected to assess the natural path. The natural path checks can include comparing the identifying angle between the crossover tangent and the natural path of a vehicle stopped at the stop bar. The angle measured from this ideal path alignment line and the crossover tangent line represents how much the natural path of the geometry will guide drivers into other lanes. With ideal path alignment, there is no angle between these two lines. Alternative configurations can be compared to those with no path overlap ranking higher than those with measurable angles. Providing tangents and eliminating path overlap support freight movements through the DDI. Exhibit 6-72 and Exhibit 6-73 show vehicle paths through a crossover. Tangents and path alignment help support lane discipline for tractor trailer vehicles, as shown in Exhibit 6-73. Crossover intersections should be designed so that a driver can proceed straight from the stop bar (i.e., not turn the steering wheel) and reach the receiving lane downstream of the crossover rather than an adjacent lane. Exhibit 6-74 shows a DDI where the vehicle paths Exhibit 6-72. Vehicle paths through a crossover. Exhibit 6-73. Truck paths through a crossover.

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). 6.7.2.5 Vista Through Crossover Vista through crossover helps describe the amount of the continued roadway that a driver can see to enforce the crossover and discourage wrong-way movements. Exhibit 6-75 depicts the vista through the crossover. The more drivers can see of the roadway before them, the more pre- pared they are in their navigation tasks. Vista through the crossover can help drivers see potential downstream queues and other roadway features. The nominal length of the vista through crossover is measured by drawing a line tangent to the center of the outermost approach lane line and the edge of traveled way and extending the line to the view to the edge of the roadway. Median width between crossovers has a direct influence on the available distance provided. Wider medians provide longer vistas. In comparing geometric configurations or refining DDI geometric features, those that maximize vista through the crossover in balance with other performance metrics should Exhibit 6-74. Crossover intersection with path overlap (3). Exhibit 6-75. Vista through crossover.

186 Diverging Diamond Interchange Informational Guide Exhibit 6-76. Median effects of vista through crossover. rate higher than configurations with shorter vistas. Exhibit 6-76 presents three DDI configu- rations with various median shapes and widths as might be expected in developing and refining a DDI for a given location. Those forms with wider medians provide longer vistas through the crossover. 6.8 References 1. Schroeder, B., L. Rodegerdts, P. Jenior, E. Myers, C. Cunningham, K. Salamati, S. Searcy, et al., NCHRP Research Report 834, Crossing Solutions at Roundabouts and Channelized Turn Lanes for Pedestrians with Vision Disabilities: A Guidebook. Transportation Research Board, Washington, D.C., 2017. 2. Kittelson & Associates, Inc. Photo Credit. 3. Google Inc. Google Earth. Accessed July 2013. 4. Alaska Department of Transportation and Public Facilities. Photo Credit. 5. Google Inc. Google Earth. Accessed March 2020. 6. Jenior, P. Photos Credit. 7. Missouri’s Experience with the Diverging Diamond Interchange. MoDOT Report No. OR10- 021, Missouri Department of Transportation, Jefferson City, MO, 2010.

Geometric Design 187 8. Cunningham, C., B. Schroeder, J. Hummer, C. Vaughan, C. Yeom, K. Salamati, D. Findley, J. Chang, N. Rouphail, S. Bharadwaj, C. Jagadish, K. Hovey, and M. Corwin. Field Evaluation of Double Crossover Diamond Interchanges. Contractor’s Draft Submittal. FHWA, Project No. DTFH61-10-C-00029, 2014. 9. Lee, K. Photo Credit. 10. Daleiden, A. Photo Credit. 11. A Policy on Geometric Design of Highways and Streets, 7th ed. AASHTO, Washington, D.C., 2018. 12. Robinson, B., L. Rodegerdts, W. Scarborough, W. Kittelson, R. Troutbeck, W. Brilon, et al., Roundabouts: An Informational Guide, 1st Edition. Report No. FHWA-RD-00-067. FHWA, U.S. Department of Transporta- tion, Washington, D.C., 2000. 13. Manual on Uniform Traffic Control Devices. FHWA, U.S. Department of Transportation, 2009. https://mutcd. fhwa.dot.gov/htm/2009/part4/part4d.htm. Accessed March 2020. 14. Google Inc. Google Earth. Accessed July 2014. 15. Staplin, L., K. Lococo, S. Byington, and D. Harkey. Highway Design Handbook for Older Drivers and Pedestrians. Report No. FHWA-RD-01-103. FHWA, U.S. Department of Transportation, Washington, D.C., May 2001.