Diverging Diamond Interchange Informational Guide, Second Edition (2021)

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20 3.1 Introduction This chapter presents guidance when considering pedestrians, bicyclists, and transit vehicles at a diverging diamond interchange (DDI). While many existing interchanges do not have facilities for pedestrians, bicyclists, or transit, almost all DDIs constructed to date include some combination of such facilities. The overall objective is to develop a design, regardless of the type of intersection, compatible with a complete street. A complete street is a facility that serves many types of users including freight, transit, and nonmotorized users. This chapter describes the unique characteristics of nonauto modes (pedestrians, bicyclists, and transit) that should be considered when analyzing and designing DDIs. The transportation professional needs to work to identify and understand the needs of these various users in order to produce a balanced design that serves them all. 3.1.1 Benefits and Challenges Selecting the DDI as an alternative intersection choice has both benefits and challenges in the areas of safety and efficiency for multimodal users. Multimodal benefits of DDIs include: • Reduced overall right-of-way footprint compared to a conventional diamond interchange, • Opportunity for two-phase traffic signal control with reduced pedestrian wait time, • Minimized crossing distances, • Simplification of conflicts to one-directional vehicular traffic, • Opportunities for bicycle lanes and multiuse paths through the interchange, and • Reversed lane direction between crossovers allows for a single transit stop facility in median serving both directions of travel. Some of the challenges of multimodal provisions at DDIs include: • Altered travel paths, often with travel in the center of the interchange between vehicular lanes, • Traffic approaching from unexpected directions, • Unfamiliar signal phasing schemes, • Uncontrolled crossing of turn lanes, and • Long clearance interval needed for bicycles in the outbound direction at crossover inter- sections if the off-ramp right turn is signalized. A DDI generally generates higher capacity per lane for motor vehicles compared to a conven- tional diamond. This frees up right-of-way that can be used for multimodal facilities in the form of sidewalks, bicycle lanes, or even transit facilities. The reduced crossing distance can make it easier to serve nonmotorized movements compared to a conventional interchange. At long crossings, the need to provide adequate C H A P T E R 3 Multimodal Considerations

Multimodal Considerations 21 pedestrian clearance may result in the pedestrian movement controlling the phase lengths, leading to longer cycle lengths and greater pedestrian delay. In contrast, vehicle movements typically control phase length at DDI signals resulting in sufficient time per phase to also serve pedestrians. Although pedestrian crossings at the crossovers are signalized, pedestrian crossings of the turn lanes to and from the freeway may not be signalized. These potentially uncontrolled crossing locations require special attention and consideration to ensure pedestrian safety. Pre-timed DDI signals can provide for extended pedestrian walk phase times, which can reduce pedestrian delay and provide added time for pedestrians with disabilities. The reduced crossing distances can also benefit bicyclists who have a reduced exposure time within the crossover intersection, thereby minimizing the chance for vehicular conflicts. Finally, through the separation and channelization of the two directions of vehicular traffic, pedestrians interact with one direction of traffic at a time. This simplifies the pedestrian gap acceptance process and reduces the risk for pedestrian-vehicle conflicts, provided pedestrians understand from which direction traffic is coming at a given crossing. 3.1.2 Anticipating Multimodal Needs, Behavior, and Patterns A fundamental challenge in developing any new intersection or interchange form is decid- ing how to best provide for pedestrian and bicycle movements and anticipating the desire lines between different origins and destinations for these modes (e.g., how they travel through the intersection or interchange). Forecast volumes for nonmotorized users are rarely available, and if they are, they typically do not capture travel patterns within the intersection or inter- change. However, the majority of DDIs constructed to date feature pedestrian and bicycle facilities. For many retrofit sites, the existing pedestrian and bicycle facilities were improved with construction of the DDI. For example, DDIs at MO-13 in Springfield, Missouri; Dorsett Road in Maryland Heights, Missouri; and Harrodsburg Road in Lexington, Kentucky all added shared-use paths through the interchange (see Exhibit 3-1, Exhibit 3-2, and Exhibit 3-3). At all three of these sites, the construction of multimodal facilities was a priority for agen- cies, garnering positive feedback from local residents and users of the facility. At interchanges, land use development can sometimes lag interchange construction, resulting in pedestrian and bicycle needs not readily apparent on opening day. Early consideration and provision for pedestrian and bicycle movements should be a priority consideration for any DDI and should be accounted for even in early design concepts. Exhibit 3-1. Inner walkway at MO-13 (Springfield, Missouri) (1).

22 Diverging Diamond Interchange Informational Guide 3.2 Facility Selection This section discusses the operational considerations at a DDI that have relevance to the pedestrian and bicyclist experience, so the reader can understand trade-offs associated with various facility types. As discussed in Chapter 1, identifying project outcomes should be conducted as early in the design process as possible. This includes identifying outcomes for pedestrian and bicycle users. These outcomes can then inform the facility selection process. Selection should occur early enough to allow for design simultaneous to the roadway design. 3.2.1 Pedestrians Pedestrian facilities at grade-separated interchanges can generally be challenging due to high vehicular volumes and a focus on providing unimpeded capacity to vehicular flow. None- theless, interchanges can be designed to be safe and comfortable for pedestrians if the designer applies intersection-level design concepts that slow traffic flow, provide proper lines of sight for pedestrians and drivers, and manage conflict points to maximize pedestrian safety. A major consideration for pedestrian facilities at a DDI is whether to provide inner or outer walkways. Several design decisions flow from this choice. Pedestrian facilities in the center of the interchange (within the median) may be preferable to minimize conflicts with left-turning Exhibit 3-2. Outer walkway at Dorsett Road (Maryland Heights, Missouri) (1). Exhibit 3-3. Shared-use path on outside at Harrodsburg Road (Lexington, Kentucky) (1).

Multimodal Considerations 23 traffic to and from the freeway and naturally allow crossing the interchange in all directions (i.e., travel along the cross road and crossing the cross road from one side to the other). For underpass DDIs, placement of bridge columns, especially at a retrofit, may restrict the choice of the inner versus outer walkway. In general, pedestrian safety and comfort can be enhanced by reducing vehicle speeds, improving sight distances between drivers and pedestrians, and appropriately locating the crosswalks. At DDIs, the required channelized right- and left-turn lanes to and from the freeway present an opportunity for pedestrian-focused designs through the use of reduced curve radii and other geometric changes. Exhibit 3-4 and Exhibit 3-5 illustrate this concept for a DDI with an inner walkway and a DDI with outer facilities, respectively. The key concepts underlying pedestrian-focused design of DDIs include four primary principles: • Tighten vehicle curve radii to reduce speeds at the crosswalk. In research, lower vehicle speeds have been linked to increased driver yielding rates and lower risk of serious injury or death for the pedestrian in the event of a crash. • Provide adequate sight distance for vehicle approaches to crosswalks by locating the cross- walk in the tangent portion of the approach or in the beginning portions of the curve. A crosswalk located in the middle of a large swooping turn is difficult for drivers to see and react to. Improved vehicle sight distance also provides enhanced pedestrian sight distance for pedestrians to make adequate gap-crossing decisions at unsignalized crossings. • Provide one or more vehicle lengths of storage downstream of the crosswalks for yield- controlled vehicle movements. Similar to the crosswalk placement at roundabouts, this separates the driver decision points of yielding to pedestrians at the crosswalk and screening for gaps at the yield sign. It also prevents drivers who are waiting at the yield line from blocking the crosswalk with their vehicle. • Locate crosswalks downstream of the stop bar for signalized vehicle turns, consistent with driver and pedestrian expectations at signalized intersections. These configurations apply equally to DDIs with inner or outer pedestrian facilities. One key difference between the two is that the turn radius for the left turns to and from freeways can be selected independent of pedestrian considerations for the inner walkway because pedestrians do not cross these turn movements with an inner walkway DDI configuration. Exhibit 3-4. Pedestrian-focused DDI—Inner Walkway.

24 Diverging Diamond Interchange Informational Guide 3.2.1.1 Inner Walkways Exhibit 3-6 gives an example of a pedestrian facility located in the center of a DDI. While on- and off-ramp movements may be signalized or unsignalized, the exhibit shows movements that would time together if all ramp movements were signalized. For pedestrian movements across unsignalized ramp movements, geometric design or enhanced crossing treatment options may be incorporated to enhance pedestrian safety (discussed in Section 3.3.2). In the case of a DDI with inner walkways, pedestrian facilities to cross into the center can be co-located with these vehicle signals at crossover movements, and a pedestrian crossing phase can be provided with the concurrent vehicle phase. The right turns may be unsignalized or sig- nalized crossings and should, in either case, be configured in a way that promotes low vehicle speeds and good sight distance to the crosswalk. A pedestrian-vehicle conflict point exists anywhere pedestrian walkways and vehicular travel lanes cross. The pedestrian-vehicle conflict points for inner crossing DDIs are illustrated in Exhibit 3-7. The advantages and challenges of inner pedestrian facilities at the street crossing and along the walkway are summarized in Exhibit 3-8 and Exhibit 3-9, respectively. 3.2.1.2 Outer Walkways Exhibit 3-10 gives an example of an outer pedestrian walkway. While on- and off-ramp move- ments may be signalized or unsignalized, the exhibit shows movements that would run together if all ramp movements were signalized. For pedestrian movements across unsignalized ramp movements, geometric design or enhanced crossing treatment options may be incorpo- rated to enhance pedestrian safety (discussed in Section 3.3.2). In the case of outer walkways at DDIs, traversing pedestrians may cross four separate vehicle turning movements (the ramps). These may be free flowing or controlled. In some existing DDIs with outer walkways, no pedestrian crossing of the cross road is provided. This is an undesirable design that limits pedestrian access. Crossing opportunities should be provided. Otherwise, pedestrians can be expected to pursue their desired lines, often at considerable risk. Exhibit 3-5. Pedestrian-focused DDI—Outer Walkway.

Multimodal Considerations 25 Exhibit 3-6. Pedestrian movements given an inner walkway at a DDI. The top figure shows movements that time with the inbound crossover movements, while the bottom figure shows movements that time with the outbound crossover movements. Exhibit 3-7. Pedestrian-vehicle conflict point diagram for a DDI with an inner walkway.

26 Diverging Diamond Interchange Informational Guide Benefits Challenges + Crossing of the cross road naturally provided at DDI for full pedestrian access – Possible crossing of free-flow right- turn movements to/from freeway + Crossing one direction of traffic at a time – Pedestrians may not know to look to the right when crossing from center + No exposure to left turns to freeway (typically free flowing) – Wait at center median dictated by length of signal phase for through traffic + Protected signalized crossing to walkway – Pedestrian signals can conflict with vehicle signals at crossovers + Pedestrian clearance time generally provided in crossover signal phasing – Out of direction travel for pedestrians not desiring to cross the cross road + Pedestrian delay to center minimized by short cycles at two-phase signals Exhibit 3-8. Inner walkway pedestrian safety and comfort at street crossings. Benefits Challenges + Side walls provide a positive barrier between vehicular movements and pedestrians. (Walls low enough to avoid “tunnel” effect could have lesser impact on pedestrian comfort.) – Design of side walls must be managed to avoid impeding sight distance + Need for only one facility inside the intersection footprint (inner walkway) can offer more enhanced features within the same right-of-way constraints – Potential discomfort from moving vehicles on both sides of walkway – Potential challenge in placing all necessary signs and signal control equipment while maintaining full pedestrian access Exhibit 3-9. Inner walkway pedestrian safety and comfort along walkway segments. A pedestrian-vehicle conflict point exists anywhere pedestrian walkways and vehicular travel lanes cross. The pedestrian-vehicle conflict points for outer crossing DDIs are illustrated in Exhibit 3-11. Note the key conflict points denoted by asterisks (the freeway on-ramps). These conflict points may be controlled or uncontrolled conflict points, depending on the design of the inter- section. Vehicles at these conflict points are accelerating, so it is critical to consider how speeds and yielding rates can be controlled through design. The advantages and challenges of outer pedestrian facilities at the street crossing and along the walkway are summarized in Exhibit 3-12 and Exhibit 3-13, respectively.

Multimodal Considerations 27 Exhibit 3-10. Outer pedestrian facilities at a DDI. The top figure shows movements that time with the inbound crossover movements, while the bottom figure shows movements that time with the outbound crossover movements. Exhibit 3-11. Pedestrian-vehicle conflict point diagram for a DDI with an outer walkway.

28 Diverging Diamond Interchange Informational Guide 3.2.2 Bicycles Three basic facility options exist for bicyclists at a DDI. These options include providing: • Shared use of the travel lane, • A marked bicycle lane through the DDI, or • A separated bicycle path or shared-use path. One or more of these options may be viable for a project and would be determined in a Stage 1 intersection control evaluation (ICE) process as detailed in Chapter 1. FHWA’s Bikeway Benefits Challenges + Crossing one direction of traffic at a time – Crossing of free-flow right-turn movements to/from freeway + Ramp crossing distances are often shorter than through traffic crossing distance due to fewer travel lanes – Conflict with left turns to freeway (typically free flowing) where high vehicle speeds are likely (acceleration to freeway) – Potential sight obstruction of pedestrian crossing left turns from behind any structures – Pedestrians may not know which direction to look when crossing turn lanes – Unintuitive traffic directions to check when crossing out of the crossover – Providing signalized crossings requires more complicated timing and potential safety risks associated with motor vehicle queuing Exhibit 3-12. Outer path/sidewalk pedestrian safety and comfort at street crossings. Benefits Challenges + Extension of existing pedestrian network (natural placement on outside of lanes) – Need for widened structure on outside for overpass + Pedestrian typically has view of path ahead (depends on sight lines and obstructions) – Potential for additional right-of-way for underpass or construction of retaining wall under bridge + Walkway does not conflict with center bridge piers (at underpass) – Need for additional lighting for underpass + Opportunity to use right-of-way outside of bridge piers (at underpass) + Does not create potential tunnel effect that could make pedestrians feel “trapped” Exhibit 3-13. Outer path/sidewalk pedestrian safety and comfort along walkway segments.

Multimodal Considerations 29 Selection Guide (2) provides facility selection guidance with respect to motor vehicle volume and operating speed, as shown in Exhibit 3-14. A shared use of the travel lane should be reserved for conditions where vehicle operating speeds are below 25 mph and vehicle volumes are below 3,000 vehicles per day (vpd). Separated bicycle facilities are recommended where actual vehicle speeds exceed 35 mph or volumes exceed 7,000 vpd. Note that, as described in Chapter 5, operating speeds at DDIs are rarely at or below 25 mph, so a shared use of the travel lane is likely not an appropriate option. Low volume DDIs, such as those installed in rural areas for safety benefits, may be appropriate for on-street marked bike lanes; however, medium-to-high volume DDIs installed for safety and operational benefits will likely exceed the volumes appropriate for on-street facilities. In all cases, the signal timing should allow a bicyclist to complete all movements and phases and provide adequate clearance time for bicyclists to clear the intersection before releasing con- flicting traffic. Bicycle lanes at a DDI provide bicyclists with dedicated road space to travel across the inter- change. As shown in Exhibit 3-15, bicycle lanes should be located to the right of the travel lanes for motorized traffic, which is generally where bicyclists and motorists expect bicyclists to travel. A wider bike-appropriate shy distance is critical where the facility is next to a barrier such as between the crossovers where a center barrier wall is present. By providing the addi- tional width, it reduces the bicyclist’s feeling of being “trapped.” Green-colored pavement and/or dashed bicycle lane lines can be used to connect the solid bicycle lane lines at intersections. The bicycle lane should only be interrupted by stop bars at Exhibit 3-14. Bicycle facility selection guidance by motor vehicle volume and operating speed (2).

30 Diverging Diamond Interchange Informational Guide the signalized crossovers. The bicycle lane should continue through the off-ramps from freeway, where motorized traffic would generally be required to yield to cross road traffic, including bicyclists. Where bicycle lanes cross off-ramps, the use of colored pavement may increase bicyclist visibility to motorists. Exhibit 3-16 and Exhibit 3-17 show a DDI bicycle lane in Reno, Nevada, that highlights some of these features. An alternative to providing bicycle lanes through a DDI is to provide separated facilities. Where an inner path is possible, the inner walkway may instead be an inner shared-use path. Bicyclists crossing at the signalized intersections would either share a crossing with pedestrians or cross in adjacent crossings. A shared-use path option can be provided as part of an outer path option as well, as depicted in Exhibit 3-18. If separated facilities are not provided on the approach to the DDI, a ramp can transition bicyclists from the roadway to the separated path (see Section 3.4.1 for example). Note that this can be accomplished in advance or in conjunction with the development of vehicle right turns to avoid a conflict. For any shared-use path design, attention should be paid to locations where bicycle-pedestrian conflict points exist, such as at ramps onto and off of the roadway. A bicycle-vehicle conflict point exists anywhere bicyclist paths and vehicular travel lanes cross. Bicycle-vehicle conflict points for an inner bicycle path are presented in Exhibit 3-19. The asterisks represent conflict points that may be along accelerating vehicle paths, subject to the design of the intersection. Exhibit 3-15. Schematic for bicycle lane placement on right side of vehicular traffic.

Multimodal Considerations 31 Exhibit 3-16. Bicycle lane extending through inbound crossover (3). Exhibit 3-17. Bicycle lanes through the crossover (3). Exhibit 3-18. Outer shared-use path at DDI in Lexington, Kentucky (4).

32 Diverging Diamond Interchange Informational Guide 3.3 Pedestrian & Bicycle Assessment Research has documented vehicular safety benefits for DDIs. The reduction in vehicular crashes relative to a traditional diamond interchange is attributable to the separation of conflict points and the elimination of high-risk and high-severity crash patterns (e.g., angle crashes between left-turn and opposing through movements). For pedestrian and bicyclist safety, insufficient data are available to draw any conclusions regarding observed crash performance of DDIs. In the absence of quantitative crash data, an investigation of conflict points provides insight into the expected safety of DDIs. Common intersection and interchange design principles apply to pedestrian and bicycle facilities and can be assessed using the method presented in the following section. 3.3.1 Assessment Methodology NCHRP Project 07-25, “Guide for Pedestrian and Bicycle Safety at Alternative Intersections and Interchanges (A.I.I.),” developed an assessment method for bicycle and pedestrian facilities at intersections and interchanges. While a summary is provided below, full details can be found in NCHRP Research Report 948: Guide for Pedestrian and Bicycle Safety at Alter native Intersections and Interchanges (5). As a surrogate for quantitative performance measures, a series of performance measures— also known as design flags—can be used to assist in the identification of potential safety, acces- sibility, operational, or comfort issues for pedestrians and bicyclists. A design flag does not necessarily represent a fatal flaw for an alternative; rather, it presents a design issue that should be addressed in the iterative development and evaluation of the alternative. These design flags are not unique to DDIs, as they may also apply to traditional intersections and interchanges. Design flags generally apply to conflict points within the intersection rather than to segments. Outputs related to safety or accessibility are generally higher priority items that need to be addressed in design refinements. Outputs related to delay, travel time, and level of comfort are generally of lesser priority relative to safety and accessibility, suggesting items of concern but not necessarily fatal flaws in the design. Both levels of priority can be used to differentiate alternatives during the ICE process, and the relative balance of these levels of priority can be customized to the context of the location. Exhibit 3-20 summarizes all flags, including their applicability (pedes- trian versus bicyclist) and the measure of effectiveness associated with the flag. Exhibit 3-19. Bicycle-vehicle conflict point diagram for a DDI with an inner bicycle path.

Multimodal Considerations 33 3.3.2 Pedestrians—Key Safety Challenges DDIs present a reduced number of conflict points for pedestrians relative to a conventional intersection or interchange. At the same time, there are design factors inherent to a DDI that should be flagged as the design is developed. They are listed below. • Nonintuitive vehicle direction—A DDI often entails crossing traffic approaching from a potentially counterintuitive direction, which presents wayfinding challenges and risks for pedestrians. While each crossing conflicts with only a single direction of vehicle traffic, pedes- trians may not intuitively know in which direction to look for vehicles. Flag # Design Flag Bicycles Pedestrians Measure of Effectiveness 1 Motor Vehicle Right Turns X Vehicle Turning Speed and Vehicle Volume 2 Uncomfortable/Tight Walking Environment X Effective Walkway Width 3 Nonintuitive Motor Vehicle Movements X Vehicle Acceleration Profile 4 Crossing Yield-Controlled or Uncontrolled Vehicle Paths X X Vehicle Turning Speed and Vehicle Volume 5 Indirect Paths X X Out of Direction Travel Distance 6 Executing Unusual Movements X X Local Expectation 7 Multilane Crossings X X Number of Lanes without Refuge 8 Long Red Times X X Delay 9 Undefined Crossing at Intersections X X Path Markings 10 Motor Vehicle Left Turns X X Vehicle Turning Speed and Vehicle Volume 11 Intersection Driveways and Side Streets X X Count of Access Points in Area of Influence 12 Sight Distance for Gap Acceptance Movements X X Sight Distance 13 Grade Change X X % Grade 14 Riding in Mixed Traffic X Vehicle Speed and Vehicle Volume 15 Bicycle Clearance Times X Vehicle Speed and Clearance Zone Length 16 Lane Change Across Motor Vehicle Travel Lane X Vehicle Speed and Vehicle Volume 17 Channelized Lanes X Vehicle Speed and Channelization Length 18 Turning Motorists Crossing Bicycle Path X Motor Vehicle Lane Configuration 19 Riding between Travel Lanes, Lane Additions, or Lane Merges X Motor Vehicle Lane Configuration 20 Off-Tracking Trucks in Multilane Curves X Turn Angle Exhibit 3-20. Summary of design flags and measures of effectiveness.

34 Diverging Diamond Interchange Informational Guide • On-ramp movements—In general, the capacity benefits provided by unsignalized on-ramps at a DDI degrade the crossing environment for pedestrians and bicyclists on separated paths. While left turns are typically free flowing, the right turn is either free flowing with an acceleration lane (Exhibit 3-21) or yield controlled at the merge point with the left turn. Yield- controlled turns are likely to have slower vehicle speeds and would therefore be easier for pedestrians to cross than free-flowing turns. Both types of crossings can be improved by ensuring that roadway geometry manages motor vehicle speeds and that sufficient sight distance is provided. The decision of whether to control these vehicle movements should be weighed during the design process. Both movements can potentially be signalized, especially if an appropriate curve radius (intended to lower vehicular speeds) is difficult to obtain in design. Any potential queuing from signal control of these movements should be accounted for as part of the iterative design process. Exhibit 3-22 presents the most likely design flags that are applicable to pedestrians at DDIs. 3.3.3 Bicycles—Key Safety Challenges Bicyclists at DDIs face challenges that either create safety risks or stress for riding through the intersection. • Provision of space—The existing bicycle facilities at DDIs have consisted of traditional bike lanes to the right of motor vehicles, passing through the crossover and back. A design principle for bicyclists is to provide a relatively straight line of travel for bicyclists through an intersection (or at least to avoid abrupt turns). Given the speed difference between motorists and bicyclists through DDIs and the crossover section, as well as the curvature throughout the entire intersection, a bike lane width or buffer larger than the typical size would improve bicyclist safety and comfort. Alternatively, separated facilities would address this issue. Exhibit 3-21. On-ramp free right and left turns with acceleration lanes.

Multimodal Considerations 35 • On-ramp movements—If free-flow movements are provided for motorists to access freeway on-ramps, then an uncontrolled diverging conflict point is introduced between bicyclists and motorists (essentially a “right hook” opportunity). This is closely related to safety concerns with turning motorists crossing bicycle paths and should be treated as such for bicyclists in the design flag assessment. • Bicycle clearance time—An approaching bicyclist must judge if there is enough time to clear the intersection before a phase change. Usually, there are two options for bicyclists to assist in this decision. 1. If a pedestrian signal is present with a countdown, the bicyclist may get a better idea of when a signal phase is about to transition. Note that this is not the design intent of countdown timers, but they may be useful to the bicyclists. The decision to cross may lead to a bicyclist still being in the intersection when the phase changes. Design Flag Description Exhibit Motor Vehicle Right Turns This flag would apply if right-turn-on-red (RTOR) were permitted. Exhibit 3-23 Uncomfortable/Tight Walking Environment Pre-existing bridges, abutments, and piers may constrain the total facility width between the crossovers. Exhibit 3-24 Nonintuitive Motor Vehicle Movements Pedestrians crossing on the outside would confront consecutive crossings with vehicle traffic arriving from the same direction. Exhibit 3-25 Crossing Yield-Controlled or Uncontrolled Vehicle Movements Movements to and from on- and off- ramps may be yield controlled and therefore create additional stress and safety concerns for pedestrians. Exhibit 3-26 Indirect Paths Outer crossing designs should still allow for pedestrian crossing of the mainline. Exhibit 3-27 Executing Unusual Movements In most local contexts, pedestrians do not expect to cross into the mainline median to continue moving along the mainline road. Exhibit 3-28 Multilane Crossings Depending on the lane configuration, pedestrians may cross multiple lanes either when crossing an on- or off-ramp or when crossing into the median. Exhibit 3-29 Motor Vehicle Left Turns This flag would apply if left-turn-on-red were permitted. Exhibit 3-30 Sight Distance for Gap Acceptance Movements At yield-controlled movements, vertical and horizontal alignments should allow for proper sight distance to pedestrian crossings. Exhibit 3-31 Grade Change Vertical curves on bridges may create challenges for pedestrians with mobility challenges or those carrying or pushing objects. Exhibit 3-32 Exhibit 3-22. Design flags applicable to pedestrians at DDIs.

36 Diverging Diamond Interchange Informational Guide Exhibit 3-23. Motor vehicle right turns design flag. Exhibit 3-24. Uncomfortable/tight walking environment design flag.

Exhibit 3-25. Nonintuitive motor vehicle movements design flag. Exhibit 3-26. Crossing yield-controlled or uncontrolled vehicle movements design flag. Exhibit 3-27. Indirect path design flag.

38 Diverging Diamond Interchange Informational Guide Exhibit 3-28. Executing unusual movements design flag. Exhibit 3-29. Multilane crossings design flag. 2. At many signals, a bicyclist will only be able to rely on the yellow clearance phase for vehicular traffic. This yellow clearance phase is designed just for drivers and generally does not exceed 5 seconds. This clearance time is almost always too short for bicyclists to clear the intersections. The disparity is exacerbated as clearance lengths increase. When signal heads are placed upstream of the intersection, bicyclists have no indication of whether the signal phase has changed once they have passed the signal heads. In consideration of the bicyclist conflict points at DDIs and the key safety challenges that accompany them, Exhibit 3-33 presents the design flags that are most likely applicable to bicyclists at DDIs.

Multimodal Considerations 39 Exhibit 3-30. Motor vehicle left turns design flag. Exhibit 3-31. Sight distance for gap acceptance movements design flag.

40 Diverging Diamond Interchange Informational Guide Exhibit 3-32. Grade change design flag. Design Flag Description Exhibit Executing Unusual Movements Similar to motor vehicles, bicyclists do not expect to cross to the left side of the road when crossing the DDI. Exhibit 3-28 Multilane Crossings Large DDIs may induce bicycles to cross a significant number of lanes at the crossover. Exhibit 3-29 Undefined Crossings at Intersections While not unique to DDIs, a lack of bicycle markings crossing ramp merge or diverge points may result in vehicles impeding upon the bicycle lane, particularly if RTOR is permitted. Exhibit 3-34 Grade Change Vertical curves on bridges may create challenges for bicycles trying to maintain speed, resulting in a large speed differential with vehicles. Exhibit 3-32 Riding in Mixed Traffic While not unique to DDIs, vehicle speeds near interchanges may be significant, especially if curves have large design radii. Exhibit 3-35 Bicycle Clearance Time Long distances between the crossover and right- turn off-ramp movement may result in conflict between bicyclists and right-turning vehicles. Exhibit 3-36 Turning Motorists Crossing Bicycle Path A high volume of turns onto the on-ramp may exacerbate vehicle-bicycle conflicts. Exhibit 3-37 Riding between Travel Lanes, Lane Additions, or Lane Merges Off-ramp designs with lane additions or downstream merges create additional stress and safety concerns for bicyclists traveling along the mainline. Exhibit 3-38 Off-Tracking Trucks in Multilane Curves Trucks moving through the crossover intersections may off-track into adjacent bicycle lanes. Exhibit 3-39 Exhibit 3-33. Design flags applicable to bicyclists at DDIs.

Multimodal Considerations 41 Exhibit 3-34. Undefined crossing at intersection design flag. Exhibit 3-35. Riding in mixed traffic design flag.

42 Diverging Diamond Interchange Informational Guide Exhibit 3-36. Bicycle clearance time design flag. Exhibit 3-37. Turning motorist crossing bicycle path design flag.

Multimodal Considerations 43 Riding between motor vehicle lanes creates safety concerns. Exhibit 3-38. Riding between travel lanes, lane additions, and lane merges design flag. Exhibit 3-39. Off-tracking trucks in multilane curves design flag.

44 Diverging Diamond Interchange Informational Guide 3.4 Intersection-Level Concepts Three design concepts are presented in this section to offer techniques for improving pedes- trian and bicycle safety and operational performance of DDIs. These concepts are not suggested as designs to be replicated as-is; rather, they illustrate the DDI options that are possible in various contexts. Note that these concepts mix design approaches. The designer must consider traffic volume and speed when matching designs and treatments to the appropriate context. Following each concept is a discussion of the flags remaining with the given design—in other words, the flags not obviated by the design that would still need to be addressed. The designs include the following: • Shared-use path/inner walkway concept, • On-street bike lane/outer walkway concept, and • Separated bike lane/inner walkway concept. Section 3.3 presents other key design flags that would be subject to site-specific concerns and are not obviously presented or addressed with any of the concepts presented below. 3.4.1 Shared-Use Path/Inner Walkway Concept The first DDI concept, shown in Exhibit 3-40, includes a shared-use path located inside the median. Bicycle access to the shared-use path is provided upstream of the DDI through the use of bicycle ramps. Downstream of the DDI, ramps return bicyclists to the roadway. The concept would be appropriate for intersections where heavy vehicle movement through the crossover may result in truck off-tracking through the crossovers. The inner shared-use path allows for free-flowing left turns onto the freeway without pedestrian or bicycle conflicts. Benefits—This design addresses the following key elements with respect to safety and comfort: • Nonintuitive motor vehicle movements design flag—By providing an inner walkway, pedes- trians cross motor vehicle movements in an alternating fashion. Vehicles arrive from the left at the first crossing, from the right at the next crossing, and so on. This conforms with the typical expectation. • Indirect path design flag—Pedestrians and bicyclists can cross the mainline by proceeding from one side of the roadway, into the center median, and then to the opposing side of the roadway. This avoids the need for pedestrians to travel to an adjacent intersection to cross the mainline. This flag may still apply for other movements as described below. Exhibit 3-40. DDI shared-use path/inner walkway design concept.

Multimodal Considerations 45 • Undefined crossings at intersections design flag—All locations where pedestrians and bicyclists cross motor vehicles are marked. This will reduce the likelihood that vehicles will encroach on the crossing areas when stopped for a signal. • Motor vehicle left turns design flag—The inner walkway design removes pedestrians from crossing the left turn at the off-ramp. • Riding in mixed traffic design flag—This design features a bike ramp off the roadway and onto the shared-use path. Bicyclists are provided a less stressful and safer path through the interchange. • Bicycle clearance time design flag—By removing bicyclists from the roadway, the bicyclists no longer must travel between the crossover intersection and the right turn from the off- ramp. This eliminates the need for additional clearance time otherwise needed for bicyclists to cover this distance. • Turning motorists crossing bicycle path design flag—The bicycle ramp to the shared-use path is placed upstream of the development of the right-turn lane onto the on-ramp. This eliminates the conflict of motorists crossing the bicycle path. • Off-tracking trucks in multilane curves design flag—Heavy vehicles may experience chal- lenges maintaining their lane when traveling through the crossover intersections. By providing an off-street shared-use path for bicyclists, the users are separated in space, avoiding the potential conflict. Challenges—Emphasizing that the design is not intended to be “ready-made,” this concept leaves several design flags as described in Exhibit 3-41. 3.4.2 On-Street Bike Lane/Outer Walkway Concept The second DDI concept, shown in Exhibit 3-42, features an outer walkway for pedestrians and on-street bicycle lanes. This concept could be implemented at locations where bridge piers or other objects in the median make an inner walkway difficult, or where local preference is to remain on the outside of the interchange. As on-street bicycle lanes are present, the design is best suited where geometric elements create a low speed environment. Benefits—This design addresses the following key elements with respect to safety and comfort: • Indirect path design flag—Pedestrians can cross the mainline by proceeding from one side of the roadway, into the center median, and then to the opposing side of the roadway. This avoids the need for pedestrians to travel to an adjacent intersection to cross the mainline. • Executing unusual movements design flag—By providing an outer walkway, pedestrians desiring to continue along the mainline can do so without crossing any mainline movement. This flag still applies to bicyclists (see challenges below). • Undefined crossings at intersections design flag—All locations where pedestrians and bicyclists cross motor vehicles are marked. This will reduce the likelihood that vehicles will encroach on the crossing areas when stopped for a signal. Challenges—Emphasizing again that the design is not intended to be “ready-made,” this concept leaves several design flags as described in Exhibit 3-43. 3.4.3 Separated Bike Lane/Inner Walkway Concept The final DDI concept, shown in Exhibit 3-44, provides a separated bike lane through the intersection located within the median between the crossovers as well as an inner walkway for pedestrians. The presence of the pedestrian walkway inside the separated bike lanes provides addi- tional separation for pedestrians from motor vehicles while moving through the median. For less confident bicyclists, the separation from both pedestrians and motor vehicles reduces stress.

46 Diverging Diamond Interchange Informational Guide Design Flag Description Motor Vehicle Right Turns DDIs tend to have high volumes of turns to and from the freeway. This design still requires pedestrians to cross the right turn from the off-ramp. This flag can be mitigated by prohibiting RTOR or by designing the curve radii to keep speeds from exceeding 25 mph. Further, the pedestrian yield point and the merge point should be separated in space. Uncomfortable/Tight Walking Environment Careful design of the inner walkway is necessary as the design progresses to ensure it provides adequate space for all users. Crossing Yield-Controlled or Uncontrolled Vehicle Movements This design has a yield-controlled right turn onto the on-ramp. Given the expected high volume of turns onto the on-ramp, the curve radii should be designed to keep vehicle speeds from exceeding 25 mph. This will mitigate the flag by increasing the likelihood of vehicles yielding to pedestrians. Indirect Path Users desiring to continue along the mainline may experience enough out of direction travel crossing to the median and back that the indirect path flag would apply. Executing Unusual Movements In most areas, DDIs are a relatively novel design. Users desiring to continue along the mainline likely do not expect to need to cross one direction of mainline traffic. Proper wayfinding design will be especially important for all users to understand how to execute their desired path. Multilane Crossings At the crossovers of this design, pedestrians must cross multiple lanes without refuge (the yellow flag threshold for pedestrians is 2–3 lanes). Sight Distance for Gap Acceptance Movements The motor vehicle right turns to the on-ramp are yield controlled and therefore require careful attention to sight distance requirements. In this design, the position of the crosswalk upstream of the center of the curve would likely provide adequate sight distance. Grade Change While not able to be evaluated from the plan view provided, interchanges can experience grade changes. This should be evaluated during the flag assessment process. Exhibit 3-41. Summary of design flags remaining with DDI shared-use path/ inner walkway design concept. Exhibit 3-42. DDI on-street bike lane/outer walkway design concept.

Design Flag Description Motor Vehicle Right Turns DDIs tend to have high volumes of turns to and from the freeway. This design still requires pedestrians to cross the right turn from the off-ramp. This flag can be mitigated by prohibiting RTOR or by designing the curve radii to keep speeds from exceeding 25 mph. Nonintuitive Motor Vehicle Movements Shared-use path users following the outer walkway will encounter motor vehicles approaching from the same direction twice in a row. This deviates from the typical expectation of encountering vehicles from alternating directions. Crossing Yield- Controlled or Uncontrolled Vehicle Movements This design has a yield-controlled right turn onto the on-ramp. Given the expected high volume of turns onto the on-ramp, the curve radii should be designed to keep vehicle speeds from exceeding 25 mph. This will mitigate the flag by increasing the likelihood of vehicles yielding to pedestrians. Executing Unusual Movements In most areas, DDIs are a relatively novel design. Users desiring to continue along the mainline likely do not expect to need to cross one direction of mainline traffic. Proper wayfinding design will be especially important for all users to understand how to execute their desired path. Multilane Crossings At the crossovers of this design, pedestrians must cross multiple lanes without refuge (the yellow flag threshold for pedestrians is 2–3 lanes). Sight Distance for Gap Acceptance Movements The motor vehicle right turns to the on-ramp are yield controlled and therefore require careful attention to sight distance requirements. In this design, the position of the crosswalk upstream of the center of the curve would likely provide adequate sight distance. Grade Change While not able to be evaluated from the plan view provided, interchanges can experience grade changes. This should be evaluated during the flag assessment process. Riding in Mixed Traffic Given the expected high volume of vehicles on an interchange, this flag must be mitigated through vehicle speed control. Geometric design should be employed to reduce speeds below 25 mph (yellow flag threshold). Bicycle Clearance Time Bicyclists need sufficient time to travel through the crossover and continue through the right turn from the off-ramp. Without signal timing details, it is not possible to determine the applicability of this flag to the design. Turning Motorists Crossing Bicycle Lanes Vehicles turning right to the on-ramp must cross the bicycle lane, resulting in a conflict. Riding between Travel Lanes, Lane Additions, or Lane Merges The addition of the right-turn pocket onto the on-ramp results in bicyclists riding between the through and right-turn lanes for an extended period. Off-Tracking Trucks in Multilane Curves Bicycles moving through the crossover may be impeded by heavy vehicles off- tracking. Careful design of lane widths and curve radii may mitigate this issue. Exhibit 3-43. Summary of design flags remaining with DDI on-street bike lane/ outer walkway design concept. Exhibit 3-44. DDI separated bike lane/inner walkway design concept.

48 Diverging Diamond Interchange Informational Guide Benefits—This design addresses the following key elements with respect to safety and comfort: • Uncomfortable/tight walking environment design flag—This design provides an exclu- sive path for pedestrians separate from both bicyclists and motor vehicles. By placing motor vehicles on either side of pedestrians in the inner walkway, pedestrians are further removed from motor vehicles. Care should be taken in design to ensure proper space is provided for pedestrians to move past each other without encroaching on the bicycle lanes. • Nonintuitive motor vehicle movements design flag—By providing an inner walkway, pedestrians cross motor vehicle movements in an alternating fashion. Vehicles arrive from the left at the first crossing, from the right at the next crossing, and so on. This conforms with the typical expectation. • Indirect path design flag—Pedestrians and bicyclists can cross the mainline by proceeding from one side of the roadway, into the center median, and then to the opposing side of the roadway. This avoids the need for pedestrians to travel to an adjacent intersection to cross the mainline. This flag may still apply for other movements as described below. • Undefined crossings at intersections design flag—All locations where pedestrians and bicyclists cross motor vehicles are marked. This will reduce the likelihood that vehicles will encroach on the crossing areas when stopped for a signal. • Motor vehicle left turns design flag—The inner walkway design removes pedestrians from crossing the left turn at the off-ramp. • Riding in mixed traffic design flag—This design features a bike ramp off the roadway and onto the shared-use path. Bicyclists are provided a less stressful and safer path through the interchange. • Bicycle clearance time design flag—By removing bicyclists from the roadway, the bicy- clists no longer must travel between the crossover intersection and the right turn from the off-ramp. This eliminates the need for additional clearance time otherwise needed for bicy- clists to cover this distance. • Turning motorists crossing bicycle path design flag—The bicycle ramp to the shared-use path is placed upstream of the development of the right-turn lane. This eliminates the conflict of motorists crossing the bicycle path. • Off-tracking trucks in multilane curves design flag—Heavy vehicles may experience chal- lenges in maintaining their lane when traveling through the crossover intersections. By pro- viding an off-street shared-use path for bicyclists, the users are separated in space, avoiding the potential conflict. Challenges—Emphasizing again that the design is not intended to be “ready-made,” this concept leaves several design flags as described in Exhibit 3-45. 3.5 Detailed Design Techniques Design flags are a means of identifying areas of an alternative that may create challenges for pedestrians or bicycles. This section addresses design responses to common DDI challenges to assist in addressing flags. This includes: • Pedestrian phase coordination, • Pedestrian channelization and wayfinding, • ADA and accessibility, • Channelized turn lanes, • Vehicle movements from counterintuitive directions, and • Indirect paths.

Multimodal Considerations 49 Design Flag Description Motor Vehicle Right Turns DDIs tend to have high volumes of turns to and from the freeway. This design still requires pedestrians to cross the right turn from the off- ramp. This flag can be mitigated by prohibiting RTOR or by designing the curve radii to keep speeds from exceeding 25 mph. Crossing Yield- Controlled or Uncontrolled Vehicle Movements This design has a yield-controlled right turn onto the on-ramp. Given the expected high volume of turns onto the on-ramp, the curve radii should be designed to keep vehicle speeds from exceeding 25 mph. This will mitigate the flag by increasing the likelihood of vehicles yielding to pedestrians. Indirect Path Users desiring to continue along the mainline may experience enough out of direction travel crossing to the median and back that the indirect path flag would apply. Executing Unusual Movements In most areas, DDIs are a relatively novel design. Users desiring to continue along the mainline likely do not expect to need to cross one direction of mainline traffic. Proper wayfinding design will be especially important for all users to understand how to execute their desired path. Multilane Crossings At the crossovers of this design, pedestrians must cross multiple lanes without refuge (the yellow flag threshold for pedestrians is 2–3 lanes). Sight Distance for Gap Acceptance Movements The motor vehicle right turns to the on-ramp are yield controlled and therefore require careful attention to sight distance requirements. In this design, the position of the crosswalk upstream of the center of the curve would likely provide adequate sight distance. Grade Change While not able to be evaluated from the plan view provided, interchanges can experience grade changes. This should be evaluated during the flag assessment process. Exhibit 3-45. Summary of design flags remaining with DDI separated bike lane/ inner walkway design concept. Design techniques for specific design flags can be found in NCHRP Research Report 948: Guide for Pedestrian and Bicycle Safety at Alternative Intersections and Interchanges (forthcoming) (5). 3.5.1 Pedestrian Phase Coordination With an inner walkway between the crossovers, the designer has the ability to time pedestrian crossings to match crossing behavior. Because the right turn onto a freeway on-ramp does not require coordination with any other phase at the DDI, the signal timing can progress the pedestrian movement across the right-turn on-ramp (if the movement is signalized) and then across the crossover movement or vice versa. The geometry of the freeway off-ramps affects pedestrian crossing delay as well. Where an inner walkway is provided, the right-turn off-ramp and adjacent crossover crossing can operate concurrently (Exhibit 3-46). Depending on the distance between crossings and the phase length, a pedestrian may be able to cross both movements in a single signal phase. A design that keeps these crossings as close as possible would promote that sequential crossing (6). Where an outer walkway is provided, the right-turn and left-turn off-ramp movements will not run concurrently (Exhibit 3-10). Thus, the distance of these crossings from one another is of less importance for pedestrian progression: pedestrians either walk slightly farther or are likely waiting for the signal phase to change (6).

50 Diverging Diamond Interchange Informational Guide Exhibit 3-46. Back-to-back off-ramp crossings during the same signal phase. 3.5.2 Pedestrian Channelization and Wayfinding Because the DDI crossover is unusual for drivers, human factors considerations are emphasized throughout the design process. Human factors considerations also apply to the pedestrian environ- ment, which is different from what pedestrians are used to at conventional interchanges. With center walkways, crossing to the center of the street is unusual compared to the typical hierarchy of street design in which pedestrian facilities are placed outside of vehicular traffic. This hierarchy is satisfied for outside walkways, but pedestrian discomfort can arise as pedestrians have to cross a freeway bridge or walk through an underpass adjacent to vehicular traffic. Depending on the design vehicle and speed of the DDI, the channelization islands separating the right- and left-turning movements can be quite large, and pedestrians need clear guidance and information on where they should and should not walk and where they should and should not cross. Cut-through island designs can be used to provide positive guidance to pedestrians as to where walkway and crossing locations are provided. A cut-through walkway can guide the pedestrian directly to the intended crossing point and can be angled to support pedestrians in viewing oncoming vehicular traffic and potential conflicts. The channelization islands at DDIs provide the opportunity for wide walkways. The cut-through walkway should be at least 8 feet wide to comfortably accommodate pedestrians, including those with wheel- chairs and other mobility devices. The actual curb ramp landing should be aligned perpen- dicular to the street centerline, which minimizes crossing distance and orients pedestrians to access ramps.

Multimodal Considerations 51 Exhibit 3-47. Channelization toward center crosswalk (4). Exhibit 3-48. Channelization toward outside crosswalk (4). As an alternative to the cut-through design, landscaping (grass or gravel) can be used to define the boundaries of the pedestrian walkway and give pedestrians a sense of place in what can be very large channelization islands. Examples of cut-through and landscaping designs are provided in Exhibit 3-47 and Exhibit 3-48, respectively. 3.5.3 ADA and Accessibility Accessibility was previously described in Chapter 2 in the broader contexts of considering a project’s contextual environment and the ability for various users to approach a desired desti- nation or potential opportunity for activity using highways and streets (including the sidewalks and/or bicycle lanes provided within those rights-of-way). The basic principles for accessible design can be divided into the pedestrian walkway and the pedestrian crossing location. For pedestrian walkways, the following considerations apply: • Delineate the walkway through landscaping, curbing, or fencing to assist with wayfinding for pedestrians with visual impairments. Examples of wayfinding provisions are shown in Exhibit 3-49 and Exhibit 3-50. Note the use of fencing under the bridge structure where landscaping is more difficult to maintain. • Provide sufficient space (length and width) and recommended slope rates for wheelchair users and other nonmotorized users, such as people pushing strollers, walking bicycles, and others.

52 Diverging Diamond Interchange Informational Guide Exhibit 3-49. Example of pedestrian wayfinding provision at DDI via curbing (4). Exhibit 3-50. Example of pedestrian wayfinding provision at DDI using an urban fence (4). • Construct an appropriate landing with flat slope and sufficient size at crossing points. For pedestrian crossing locations, these additional considerations apply: • Provide curb ramps and detectable warning surfaces at the end of curb ramp and transition to the street. • Align the curb ramp landing to the intended crossing direction. • Crosswalk width through the intersection should be wide enough to permit pedestrians and wheelchairs to pass without delay from opposing directions, and the medians should provide sufficient storage for all nonmotorized users to safely wait when two-stage crossings are required. • Pedestrians with vision, mobility, or cognitive impairments may benefit from targeted outreach and additional informational material created with these specific users in mind. These outreach materials should include information on crosswalk placement and intended behavior, as well as answers to frequently asked questions. For pedestrians with visual impair- ments, materials need to be presented in an accessible format with a sufficient description of all features of the DDI crossing.

Multimodal Considerations 53 • Provide audible speech messages to communicate directionality of traffic (from left or from right) at all crossing points. Audible speech messages should be used where spacing between accessible pedestrian signal (APS) devices is less than 10 feet or where additional narrative for the expected direction of traffic is needed (i.e., “traffic from left” or “traffic from right”). • Provide accessible pedestrian signals with push-button locator tones at signalized crossings. • Locate push buttons to be accessible by wheelchairs and adjacent to the crossing with a minimum separation of 10 feet. At the DDI, locating the APS may pose a challenge on the median island for pedestrian facilities located in the center. Exhibit 3-51 shows an example of an undesirable pedestrian push-button installation with the push button for the two directions on the same pole. The lack of separation may make it difficult for pedestrians (especially those with vision disabilities) to distinguish which push button is intended for which crossing. Further, the example shown does not provide APS devices or any audible information about the crossing. Given that the nose of the median island does not provide adequate room to allow for the pedestrian push buttons to be on separate poles and sufficiently separated, it is recommended that the pedestrian push buttons in the median be separated diagonally, as shown in Exhibit 3-52. Locating the push buttons downstream of the crosswalk provides audible separation between the APS messages and the oncoming traffic for which visually impaired pedestrians are listening. In general, wider islands are strongly recommended to provide a true refuge area of at least 6 feet in the direction of pedestrian travel. This ensures a minimum of 2 feet between the detectable warning surfaces and adequate storage for wheelchair users. If the two APS devices are less than 10 feet apart, speech messages with customized wording specific to the DDI are required to be played after activating the push button. During the “Wait” interval, one potential wording [for speech push-button information message, see MUTCD 4E.13, par 9 & 10 (7)] may be: “Wait to cross eastbound lanes Airport Road at Highway 26. Traffic coming from your left.” During the “Walk” interval, the message would be: “Eastbound lanes Airport Road; walk sign is on to cross eastbound lanes Airport Road.” An expert in Exhibit 3-51. Undesirable use of single pole with two pedestrian push buttons, no APS, and insufficient separation of the two detectable warning surfaces (4).

54 Diverging Diamond Interchange Informational Guide accessibility installations may need to be consulted for specialized applications and signal installations at a DDI to ensure that the crossings are accessible to and usable by all pedestrians as required by ADA. 3.5.4 Turning Movements at Ramps Channelization of all turns to and from the freeway is used to discourage wrong-way maneu- vers and to move ramp terminal intersections away from the crossover intersection. Channel- ization, especially for unsignalized turns, could create pedestrian safety concerns due to the potential for high speeds and sight distance limitations. Many DDI designs have been associated with upgrades to pedestrian facilities and/or shared-use paths, and best practices for the design of these facilities are still developing. Considerations include location of the pedestrian facilities (outer versus inner), unintuitive traffic directions for crossing, radius and speed of turning movements, and whether to use a form of signalization at on-ramps. Attention to the placement, visibility, and vehicular speeds is necessary when considering the pedestrian crossing the on-ramp where speeds are higher and where there may be limited sight distance. This applies to the right-turn on-ramp regardless of the pedestrian facility selected and to the left-turn on-ramp if pedestrian facilities are placed on the outside. Exhibit 3-53 shows one example of a left-turn crossing, viewed from the raised island and in line with one of the two crossing points for the unsignalized left turn onto the freeway. The image shows an exclusive left-turn lane approaching the crosswalk, with motor vehicles accelerating toward the freeway on-ramp. The exhibit further shows potential visibility limita- tions for pedestrians crossing from the bridge to exit the DDI (i.e., toward the photographer). The waiting area is obscured by a shadow in the photo, but even at other times of day, the line of sight between the waiting position and the approaching truck in the left-turn lane is Exhibit 3-52. DDI splitter island with diagonal pedestrian signals.

Multimodal Considerations 55 Exhibit 3-53. Example of pedestrian crossing at free-flow left onto freeway (8). partially obstructed by the barrier wall on the bridge structure. Free-flowing vehicle movements, elevated speeds, acceleration, and insufficient sight distance can contribute to low yielding and an increased chance of conflicts at these crossing locations. To overcome visibility and sight distance challenges, several potential treatments could be considered, including: • Revising the left-turn geometry toward a pedestrian-focused DDI design with reduced turn radii, reduced vehicle speeds, and improved sight distances, as described above. • Relocating the crosswalk to farther upstream in the turn-lane for improved sight distance, which may require a slightly longer crossing. • Adding raised crosswalks or other geometric modifications to control vehicular speeds in the vicinity of the crosswalk. • Installing signalization with standard signal heads, rectangular rapid-flashing beacons (RRFBs), or pedestrian hybrid beacons (PHBs) to alert drivers of the presence and crossing intent of a pedestrian. • Providing a pedestrian-activated signal to supply a crossing opportunity with a steady red phase for vehicular traffic. • Moving pedestrian facilities to inside the median (resolves left-turn movements, but right- turn movements may still need additional treatment). While this discussion has focused on the channelized left turn to the freeway, similar consider- ations should be applied to channelized right-turn lanes to and from the freeway, as well as to the channelized left turn from the freeway (if yield controlled). Relative to other intersection forms, sight distance is a challenge at DDIs. 3.5.5 Vehicle Movements from Counterintuitive Directions Regardless of whether inner or outer pedestrian facilities are provided, a unique issue with pedestrian crossings at DDIs is the propensity to look the wrong direction for gaps in traffic. One treatment that could be considered, in addition to supplemental signing and/or speech messages used with APS devices, is an embedded pavement marking, such as the one shown in Exhibit 3-54. Other marking possibilities that would be more accessible to pedestrians who cannot read English or have vision impairments could also be explored. This treatment could be helpful to pedestrians and is relatively inexpensive to install and maintain. The installation process may require that a small section of pavement is removed prior to marking installation. This provides protection against snowplows and wheel friction, which reduces the marking’s maintenance needs.

56 Diverging Diamond Interchange Informational Guide 3.5.6 Indirect Paths DDI design and signal phasing provide a natural time and location for pedestrian access across the mainline. Yet, some existing DDIs with outer walkways do not include provisions for pedestrians to cross the cross road at a DDI (Exhibit 3-55). If a pedestrian needs to cross the cross road, they are expected to travel to the next intersection to do so. This contradicts the design objectives to provide access, reasonable comfort, and acceptable travel time to all pedestrians. It can be expected that a person will find a desire line to cross, even if it is at the expense of safety, rather than detour to the next intersection and back for a street crossing. Crossings should be provided and can be accomplished with outer walkways. Concepts in Section 3.4.2 demonstrate how this can be reasonably achieved. 3.6 Transit 3.6.1 Buses on Cross Road Buses operating on the cross road will benefit from the reduced number of signal phases and potentially lower delay when traveling through a DDI. In general, bus stops are not placed within interchanges unless there is transit service within the freeway corridor that stops at the interchange. Exhibit 3-54. Engraved pedestrian pavement marking—“Look Left” (4). Exhibit 3-55. DDI with outer walkway and no arterial crossing opportunities.

Multimodal Considerations 57 3.6.2 Transit on Freeway If rail transit or a busway is located within the freeway right-of-way and a station is provided at the interchange, a wider median on the cross road would allow buses on the cross road to stop directly above or below the station. This would facilitate passenger transfers, with pas- sengers boarding and alighting directly from a median walkway with an elevator and stairs connecting the walkway to the freeway station platform. The added median width would pro- vide room for both the elevator and stairs and for bus pullouts on the far side of the station access points to serve the bus stops. Another transit possibility is that express bus service is provided on the freeway and transit service is also provided on the cross road, with a transfer between the two services. At a con- ventional diamond interchange, the typical approach would be to have the express bus service exit using the off-ramp, cross the cross road at the ramp terminal intersection, and serve a far-side stop before re-entering the freeway via the on-ramp. Because a DDI does not support through movements between off- and on-ramps, an alternative configuration would be needed. One option would be a “freeway flyer” stop where a bus-only collector-distributor road is provided adjacent to the freeway between the ramps, providing a safe location for buses to stop and serve passengers. Transfers between the freeway express and the cross road transit service would be facilitated via the inner walkway in much the same manner as described previously. 3.6.3 Light-Rail on Cross Road Median-running light-rail can remain in the median and go “straight” at the crossover inter- sections. Light-rail remaining in the median would be served by a dedicated signal phase at crossover intersections. Extra clearance between the light-rail tracks and the adjacent travel lane may be required to allow for a train’s dynamic envelope. Some form of positive separation may be desirable to discourage vehicles from encroaching onto the tracks when going through the curves at the crossover points. An example of a median-running light-rail line through a DDI is shown in Exhibit 3-56. This light-rail line remains in the median through the DDI and has a dedicated (preemptive) signal phase to clear the main two crossover intersections of vehicles at both crossovers. Geometric design considerations for a DDI with a center-running light-rail include: • Crossover requires longer tangent length to clear rail line on straight trajectory. • Spacing of the crossovers needs to be long enough to store the entire length of the train, unless signal phasing allows the light-rail to cross the entire DDI without stopping. Exhibit 3-56. Aerial view of DDI with center-running light-rail at I-494 and 34th Avenue in Bloomington, Minnesota (9).

58 Diverging Diamond Interchange Informational Guide • Outer pedestrian facilities or two inner walkways (with the light-rail tracks in the middle) need to be provided. • Pedestrian crossings of the cross road are possible, but placement of a median refuge area may be challenging due to the presence of the light-rail tracks. • Drivers may be more likely to feel they are on the “wrong” side of the road when they see train movements to their right when between crossover intersections. Traffic signal considerations for a DDI with center-running light-rail include: • Additional movements to serve light-rail line through the crossover. These movements can be served in a phase concurrent with both right and left turns from the freeway. • Signal stop bars moved further away from crossover to prevent drivers from stopping too close to the light-rail line. • Advance pavement markings and signing alerting drivers of the rail crossing. • Potential use of supplemental blank-out sign displaying a train graphic and message, such as “Light-Rail Crossing,” when a train is approaching. 3.7 References 1. 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. 2. Schultheiss, B., D. Goodman, L. Blackburn, A. Wood, D. Reed, and M. Elbech. Bikeway Selection Guide. FHWA, U.S. Department of Transportation, 2019. 3. Lee, K. Photo Credit. 4. Google Inc. Google Earth. Accessed July 2014. 5. Kittelson & Associates, Institute for Transportation Research and Education, Toole Design Group, Accessible Design for the Blind, and ATS Americas. NCHRP Research Report 948: Guide for Pedestrian and Bicycle Safety at Alternative Intersections and Interchanges. Transportation Research Board, Washington, D.C. 6. Chlewicki, G. “Improving Pedestrian Operations at Innovative Geometric Designs.” Presented at the 5th Urban Street Symposium, Raleigh, NC, 2017. 7. Manual on Uniform Traffic Control Devices. FHWA, U.S. Department of Transportation, 2009. 8. Daleiden, A. Photo Credit. 9. MAC and Kimley-Horn and Associates, Inc. Photo Credit.