Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges (2021)

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9-1 9.1 Introduction The diverging diamond interchange (DDI) is an alternative to the conventional diamond interchange or other alternative interchange forms. The primary difference between a DDI and a conventional diamond interchange is the design of directional crossovers on either side of the interchange. This eliminates the need for left-turning vehicles to cross the paths of approaching through vehicles. By shifting cross-street traffic to the left side of the street between the signalized crossover intersections, vehicles on the crossroad making a left-turn onto freeway on-ramps or from freeway off-ramps no longer conflict with opposing through vehicles. The DDI design reduces the number of vehicle-to-vehicle conflict points. It has been shown to improve the operations of turning movements to and from the freeway facility and to reduce the number of vehicle-to-vehicle crashes compared to a conventional diamond interchange by 24% to 45% (1, 2, 3, 4, 5). The DDI also reduces the severity of such crashes, because conflicts between left-turning movements and the opposing through movement are eliminated. However, no quantitative factors have been developed for pedestrian- or bicycle-related safety. Exhibit 9-1 provides an example of a DDI and highlights the key features of this interchange design (1). The street segment between the crossovers can be designed as an underpass or overpass, depending on the site characteristics. The interchange design will be directly affected by whether the arterial passes over or under the limited access facility. The majority of DDIs evaluated have reconstructed existing diamond interchanges, and the decision to go over or under the limited access facility had already been determined. Pedestrian movements at DDIs can either be on the inside of (between) travel lanes or the outside (perimeter), with crossings being signalized or uncontrolled. Bicycle movements tradi- tionally have been accommodated with shared use of the travel lane or through a shared-use path facility, but DDI design features can pose safety challenges for nonmotorized modes and need to be evaluated carefully as part of the design process. This chapter conveys the geometric, operations, and safety considerations, as well as design flags, specific to a DDI design. In light of these characteristics, this chapter also provides tech- niques and treatments at a DDI to help meet the design objectives presented in this guide: maxi- mizing safety, providing access and accessibility, managing delay and travel time, and providing reasonable comfort. 9.2 Multimodal Operations This section discusses the operational considerations at a DDI with relevance to the pedestrian and bicyclist experience, so the reader can understand tradeoffs associated with design decisions. C H A P T E R 9 Diverging Diamond Interchanges (DDIs)

9-2 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges 9.2.1 Motor Vehicles DDIs generally have a reduced number of signal phases relative to conventional diamond interchanges and provide greater vehicular capacity and throughput. The following general design features are commonplace at existing DDIs: • Two-phase signals that reduce lost time at the interchange, allowing for shorter cycle lengths and increasing capacity relative to conventional diamond interchanges; • Free-flowing left-turns (and right-turns) onto the freeway (on-ramps); • Ability to coordinate through traffic or left-turns from the freeway; and • Comparatively high capacity for the off-ramp movements from the freeway (particularly if right and left-turns on red are allowed). The free-flowing on-ramps and permitted right and left-turns on red are design features common among existing designs that support improved vehicular capacity but are in tension with maximizing pedestrian and bicyclist safety, depending on the facility locations and design (the free-flowing left-turn does not conflict with inside crossings; see Section 9.2.2.1). The performance objectives for the intersection will help to decide which characteristics are part of a proposed design. These elements should be determined in Stage 2 of an ICE process. If a DDI’s vehicular operational performance is tied to free-flowing turn movements that raise flags for pedestrian safety, resulting in the need for movements to be controlled, its selection as an alternative may not be realistic. Because mainline through movements conflict at DDI crossovers, a decreased green time may be experienced by pedestrians and bicyclists traveling along the mainline. Shorter cycle lengths and geometric considerations can mitigate some delays. 9.2.2 Pedestrians Pedestrian facilities at interchanges can be challenging due to high vehicular volumes and a focus on providing unimpeded capacity to vehicular flow. Nonetheless, interchanges can 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. For pedestrians, the DDI allows for reduced signal phasing (shorter cycle lengths), channeliza- tion of movements (crossing of one direction of travel at a time), signalization, and right-of-way Exhibit 9-1. Key characteristics of a DDI.

Diverging Diamond Interchanges (DDIs) 9-3 availability for pedestrian facilities. The DDI requires additional stages for pedestrian crossings, which can result in additional stops and delays (6). Several special considerations apply to the DDI, which are summarized below. 9.2.2.1 Inner Versus Outer Walkway A major consideration for pedestrian facilities at a DDI is whether to provide inner or outer walkways. Several design decisions flow from this choice. For an overpass DDI, pedestrian facili- ties in the center of the interchange (within the median) may be preferable so as to minimize conflicts with left-turning traffic to and from the freeway and allow crossing the interchange in all directions (i.e., travel along the arterial and crossing the arterial from one side to the other). For underpass DDIs, the inner versus outer options may not be a choice, depending on the placement of bridge columns. Exhibits 9-2 and 9-3 give examples of a pedestrian facility in the center of a DDI, and of an outside pedestrian walkway, respectively. The exhibits further highlight which crossings are constrained in time by signal phasing, as well as which may not be. For pedestrian crossings without signals, geometric design or enhanced crossing treatment options may be incorporated to enhance pedestrian safety (discussed in Section 6.3.1). With the overpass DDI having 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 Exhibit 9-2. Pedestrian movements given an inner walkway at a DDI. The top figure shows movements which run in the first phase. The bottom figure shows movements which run in the second phase.

9-4 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges or signalized crossings and, in either case, can be configured to promote low vehicle speeds and good sight distance to the crosswalks. With outside walkways at DDIs, traversing pedestrians may cross four separate vehicle turn- ing movements (the ramps). These may be free-flowing or controlled. In some existing DDIs with outer walkways, no pedestrian crossing of the arterial street is provided. This limits pedes- trian access. As presented in Chapter 1, unless the intention is to restrict access for the life of the project, crossing opportunities should be provided. Otherwise, pedestrians will likely pursue their desired lines, often at considerable risk. The advantages and challenges of inside and outside pedestrian facilities are summarized in Exhibits 9-4 and 9-5, respectively. 9.2.2.2 Pedestrian Phase Coordination With an inner walkway between the crossovers, the designer can time pedestrian crossings to match crossing behavior. Because the right-turn onto a freeway on-ramp does not require coor- dination with any other phase at the DDI, the signal timing can progress the pedestrian move- ment across the right-turn on-ramp 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 9-6). Depending on the distance between crossings and the phase length, a pedestrian may cross both movements in a single signal phase. A design that keeps these cross- ings close would promote that sequential crossing (7). Exhibit 9-3. Pedestrian facilities on the outside of DDI. The top figure shows movements that run in the first phase. The bottom figure shows movements that run in the second phase.

Exhibit 9-4. Inner walkway pedestrian safety and comfort. Advantages Challenges Street Crossings • Crossing of the arterial street naturally provided at DDI for full pedestrian access • Crossing one direction of traffic at a time • No exposure to left-turns to the freeway (typically free-flowing) • Protected signalized crossing to the walkway • Pedestrian clearance time generally provided in crossover signal phasing • Pedestrian delay to center minimized by short cycles at two- phase signals • The possible crossing of free-flow right-turn movements to/from the freeway • Pedestrians may not know to look to the right when crossing from center • Wait at center island dictated by the length of signal phase for through traffic • Pedestrian signals can conflict with vehicle signals at crossovers • Out-of-direction travel for pedestrians not desiring to cross the arterial Walkway Facility • Side walls provide a positive barrier between vehicular movements and pedestrians. (Walls low enough to avoid the “tunnel” effect could have a lesser impact on pedestrian comfort.) • Need for only one facility inside the intersection footprint (inner walkway) can offer more enhanced features within the same right-of- way constraints. • Design of side walls must be managed to avoid impeding sight distance • Potential discomfort from moving vehicles on both sides of the walkway • Potential challenge placing all necessary signs and signal control equipment while maintaining full pedestrian access Exhibit 9-5. Outside path/sidewalk pedestrian safety and comfort. Advantages Challenges Street Crossings • Crossing one direction of traffic at a time • Ramp crossing distances are often shorter than through traffic crossing distances due to fewer travel lanes • Crossing of free-flowing right-turn movements to/from the freeway • Conflict with left-turns to the freeway (typically free-flowing), where high vehicle speeds are likely (acceleration to the 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 Walkway Facility • Extension of the existing pedestrian network (natural placement on outside of lanes) • Pedestrian typically has a view of the path ahead (depends on sightlines and obstructions) • The walkway does not conflict with center bridge piers (at underpass) • Opportunity to use right-of-way outside of bridge piers (at underpass) • Does not create a potential tunnel effect that could make pedestrians feel “trapped” • Need for widened structure on the outside for overpass • Potential for additional right-of-way for underpass or construction of retaining wall under the bridge • Need for additional lighting for the underpass

9-6 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges Where an outer walkway is provided, the right-turn and left-turn off-ramp movements will not run concurrently (Exhibit 9-3). 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). 9.2.2.3 Left-Turn at Exit Ramp When pedestrian facilities are provided on the outside, signalization is a preferable control strategy for exit ramp left-turns because it provides a controlled crossing opportunity. For DDIs with inner walkways, and thus no pedestrian crossings at the left-turn exit ramp (see Exhibit 9-6), refer to the MUTCD for general guidance on choosing traffic control devices. 9.2.3 Bicycles Three basic options exist for accommodating bicyclists at a DDI. These options are to provide the following: • A marked bicycle lane through the DDI; • A separated bicycle path or shared-use path; or • Shared use of the travel lane. One or more of these options may be viable for a project and would be determined in a Stage 1 ICE process (see Chapter 4). In all cases, the signal timing should accommodate a cyclist through all movements and phases and provide adequate time for bicyclists to clear the intersection before releasing conflicting traffic. Exhibit 9-6. Back-to-back off-ramp crossings during the same signal phase.

Diverging Diamond Interchanges (DDIs) 9-7 9.2.3.1 Bicycle Lane Guidance Bicycle lanes at a DDI provide bicyclists with dedicated road space to travel across the inter- change. 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. Separated bicycle lanes are recommended where actual vehicle speeds exceed 35 mph or in high-volume situations, as explained in Chapter 3. Consistent with the guidance in Chapter 5, a wider bike shy distance is critical where the facility is next to a barrier. The bicycle lane should continue through the off-ramps from the freeway where motorized traffic would generally be required to yield to arterial traffic, including cyclists (see Exhibit 9-16). Where bicycle lanes cross exit ramps, using colored pavement may increase bicyclist visibility to motorists. When bicycle lanes are provided, it is generally preferred to locate them to the right of motor- ized vehicle traffic, consistent with generally expected bicyclist behavior. At a DDI with a center barrier wall adjacent to a bicycle lane, the lane should provide adequate width to avoid cyclists feeling “trapped.” Exhibit 9-16 provides an example of bicycle lanes at a DDI. 9.2.3.2 Separated Bicycle Facilities 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. Exhibits 9-14 and 9-18 provide examples of this design option. Bicyclists crossing at the signalized intersections would either share a crossing with pedestrians or cross in adjacent crossings; the progression concerns would be largely the same as those for pedestrians (discussed in Section 9.2.2.2). If separated facilities are not provided on the approach to the DDI, a ramp can transition bicyclists from the roadway to the separated path. This can be accomplished in advance or with the development of vehicle right-turns to avoid a conflict. (This would be the turning motorists crossing bicycle path conflict, discussed in Section 4.4.18). A shared-use path option can be provided as part of an outer path option, as depicted in Exhibit 9-7. 9.3 Safety and Comfort 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 eliminating high-risk and high-severity crash patterns (e.g., angle crashes between Exhibit 9-7. Outer shared-use path at DDI in Lexington, Kentucky. Source: Google.

9-8 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges left-turn and opposing through movements). For pedestrian and bicyclist safety, insufficient data are available to draw any conclusions regarding the crash performance of DDIs. Absent quantitative crash data, an investigation of conflict points provides insight into the expected safety of DDIs. 9.3.1 Conflict Points A pedestrian-vehicle conflict point exists anywhere pedestrian walkways and vehicular travel lanes cross, and a bicycle-vehicle conflict point exists anywhere bicyclist paths and vehicular travel lanes cross. The pedestrian-vehicle conflict points for inside crossing DDIs and outside crossing DDIs are illustrated in Exhibits 9-8 and 9-9. The key conflict points are denoted by asterisks (the freeway on-ramps). These conflict points may be accelerating conflict points and may be controlled or uncontrolled conflict points, depending on the design of the intersection. Exhibit 9-8. Pedestrian-vehicle conflict point diagram for a DDI with an inside walkway. Exhibit 9-9. Pedestrian-vehicle conflict point diagram for a DDI with an outside walkway.

Diverging Diamond Interchanges (DDIs) 9-9 Bicycle-vehicle conflict points for an inside bicycle path are presented in Exhibit 9-10. The asterisks represent conflict points that may be along accelerating vehicle paths, subject to the design of the intersection. 9.3.2 Pedestrians–Key Safety Challenges The conflict points diagrams present a reduced number of conflict points for pedestrians at a DDI relative to a conventional intersection or interchange. Design factors inherent to a DDI that should be flagged in Stage 2 of the ICE process as the design is developed are as follows: • A DDI often entails crossing traffic approaching from a potentially counterintuitive direction, which brings wayfinding challenges and risks for pedestrians. Although each crossing conflicts with only a single direction of vehicle traffic, pedestrians may not intuitively know in which direction to look for vehicles. • The capacity benefits provided by free-flowing entrance ramps at a DDI degrade the cross- ing environment for pedestrians and bicyclists on separated paths. These crossings can be improved by ensuring geometry manages motor vehicle speeds and that sufficient sight dis- tance is provided. Whether to control these vehicle movements should be weighed as part of Stage 2 of the ICE process. • Assuming the left-turn entry ramp is unobstructed, the right-turn entry ramp has typically taken one of two basic designs: yield control with no acceleration lane or free turn with accel- eration lane (see Exhibit 9-11). The acceleration, if provided, would allow motorists to achieve merge speeds after the pedestrian crossing. – Yield control with no acceleration lane. This choice of traffic control has often been made in consideration of conflicting traffic volumes, the downstream merge point onto the limited access facility, and right-of-way. It is most often used where low to moderate turn- ing movements are present for the left- or right-turn. The left- and right-turn lane can be shared or exclusive. With the yield control option, the sight distance and the curve are important to encourage crossing pedestrians to yield, and the crossing point and vehicle yield point are separated in space to separate the decision points. – Free turn with acceleration lane. This form of traffic control is typically preferred if entry ramp volumes are high for turning movements or if queues from turning traffic spill back Exhibit 9-10. Bicycle-vehicle conflict point diagram for a DDI with an inside bicycle path.

9-10 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges to an arterial through lane. The downstream merge point onto the limited access facility would be designed no differently than other interchange designs. The left and right-turn lane can be shared or exclusive. Both movements can 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 in Stage 2 of an ICE process. Exhibit 9-12 presents the design flags applicable to pedestrians, along with the location of their discussion and applicable treatments (design flags and treatments whose discussion applies across alternative intersection types are in Chapter 4). 9.3.3 Bicycles–Key Safety Challenges Bicyclists at DDIs face challenges that either create safety risks or stress for riding through the intersection. Key challenges are as follows: • 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. Recall that in Chapter 3 a design principle for bicyclists was 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. Separated facilities would address this issue. • On-ramp movements. If free-flow movements are provided for motorists to access free- way on-ramps, then an uncontrolled diverging conflict point is introduced between bicy- clists and motorists (essentially a “right hook” opportunity). This is closely related to safety Exhibit 9-11. Entry ramp free right and left-turns with acceleration lanes.

Diverging Diamond Interchanges (DDIs) 9-11 Design Flag Description Mode/Travel Path Motor Vehicle Right- Turns (Section 4.4.1) This flag would apply if right-turn- on-red were permitted. Pedestrians, all crossings Uncomfortable/Tight Walking Environment (Section 4.4.2) Pre-existing bridges, abutments, and piers may constrain the total facility width between the crossovers. Pedestrians, all crossings Nonintuitive Motor Vehicle Movements (Section 4.4.3) Pedestrians crossing on the outside would confront consecutive crossings with vehicle traffic arriving from the same direction. Pedestrians, outside crossing Crossing Yield- Controlled or Uncontrolled Vehicle Paths (Section 4.4.4) Movements to and from on- and off- ramps may be yield-controlled and therefore create additional stress and safety concerns for pedestrians. Pedestrians, all crossings Indirect Paths (Section 4.4.5) Outside crossing designs should still allow for the pedestrian crossing of the mainline. Pedestrians, outside crossings Executing Unusual Movements (Section 4.4.6) In most local contexts, pedestrians do not expect to cross into the mainline median to continue moving along the mainline road. Pedestrians, inner crossing Multilane Crossings (Section 4.4.7) Depending on the lane configuration, pedestrians may cross multiple lanes either when crossing an on- or off-ramp or when crossing into the median. Pedestrians, all crossings Motor Vehicle Left- Turns (Section 4.4.10) This flag would apply if left-turn-on- red were permitted. Pedestrians, outside crossings Sight distance for gap acceptance movements (Section 4.4.12) At yield-controlled movements, vertical and horizontal alignments should allow for proper sight distance to pedestrian crossings. Pedestrians, all crossings Grade Change (Section 4.4.13) Vertical curves on bridges may create challenges for pedestrians with mobility challenges or those carrying or pushing objects. Pedestrians, all crossings Exhibit 9-12. Design flags applicable to pedestrians at DDIs.

9-12 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges concerns with the channelized turn-lane design 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 cyclists 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. 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. 2. At many signals, a bicyclist can only rely on the yellow clearance phase for vehicular traffic. This yellow clearance phase is designed just for drivers and rarely exceeds 5 seconds. This clearance time is usually too short for cyclists to clear the intersections. The disparity is exacerbated as clearance lengths increase. When signal heads are placed at the entrance to 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 9-13 presents the design flags applicable to bicyclists, along with Exhibit 9-13. Design flags applicable to bicyclists at DDIs. Design Flag Description Mode/Travel Path Executing Unusual Movements (Section 4.4.6) Similar to motor vehicles, bicyclists do not expect to cross to the left side of the road when crossing the DDI. Bicyclists, all movements Multilane Crossings (Section 4.4.7) Large DDIs may tempt bicycles to cross many lanes at the crossover. Bicyclists, all movements Undefined Crossings at Intersections (Section 4.4.9) Although not unique to DDIs, a lack of bicycle markings crossing ramp merge or diverge points may result in vehicles impeding on the bicycle lane. Bicyclists, all movements Grade Change (Section 4.4.13) Vertical curves on bridges may create challenges for bicycles trying to maintain speed, resulting in a large speed differential with vehicles. Bicyclists, all movements Riding in Mixed Traffic (Section 4.4.14) Although not unique to DDIs, vehicle speeds near interchanges may be significant, especially if curves have large design radii. Bicyclists, all movements Bicycle Clearance Time (Section 4.4.15) Long distances between the crossover and right-turn off-ramp movement may result in conflict between bicyclists and right-turning vehicles. Bicyclists, all movements Turning Motorists Crossing Bicycle Paths (Section 4.4.18) A high volume of turns onto the on-ramp may exacerbate vehicle-bicycle conflicts. Bicyclists, all movements Riding between Travel Lanes, Lane Additions, or Lane Merges (Section 4.4.19) Off-ramp designs with lane additions or downstream merges create additional stress and safety concerns for bicyclists traveling along the mainline. Bicyclists, all movements Off-Tracking Trucks in Multilane Curves (Section 4.4.20) Trucks moving through the crossover intersections may off-track into adjacent bicycle lanes. Bicyclists, all movements

Diverging Diamond Interchanges (DDIs) 9-13 the location of their discussion and applicable treatments (design flags and treatments whose discussion applies across alternative intersection types are in Chapter 4). 9.4 DDI Level Concepts The three design concepts in this section were developed to present techniques for improv- ing pedestrian and bicyclist safety and operational performance of DDIs. These concepts are not suggested as designs to be replicated as is; rather, they illustrate the DDI options possible in various contexts. These concepts mix design approaches. The designer must consider traffic volume and speed when matching designs and treatments to the appropriate context (discussion in Chapter 3, Section 3.3.2). Following each concept is a discussion of the flags remaining with the design—the flags not obviated by the design that would still need to be addressed. The designs are as follows: • DDI Shared-Use Path/Inner Walkway Concept • DDI On-Street Bike Lane/Outer Walkway Concept • DDI Separated Bike Lane/Inner Walkway Concept Section 9.3 presented other key design flags that would be subject to site-specific concerns and are not obviously presented or addressed with the concepts presented below. 9.4.1 DDI Shared-Use Path/Inner Walkway Concept The first DDI concept, shown in Exhibit 9-14, includes a shared-use path inside the median. Bicycle access to the shared-use path is provided upstream of the DDI with bicycle ramps. Downstream of the DDI, ramps return bicyclists to the roadway. The concept would be appro- priate for intersections where heavy vehicle movement through the crossover may result in truck off-tracking through the crossovers. The inside shared-use path allows for free-flowing left-turns onto the freeway without pedestrian or bicycle conflicts. 9.4.1.1 Benefits This design addresses these key elements regarding safety and comfort: • Nonintuitive Motor Vehicle Movements design flag: By providing an inner walkway, pedes- trians cross motor vehicle movements in an alternating fashion. At the first crossing, vehicles arrive from the left, and at the next crossing, vehicles arrive from the right, and so on. This conforms to the typical expectation. Exhibit 9-14. DDI shared-use path/inner walkway design concept.

9-14 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges • Indirect Path design flag: Pedestrians and bicyclists can cross the mainline by proceeding from one side of the roadway, onto the center island, 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 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 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 challenges 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, avoid- ing the potential conflict. 9.4.1.2 Challenges Emphasizing again that the design is not intended to be “ready-made,” this concept leaves several design flags as described in Exhibit 9-15. 9.4.2 DDI On-Street Bike Lane/Outer Walkway Concept The next DDI concept, shown in Exhibit 9-16, features an outer walkway for pedestrians and on-street bicycle lanes. This concept could be implemented 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. 9.4.2.1 Benefits This design addresses these key elements regarding safety and comfort: • Indirect Path design flag: Pedestrians can cross the mainline by proceeding from one side of the roadway, onto the center island, 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 wanting 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. 9.4.2.2 Challenges Emphasizing again that the design is not intended to be “ready-made,” this concept leaves several design flags as described in Exhibit 9-17.

Diverging Diamond Interchanges (DDIs) 9-15 Design Flag Description Mode/Travel Path Motor Vehicle Right- Turns (Section 4.4.1) 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 right-turn-on-red 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. Pedestrians, all crossings Uncomfortable/Tight Walking Environment (Section 4.4.2) The careful design of the median walkway is necessary as the design progresses to ensure it provides adequate space for all users. Pedestrians, center median crossing Crossing Yield- Controlled or Uncontrolled Vehicle Paths (Section 4.4.4) 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. Pedestrians and Bicyclists, all crossings Indirect Path (Section 4.4.5) Users wanting 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. Pedestrians and Bicyclists, inner median crossing Executing Unusual Movements (Section 4.4.6) In most areas, DDIs are a relatively novel design. Users wanting 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. Pedestrian and Bicyclists, inner crossing Multilane Crossings (Section 4.4.7) At the crossovers of this design, pedestrians must cross multiple lanes without refuge (the yellow flag threshold for pedestrians is 2-3 lanes). Pedestrians, all crossings Sight Distance for Gap Acceptance Movements (Section 4.4.12) 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. Pedestrians and Bicyclists, all crossings Grade Change (Section 4.4.13) 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. Pedestrians and Bicyclists, inner median crossing Exhibit 9-15. Summary of design flags remaining with DDI shared-use path/inner walkway design concept.

9-16 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges Exhibit 9-16. DDI on-street bike lane/outer walkway design concept. Exhibit 9-17. Summary of design flags remaining with DDI on-street bike lane/outer walkway design concept. Design Flag Description Mode/Travel Path Motor Vehicle Right- Turns (Section 4.4.1) 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 right-turn-on-red or by designing the curve radii to keep speeds from exceeding 25 mph. Pedestrians, all crossings Nonintuitive Motor Vehicle Movements (Section 4.4.3) Shared-use path users following the outside 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. Pedestrians, outside crossing Crossing Yield- Controlled or Uncontrolled Vehicle Paths (Section 4.4.4) 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. Pedestrians, all crossings Executing Unusual Movements (Section 4.4.6) In most areas, DDIs are a relatively novel design. Users wanting 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. Bicyclists, all movements Multilane Crossings (Section 4.4.7) At the crossovers of this design, pedestrians must cross multiple lanes without refuge (the yellow flag threshold for pedestrians is 2-3 lanes). Pedestrians, crossover crossing

Diverging Diamond Interchanges (DDIs) 9-17 Exhibit 9-17. (Continued). Bicycle Clearance Time (Section 4.4.15) 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. Bicyclists, all movements Turning Motorists Crossing Bicycle Path (Section 4.4.18) Vehicles turning right to the on- ramp must cross the bicycle lane resulting in a conflict. Bicyclists, all movements Riding between Travel Lanes, Lane Additions, or Lane Merges (Section 4.4.19) Adding the right-turn pocket onto the on-ramp results in bicyclists riding between the through and right-turn lanes for an extended period. Bicyclists, all movements Off-Tracking Trucks in Multilane Curves (Section 4.4.20) 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. Bicyclists, all movements Design Flag Description Mode/Travel Path Sight Distance for Gap Acceptance Movements (Section 4.4.12) The motor vehicle right- and left- 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. Pedestrians and Bicyclists, all crossings Grade Change (Section 4.4.13) 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. Pedestrians and Bicyclists, inner median crossing Riding in Mixed Traffic (Section 4.4.14) Given the expected high volume of vehicles on an interchange, this flag must be mitigated through vehicle speed control. The geometric design should be used to reduce speeds below 25 mph (yellow flag threshold). Bicyclists, all movements 9.4.3 DDI Separated Bike Lane/Inner Walkway Concept The final DDI concept, shown in Exhibit 9-18, provides a separated bike lane through the intersection within the median between the crossovers as well as an inner walkway for pedes- trians. The pedestrian walkway inside the separated bike lanes provides additional 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. 9.4.3.1 Benefits This design addresses these key elements regarding 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

9-18 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges vehicles on either side of pedestrians in the inner walkway, pedestrians are further removed from motor vehicles. Care should be taken in the 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, pedes- trians cross motor vehicle movements in an alternating fashion. At the first crossing, vehicles arrive from the left, and at the next crossing, vehicles arrive from the right, and so on. This conforms to the typical expectation. • Indirect Path design flag: Pedestrians and bicyclists can cross the mainline by proceeding from one side of the roadway, onto the center island, 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 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 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 pro- viding an off-street shared-use path for bicyclists, the users are separated in space, avoiding the potential conflict. 9.4.3.2 Challenges Emphasizing again that the design is not intended to be “ready-made,” this concept leaves several design flags as described in Exhibit 9-19. Exhibit 9-18. DDI separated bike lane/inner walkway design concept.

Diverging Diamond Interchanges (DDIs) 9-19 Design Flag Description Mode/Travel Path Motor Vehicle Right- Turns (Section 4.4.1) 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 right-turn-on-red or by designing the curve radii to keep speeds from exceeding 25 mph. Pedestrians and Bicyclists, all crossings Crossing Yield- Controlled or Uncontrolled Vehicle Paths (Section 4.4.4) 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. Pedestrians, all crossings Indirect Path (Section 4.4.5) Users wanting 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. Pedestrians and Bicyclists, inner median crossing Executing Unusual Movements (Section 4.4.6) In most areas, DDIs are a relatively novel design. Users wanting 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. Pedestrians, inner crossing Multilane Crossings (Section 4.4.7) At the crossovers of this design, pedestrians must cross multiple lanes without refuge (the yellow flag threshold for pedestrians is 2-3 lanes). Pedestrians and Bicyclists, all crossings Sight Distance for Gap Acceptance Movements (Section 4.4.12) 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. Pedestrians and Bicyclists, all crossings Grade Change (Section 4.4.13) 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. Pedestrians and Bicyclists, inner median crossing Exhibit 9-19. Summary of design flags remaining with DDI separated bike lane/inner walkway design concept.

9-20 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges Exhibit 9-20. Undesirable use of single pole with two pedestrian pushbuttons, no APS, and insufficient separation of the two detectable warning surfaces. Source: Google. 9.5 Detailed Design Techniques The design flag procedure and corresponding flags are outlined in Chapter 4, and techniques are discussed in Chapter 5. Because many flags and techniques appear in the same fashion at many A.I.I.s, those common flags and design treatments/techniques are discussed solely in Chapter 5. Discussion in this section is limited to unique characteristics or design responses to assist in addressing flags in a DDI. This includes: • ADA and accessibility; • Channelized turn lanes; • Sight distance; and • Indirect paths. 9.5.1 ADA and Accessibility Because the DDI crossover may be novel for drivers, human factors considerations are emphasized throughout the design process. Human factors considerations also apply to the pedestrian environment, because the DDI environment differs from what pedestrians are accustomed to using at conventional interchanges. At the DDI, the provision of an accessible pedestrian signal (APS) may pose a challenge on the median island for pedestrian facilities in the center. Exhibit 9-20 shows an example of an undesirable pedestrian pushbutton installation with the pushbutton 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 pushbutton 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 the pedes- trian pushbuttons to be on separate poles and sufficiently separated, it is recommended that the pedestrian pushbuttons be separated diagonally, with the pushbuttons consistently on the

Diverging Diamond Interchanges (DDIs) 9-21 downstream side of the crossing to provide audible separation between the APS messages and oncoming traffic, as shown in Exhibit 9-21. Wider islands are strongly recommended to provide a true refuge area of at least 6 feet in the direction of pedestrian travel. This ensures at least 2 feet between the detectable warning surfaces, as well as 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. One potential for such wording after activating the push- button (the pushbutton information message, see MUTCD 4E.13, par 9 & 10) may be: “Wait to cross eastbound lanes Airport Rd. at Highway 26. Traffic coming from your left.” (8) During the Walk interval, the message would be: “Eastbound lanes Airport Rd, walk sign is on to cross eastbound lanes Airport Rd.” An expert in 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 the ADA. 9.5.2 Channelized Turn Lanes Channelization of all turns to and from the freeway is used to discourage wrong-way maneuvers and to move ramp terminal intersections away from the crossover intersection. Channelization, 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 the location of the pedestrian facilities (outside versus inside), unintuitive traffic direc- tions for crossing, radius and speed of turning movements, and whether to signalize turns at on-ramps–especially left-turns onto on-ramps with walkways along the outside of the DDI where speeds are higher and there may be limited sight distance. Exhibit 9-21. DDI splitter island with diagonal pedestrian signals.

9-22 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges If pedestrian facilities are placed on the outside, attention is needed regarding the place- ment, visibility, and vehicular speeds for the pedestrian crossing at the left-turn to the on-ramp. Exhibit 9-22 shows one example of such a left-turn crossing, viewed from the middle of the DDI. The image shows an exclusive left-turn lane approaching the crosswalk, with motor vehicles accelerating toward the freeway entrance ramp. The exhibit further shows potential visibility limitations for pedestrians crossing from the middle of the DDI. 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 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, as well as an increased chance of conflicts at these crossing locations. To overcome visibility and sight distance challenges, several potential treatments could be considered, including the following: • 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 distances, which might require a slightly longer crossing; • Adding raised crosswalks or other geometric modifications to control vehicular speeds near the crosswalk; • Installing rectangular rapid-flashing beacons (RRFBs) or other pedestrian-activated devices 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; and • Moving pedestrian facilities to inside the median (resolves left-turn movements, but right- turn movements may still need additional treatment). Although this discussion has focused on the channelized left-turn to the freeway, similar con- siderations 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. 9.5.3 Sight Distance Sight distance is discussed in Chapters 4 and 5 but has a special application at DDIs due to traffic from potentially counterintuitive directions at pedestrian crossings. Exhibit 9-22. Example of a pedestrian crossing at free-flow left onto the freeway.

Diverging Diamond Interchanges (DDIs) 9-23 Whether or not pedestrian facilities are provided along the outside or in the center median, 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 in Exhibit 9-23. Other marking possibilities that would be more accessible to pedes- trians who cannot read English could also be explored. This treatment could help pedestrians and is relatively inexpensive to install and maintain. The installation process may require that a small section of pavement be removed before marking installation. This protects snowplows and wheel friction, which reduces the marking’s maintenance needs. 9.5.4 Visibility of Traffic Control Devices Pedestrian crossings at a DDI can be signalized or unsignalized. For inner walkways, the crossings from the channelization island to the inner walkway are signalized, while the cross- ings from the island across the on-ramps are often unsignalized. Visibility of any traffic control devices is critical (and required), per MUTCD, but DDIs call for special attention to this specific element. The visibility of the pedestrian signals should be explored in the early stages of the design phase of the DDI, especially with the narrow “noses” at the ends of the inner walkway. Sight- lines to and from the inner walkway can be further obstructed by vehicles using the crossovers, as well as by poles and signage in the median. The actual pedestrian signal may not be visible when approaching the crossing in the inner walkway, and it may be unclear what the intended crossing direction is. Signal pole placement and orientation should be screened and carefully selected to ensure that pedestrians can find and understand the intention of these traffic control devices intuitively. Supplemental signs specifically for pedestrians and cyclists may be necessary when signs for drivers are not adequate for other roadway users. An APS with a locator tone is required to help pedestrians with vision disabilities locate the pushbutton. 9.5.5 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 Exhibit 9-23. Engraved pedestrian pavement marking – “Look Left.” Source: Google.

9-24 Guide for Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges for pedestrians to cross the arterial roadway at a DDI (Exhibit 9-24)–if a pedestrian needs to cross the arterial, 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 Sec- tion 9.4 demonstrate how this can be reasonably achieved. 9.6 References 1. Schroeder, B., C. Cunningham, B. Ray, A. Daleiden, P. Jenior, and J. Knudsen. August 2014. Diverging Diamond Interchange Informational Guide. Report FHWA-SA-14-067. FHWA, Washington, DC. 2. Nye, T., C. Cunningham, and E. Byrom. 2019. “National-Level Safety Evaluation of Diverging Diamond Interchanges.” Transportation Research Record. 3. FHWA Field Evaluation of Diverging Diamond Interchanges. 2015. 4. Claros, B. R., et al. 2015. “Safety Evaluation of Diverging Diamond Interchanges in Missouri.” Transportation Research Record 2486, pp. 1-10. 5. Hummer J. et al. 2016. “Safety Evaluation of Seven of the Earliest Diverging Diamond Interchanges Installed in the United States.” Transportation Research Record 2583. 6. Molan, A. M., J. Hummer, and K. Ksaibati. 2019. “Modeling Safety Performance of the New Super DDI Design in Terms of Vehicular Traffic and Pedestrian.” Accident Analysis & Prevention. Vol 127, pp. 198-209. 7. Chlewicki, G. 2017. “Improving Pedestrian Operations at Innovative Geometric Designs.” Presented at the 5th Urban Street Symposium, Raleigh, NC. 8. FHWA. 2009. Manual on Uniform Traffic Control Devices (MUTCD). USDOT, Washington, DC. Exhibit 9-24. DDI with an outer walkway and no arterial crossing opportunities.

Abbreviations and acronyms used without definitions in TRB publications: A4A Airlines for America AAAE American Association of Airport Executives AASHO American Association of State Highway Officials AASHTO American Association of State Highway and Transportation Officials ACI–NA Airports Council International–North America ACRP Airport Cooperative Research Program ADA Americans with Disabilities Act APTA American Public Transportation Association ASCE American Society of Civil Engineers ASME American Society of Mechanical Engineers ASTM American Society for Testing and Materials ATA American Trucking Associations CTAA Community Transportation Association of America CTBSSP Commercial Truck and Bus Safety Synthesis Program DHS Department of Homeland Security DOE Department of Energy EPA Environmental Protection Agency FAA Federal Aviation Administration FAST Fixing America’s Surface Transportation Act (2015) FHWA Federal Highway Administration FMCSA Federal Motor Carrier Safety Administration FRA Federal Railroad Administration FTA Federal Transit Administration HMCRP Hazardous Materials Cooperative Research Program IEEE Institute of Electrical and Electronics Engineers ISTEA Intermodal Surface Transportation Efficiency Act of 1991 ITE Institute of Transportation Engineers MAP-21 Moving Ahead for Progress in the 21st Century Act (2012) NASA National Aeronautics and Space Administration NASAO National Association of State Aviation Officials NCFRP National Cooperative Freight Research Program NCHRP National Cooperative Highway Research Program NHTSA National Highway Traffic Safety Administration NTSB National Transportation Safety Board PHMSA Pipeline and Hazardous Materials Safety Administration RITA Research and Innovative Technology Administration SAE Society of Automotive Engineers SAFETEA-LU Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users (2005) TCRP Transit Cooperative Research Program TDC Transit Development Corporation TEA-21 Transportation Equity Act for the 21st Century (1998) TRB Transportation Research Board TSA Transportation Security Administration U.S. DOT United States Department of Transportation

TRA N SPO RTATIO N RESEA RCH BO A RD 500 Fifth Street, N W W ashington, D C 20001 A D D RESS SERV ICE REQ U ESTED G uide for Pedestrian and Bicyclist Safety at A lternative and O ther Intersections and Interchanges TRB ISBN 978-0-309-67353-2 9 7 8 0 3 0 9 6 7 3 5 3 2 9 0 0 0 0