Diverging Diamond Interchange Informational Guide, Second Edition (2021)

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215 This chapter provides an overview of the general considerations of constructing and main- taining a DDI. It also discusses law enforcement considerations. While there are differences in constructing, maintaining, and providing law enforcement at a DDI compared to a conven- tional diamond interchange, none of the differences is likely large enough to overshadow the safety and operational effects discussed in previous chapters. 8.1 Construction Staging Implementing a DDI poses some challenges in maintaining traffic flow during construction; however, the thought processes are no different than those for other designs. The sequencing of construction can use several strategies depending on a variety of factors, including but not limited to the following: • Is the facility an overpass or underpass? • If the facility is an overpass, how many structures are being built and are any existing structures already in place? • Are there nearby detour routes? • What are the traffic demands of the facility? Each of these factors also has significant effects on the costs of implementing a DDI design. As there are many staging options that could be considered, this section will provide some options that could be considered based on the design constraints of the site. Regardless of the staging option selected, maintenance of pedestrian and bicycle access should be an integral part of the maintenance of traffic plans. Compared to other interchange forms, such as partial cloverleaf designs, tight diamond inter- changes (TDI) and single-point diamond interchanges (SPDI), the DDI typically takes much less time to construct, particularly for upgrades of existing diamond interchanges. This is espe- cially true if the design is a retrofit that allows for the use of the existing bridge structure. The Missouri Department of Transportation conducted a constructability analysis of a TDI and SPDI versus a DDI, and they concluded that DDI construction would last a single season and TDI or SPDI construction would last two seasons or more (1). This is consistent with findings from other agencies. The primary consideration for construction staging is whether the interchange is new construction, whether additional structures are needed, or if the interchange is a true retrofit design using existing structures. Sites with retrofit designs are most common to date. Questions to consider include: • Can the interchange be closed? • Is the existing pavement going to be used or replaced? C H A P T E R 8 Construction and Implementation

216 Diverging Diamond Interchange Informational Guide • Is additional cross-section width necessary to accommodate future traffic (i.e., motor vehicles, bicycles, and pedestrians)? • When are the best times to switch traffic between various stages of the project? A site constructed in locations where no at-grade intersection or grade-separated interchange currently exists will cause the least disruption to traffic. A site that can be fully closed by detouring traffic to nearby intersections or interchanges provides the most efficient arrangement for construction but creates potential out-of-direction travel for users. For most agencies, DDIs will be constructed while accommodating most of the existing traffic demands already in place (2). If original pavement will be used in place, switching traffic to a DDI configuration early can be accommodated. Oftentimes, simply resurfacing can facilitate the traffic switch. Switching traffic early will usually reduce the amount of temporary pavement needed during construction. Early traffic conversion can also assist by providing space for timely construction of mast-arm, lighting, and signal cabinet foundations. Considerations should be made regarding the final alignment of left-turn off-ramps and the conversion to a DDI lane configuration. If construction of the final alignment of the ramps precedes the conversion from right-hand drive to left-hand drive between the crossovers, care is needed to ensure drivers—then still on the right side of the roadway—do not mistake the off-ramp for an on-ramp. Switching traffic later is most often done when considerable pavement replacement is necessary. The latter method is most commonly implemented and usually uses half-at-a-time construction phasing. Similar to traditional diamond interchange design considerations, portions of the available cross section must be cordoned off using raised barriers to allow construction activities to take place. Consideration is needed when determining in which stage median pedestrian facili- ties will be constructed if present. Where new structures must be completed, construction with less interference to normal traffic operations is usually possible. Structures can be built in place or constructed off-site and driven in using a self-propelled modular transporter (SPMT). An example of a pre-constructed concrete girder system being driven to the site is provided in Exhibit 8-1. This technique has been used with great success for many bridge projects in Utah (3). Although these bridge design techniques will usually require more upfront costs, the Utah Department of Transportation (UDOT) is noticing costs decreasing as contractors become more comfortable with the technique. Recent transport and installation of a parallel bridge took less than 6 hours to complete. Exhibit 8-1. SPMT brings superstructure to DDI location (3).

Construction and Implementation 217 Exhibit 8-2 shows the construction staging used at one of the UDOT sites where a prefabricated bridge was rolled into place. The middle picture shows the short transition in which the struc- ture was installed in less than 1 day. The staging of newly constructed bridge designs is not dependent on the technique used to build the structure. In fact, the illustration provided in Exhibit 8-2 could easily represent a more typical design that constructs the bridge in place. Staging techniques just prior to the opening of the DDI are also dependent on whether the site is a retrofit or new construction. For retrofit designs, the entire interchange will likely need to be closed for a short period of time. A survey of expert practitioners recommends conver- sion to the DDI traffic pattern be completed under full closure. This is recommended, in part, to assist drivers in recognizing the lane configuration change (4). In some designs, the right-turn movements on and off the limited access facility are allowed while the crossovers are tied in and striped. This is usually done in a period of 2 to 3 days over a weekend. A different tie-in method used may include one of the crossovers being closed at a time, allowing one of the left-turn movements from the limited access facility to remain in place. When the first crossover is complete, the other crossover is tied in using a similar technique. This method can be employed over a short period of time (e.g., over a weekend) as well. For designs with new structures, a common timeframe is needed for the tie-ins. 8.2 Cost Estimates One of the primary advantages of the DDI is the reduced costs associated with the design and construction compared to those for other typical interchange designs. In fact, this is the primary reason DDIs have taken a strong foothold in the transportation community in the past 5 years. In addition, the footprint of the DDI can often fit within the existing right-of-way and on an existing bridge, making it less expensive and faster to construct compared to other interchange forms previously noted in this chapter. Structural costs are the primary driving Exhibit 8-2. Construction staging using pre-cast construction methods (5).

218 Diverging Diamond Interchange Informational Guide factor in how much interchanges cost, making the DDI particularly attractive if being considered as a retrofit of an existing structure. The actual costs of designing and constructing a DDI are highly variable based on the factors described above and other site-specific elements, particularly if a design is newly constructed versus a retrofit. Construction costs at several facilities constructed to date are provided in Exhibit 8-3. Among several retrofit DDIs constructed, costs ranged from approximately $3 million to $8.5 million. The more expensive retrofit design at National Avenue incorporated an under- pass facility at the adjacent intersection, while the DDI at Timpanogos Highway added an additional bridge to the interchange. Among several new DDIs constructed to date, costs ranged from $14 million to $22 million. 8.3 Maintenance General maintenance considerations for a DDI are similar to those for other interchange forms, though the following should be considered: • Lighting—High-mast systems are often more expensive to deploy yet easier to maintain as they are usually installed in locations with easy access and outside the lane lines. Maintenance is done by lowering the luminaire ring from the mast head to the base using a winch and motor, making them accessible at the ground or by using a small cherry picker. A number of DDIs have also successfully implemented davit- and post-top lighting options, which may be more familiar for maintenance crews. • Pavement markings—Pavement markings will need to be inspected and maintained more frequently than with normal interchange designs. The unique crossover design should be well marked at all times to make sure the lanes are easily seen to help prevent path overlap. Pavement marking wear will take place much faster as the full lane widths are often used when negotiating the reverse curves. Interchange Location Open to Traffic Construction Cost Retrofit Bessemer St. and US-129 Alcoa, TN 2010 $2.9 million Yes MO-13 and I-44 Springfield, MO 2009 $3.2 million Yes Winton Rd. and I-590 Rochester, NY 2012 $4.5 million Yes National Ave. and US-60 Springfield, MO 2012 $8.2 million Yes Timpanogos Hwy. and I-15 Lehi, UT 2011 $8.5 million Yes Mid Rivers and I-70 St. Peters, MO 2013 $14 million No CR-120 and Hwy 15 St. Cloud, MN 2013 $17.5 million No Pioneer Crossing and I-15 American Fork, UT 2010 $22 million No Exhibit 8-3. Construction cost estimates (5).

Construction and Implementation 219 Through expert surveys and interviews, Brown et al. developed maintenance of traffic plans for various lane closure configurations during routine maintenance (2). 8.4 Snow Removal Consistent for all types of interchanges, snow removal strategies focus on systematically push- ing snow to the outside of the street. Many snowplows are designed to push snow to the right. Between the crossovers, this results in snow piling in the median. For climates where the snow builds throughout the season, a plan should be in place to either allow for adequate snow stor- age space next to the median or to provide for removal as needed. Snow should not be regularly stored in bicycle lanes or on sidewalks. The DDI designer needs to work collaboratively with snowplow operators to provide end treatments that delineate curb locations (i.e., surface-mounted delineators). Snowplow opera- tors will need to become familiar with the DDI configuration and develop a sequence for plow- ing the different travel paths. Through lanes are typically plowed as part of a corridor. Snow removal plans, as well as general maintenance plans, should also be developed for pedestrian and bicycle facilities. Often, these removal plans mirror current local agreements at nearby facilities. If pedestrian and bicycle facilities are located within the median, maintenance crews should be notified as the unique location may be difficult to identify under heavy snowfall. 8.5 Law Enforcement Needs The channelization of vehicle movements at a DDI creates a relatively “self-enforcing” inter- change with research showing most drivers traveling at or below the speed limit (5). Unlike many other alternative intersections and interchanges, there are no desired movements prohib- ited with signing and pavement marking alone. Unique law enforcement needs of a DDI are primarily related to the opening period. Enforce- ment during this period could help drivers become familiar with the crossed-over nature of the arterial and reduce unintentional wrong-way maneuvers. Enforcement may be most beneficial during low volume, nighttime hours when there is a decreased likelihood of opposing vehicles that would naturally help drivers avoid a wrong-way maneuver. 8.6 References 1. Missouri’s Experience with the Diverging Diamond Interchange. MoDOT Report No. OR10- 021. Missouri Department of Transportation, Jefferson City, MO, 2010. 2. Brown, H., C. Sund, P. Edara, T. Cope, and A. Khezerzadeh. Maintenance of Traffic for Innovative Geometric Design Work Zones. InTrans Project 06-277. Smart Work Zone Deployment Initiative, Ames, Iowa, 2015. 3. Walker, D. and S. Haines. True Pioneer: Utah crossing combines new crossing, new delivery. Roads&Bridges, March 2010, pp. 21–26. https://sgcweb.s3.wasabisys.com/roadsbridges/s3fs-public/22_DDI%20in%20Utah. indd.pdf. Accessed August 19, 2019. 4. Brown, H., T. Cope, A. Khezerzadeh, C. Sun, and P. Edara. Maintenance of Traffic for Innovative Geometric Design Work Zones. Transportation Research Record: Journal of the Transportation Research Board, No. 2556, 2016, pp. 49–64. 5. 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.