Development of functional geometrics are central to safe and efficient operation of a DDI. The most common design vehicle used at DDI facilities is the WB-67. In more urbanized areas, it may be appropriate to use smaller design vehicles, such as the WB-62TX or WB40. The anticipated truck movements, staying in-lane through the DDI, should be determined and AutoTURN (or a similar tool) used to assess vehicle encroachments within the DDI. The largest truck movements may be accommodated by using a roll down curb in turning areas. The design speed of the DDI affects the reverse curve radii through the two intersection crossovers and should typically range from 25 to 35 mph ( and for corresponding curve radii values). The DDI profile should be relatively flat to increase driver sight distance. A tangent section is recommended before and after the crossovers to minimize the likelihood of wrong way maneuvers into opposing lanes, and the recommended crossover angle is 40-50 degrees or greater (see and ). A consolidated ramp terminal intersection with crossover is desired over separated ramp terminal intersection elements (see ).

Curve radii approaching and following the DDI crossover generally range from 150-ft to 300-ft. At the crossover, the greater the crossing angle, the less “different” the intersection will seem. FHWA’s Intersection and Interchange Geometrics (IIG) Workshop, May 2016, recommends a crossover angle of 40 to 50 degrees. The 2016 IIG Workshop also noted the following regarding low crossover angles:

Drivers approaching or departing an intersection must have an unobstructed view of traffic control devices and sufficient length along the crossroad to safely navigate the intersection. Insufficient sight distance is a significant factor in street crashes and near collisions. As with any other intersection, DDI intersections must provide stopping sight distance (SSD) and intersection sight distance (ISD). Sight distances must be checked for these conflict areas: walls, railings, tall landscaping, or other obstructions that may limit sight distance. Intersection sight distance may also be limited by barriers or other obstacles between the crossovers.

Due to their curvilinear alignment, DDIs have special considerations that need to be reviewed and coordinated closely with the horizontal alignment. Primary considerations for vertical alignment at DDIs include:

Refer to for additional information regarding sight distance considerations.

There are three alignment alternatives resulting in a minimum cross-section along the cross road regardless of whether the facility is an over-or under pass. There are two types of alignments: Symmetrical Alignment and the Shifted Alignment. shows an example of symmetrical and shifted alignments. This results in distances of 600-ft to 750-ft between crossovers. If the distance between crossovers can be reduced, it can boost traffic operations and limit the amount of ROW needed.

Eliminating a few reverse curves can reduce the spacing between crossovers. shows alignment alternatives where the number of reverse curves has been reduced with a resultant increase in median width. This results in distances of 400-ft to 500-ft between crossovers. Chapter 7 of provides additional information on horizontal alignment options.

Auxiliary lanes can aid weaving traffic at a DDI interchange, smooth traffic flow, provide added capacity, and improve overall safety. through show examples of auxiliary lanes at a DDI interchange. Chapter 7 of provides additional information on auxiliary lanes.

Ramp Terminal geometry is discussed in , Section 6.4.8 and is shown in .

The contemporary configuration shown in is considered to be the safer solution for DDI ramp terminal geometry based on the enhanced reduction in vehicle speed while offering greater view angles that require drivers to turn their heads to less of a degree while trying to identify oncoming traffic.

DDIs have many benefits for pedestrians which include:

DDIs provide the option to direct pedestrians to either the outside of the intersection or to a center walkway. The wide area between opposing traffic allows the opportunity for a large sidewalk down the center that can be bordered by concrete barrier for additional protection and channelization for pedestrians. Pedestrian facilities on the inside minimize conflicts with traffic turning left to and from the freeway and allow the crossing of the interchange in all directions. When placing the pedestrian sidewalk along the outside, important considerations are: the location of the crosswalk with respect to the bridge structure or other sight obstructions to maintain good visibility for both the pedestrians and vehicles and designing the turning radii to reduce speeds in the vicinity of pedestrians. and depict the inside and outside pedestrian sidewalk options, respectively.

For bicyclists, the design should focus on minimizing bicycle conflicts with motor vehicles, providing adequate lateral space between vehicles and bicycles, minimizing speed differential between bicycle and vehicles, and managing bicycle-pedestrian conflicts. The 2 primary options for bicyclists on a cross street through a DDI are:

Ultimately, a thorough site assessment, an assessment of anticipated bicycle and pedestrian volumes, and an assessment of projected origins and destinations for pedestrians and bicyclists should be conducted to determine the preferred method of movement through the DDI.

DDI intersections require full access control through the interchange. A traffic simulation should be conducted to determine the impacts and any needed mitigation for adjacent and nearby intersections in the corridor. Possible disadvantages of a DDI include the following:

Refer to the for additional guidance with respect to access management.

DDIs have safety benefits due to reduced conflict points, especially crossing conflict points, resulting in fewer severe crashes. They also decrease the likelihood of wrong-way maneuvers onto the intersecting expressway. Caution should be exercised when allowing right-turn-on-red and left-turn-on-red maneuvers, as drivers may look to the wrong side of the road for oncoming traffic. For additional information on safety, refer to the .

DDI operations may vary slightly based on location and traffic demand. There are three potential phasing schemes:

Signal timing and phasing of the adjacent intersections must be coordinated to provide appropriate capacity to the DDI. Refer to the for more information on operations.

DDIs require clear traffic control devices to guide drivers through the interchange. The four main objectives of traffic control devices in DDIs are:

Refer to for more information on traffic control devices. The remains the primary source for traffic control device standards.

At a minimum, safety lighting should be provided for DDIs in rural locations. Complete illumination systems should be considered at all DDIs to enhance visibility and safety. Refer to TxDOT’s for additional information and design standards.