Diverging Diamond Interchange (DDI)
A diverging diamond interchange (DDI) is a type of diamond interchange in which the two directions of traffic cross to the opposite side as they pass over/under the limited-access roadway.
This type of interchange is unusual, in that it requires traffic to briefly drive on the opposite side of the road from what is customary for the jurisdiction.
Definitions
- Limited-access segments: the segments used to model the limited-access roadway
- DDI segments: the non-ramp segments used to model the surface road passing over/under the limited access segments.
Segments
DDI
DDI segments are modeled as a divided road, meaning all DDI segments must be set to be one-way.
Ramp
Ramp segments are modeled with one-way ramp segment for the on-/off-ramps from the limited access freeway.
All on-/off-ramp segments are split into two separate one-way ramp segments before attaching to the DDI segments (one ramp for each of the two possible directions of travel on the surface road).
At-Grade Intersections
Note that -- as with all at-grade intersections! -- junction points must be added at both of the two points where the lanes cross.
The junction points allow the routing engine to more accurately calculate the transit time impacts of the two signaled, at-grade intersections in the DDI.
Turn Restrictions
All turns for all segments at the DDI should be enabled.
Elevations
The two bridge segments are set at one elevation higher than the highest elevation of segment which passes underneath the bridge (for example, if the limited access roadway is elevation 0 ("E0", or "Ground")), the bridge segments are set to elevation 1 ("E1"). All other ramp and DDI segments should be set to elevation zero (E0, aka "ground").