User:TerryPurdue/Junction Style Guide/Interchanges/

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

The DDI should be modeled as a divided road, meaning all segments must be set to be one-way.

Ramps

One-way ramp segment are used to model the on-/off-ramps from the limited access freeway.

All on-/off-ramp segments split into two separate one-way ramp segments that attach 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 are added to allow the routing engine to properly calculate the transit time delays resulting from the two intersections with traffic signals.

Turn Restrictions

All turns for all segments at the DDI should be enabled.

Elevations

Elevations should be modeled to reflect reality. The two bridge segments should be set at elevation 1 (E1), and all other segments should be set to elevation zero (E0, aka "ground").