Innovative Intersections and Interchanges
What Is A DDI?
- A grade-separated interchange design where arterial (a major route that carries traffic between highways and smaller routes) traffic crosses to the other side of the roadway between the freeway ramps
- Vehicles can turn left onto and off freeway ramps without stopping or crossing opposing lanes of traffic
- Right turns on and off the freeway ramps occur either before or after the crossover intersections, when traffic is on the “correct” side of the road
- Both crossover intersections are signalized
- Interchange can be designed as an overpass or underpass
Where Should It Be Considered?
- At locations:
- With heavy left-turn traffic volumes onto and off the freeway ramps
- Without adjacent traffic signals or nearby driveways
- Where there is limited roadway width for left-turn lanes between ramp intersections and limited right-of-way to expand
- Improved safety: Reduces the number of points where vehicles may cross paths
- Increased efficiency: Crossovers can operate with only two traffic signal phases, which allows the interchange to handle a greater volume of traffic and operate with fewer delays
- Easier access to freeway: Design allows traffic to enter and exit the freeway without crossing opposing lanes of traffic
- Cost effective: Since there are no left-turn lanes on the arterial, a DDI can have a narrower cross section and may be more cost effective than a retrofit or new interchange construction
How to Navigate
The number of conflict points (locations where vehicle travel paths intersect) is one metric that can be used to evaluate the safety of an innovative intersection or interchange.
There are three categories: crossing, merging or diverging.
In general, merging and diverging conflict points — where vehicles are moving in the same direction — are associated with less severe crash types than crossing conflict points where vehicles are moving in opposite directions.
The diagrams below compare possible vehicle travel movements and associated conflict points at a conventional diamond interchange to a DDI.
These diagrams represent a general case, with one travel lane in each direction, and do not take into account pedestrian or bicycle movements at an intersection or interchange.
When compared to a conventional diamond interchange, a DDI has four fewer crossing conflict points.
Conventional Diamond Interchange: Conflict Points
DDI: Conflict Points