Diverging Diamond Interchanges (DDI)
McTrans’ Streets module now includes the analysis of diverging diamond interchanges (DDI), adding this to the list of interchange ramp terminal designs to implement the new procedure provided within Highway Capacity Manual, 6th edition (HCM6).
The DDI design allows for all ramp movements to and from the freeway to be accommodated with free-flowing interaction with the arterial, thus avoiding the conflicting crossing movements within a traditional diamond interchange design. Arterial through-movement crossover at the ramp junctions provides for these direct merges and allows for significantly increased capacity through the interchange without adding lanes or having to rebuild the bridge.
The DDI procedure includes saturation flow rate adjustments for the crossover movements and lane utilization for this design. Additional lost time is computed to account for the clearance overlap time required for the recessed approaches, common with DDI ramp-arterial intersections.
Extra Distance Travel Time (EDTT) is included along with Control Delay (d) in the computation of
Experienced Travel Time (ETT). The ETT defines level of service for each origin-destination pair consistent with other Interchange Ramp Terminals procedures. The Interchange Results Report is shown here.
Freeways
When building the segments for analyzing a freeway facility, a new graphic has been added to the Segments input screen to allow the user to better visualize the facility and the relationship among segments during the coding process. The HCM Example Problem 1 graphic is shown here.
For more information, contact Bill Sampson, director of McTrans at bsampson@ufl.edu or 352-392-0378.