March 10. [Opinion] There is no surprise that accidents are down after the Catawba overpass was “inverted.” The DDI design is known to slow traffic speed and improve safety. However, the often-stated claim that it improves all traffic flow is not supported by studies or experience. The DDI design prioritizes highway access over local through-traffic.

See for example “Missouri Dept. of Transportation Performance Evaluation” link:
https://ntlrepository.blob.core.windows.net/lib/36000/36000/36038/or11012.pdf

(The Executive Summary states: “Through movements within the DDI experienced a slight increase in travel time.”)

As your article correctly stated, the lack of stacking room (due to the vicinity of traffic lights on the east and west sides of the overpass) adds to this through-traffic penalty. The I-85 DDI overpasses near Concord don’t have adjacent traffic intersections and perform much better.

—Jack Hangen, BEE, MEE
Cornelius