2012 Issue

42 Introduction The Diverging Diamond Interchange (DDI) is a relatively new concept for handling traf- fic. Before 2009 there were no interchanges of this type in the United States. Since then, four of these innovative interchanges have been constructed and are now operating in Utah: • I-15/Pioneer Crossing in American Fork (August 2010), • I-15/Timpanogos Highway in Lehi (October 2011), • SR-201/Bangerter Highway in Salt Lake City (October 2011), and • I-15/500 East in American Fork (No- vember 2011). The design, construction, and operation of these DDIs have provided a wealth of information on how to meet the needs of the traveling public. While this interchange type is not a fix-all solution for every loca- tion, the DDI handles traffic exceptionally well when properly planned and designed. Background The DDI was first introduced in the United States by Gilbert Chlewicki at the 2nd Ur- ban Street Symposium held in Anaheim, California in 2003. The interchange he presented had the two directions of traffic on an arterial cross over each other before the freeway and then return to the regular side after. Figure 1 provides an example layout of a DDI. In this figure, the through movements on the arterial are illustrated in yellow and ramp movements in red. Both of the crossover intersections on the arterial use two-phase traffic signals. For example, Direction A goes followed by Direction B and then back to Direction A again and so on. The on-ramp movements are free-flow, while the off-ramp movements may be signalized, depending on local conditions. The main goal and benefit of the DDI is to better accommodate left-turnmovements, which enables the elimination phase in the signal cycle. 1 This in turn improves both safety and efficiency. A DDI has a higher capacity for left-turn movements when compared to the other interchange types. Separate left-turn lanes are not necessary for the DDI because left- turning traffic is on the outside lane after moving through the crossover intersection, where they can then turn left onto the on- ramp without any signal or other type of control. Proper Application of a DDI A DDI works best when the through traf- fic volume is proportionally lower than The Diverging Diamond Interchange By Michael Seely, P.E. and Luke Seegmiller , P.E.