2012 Issue

45 drivers. The operation of an intersection is often reported using a letter grade fromA to F. This grade represents the Level of Service (LOS) provided, and can be determined using equations found in the Highway Capacity Manual (HCM), or is measured directly using a traffic model. LOS ranges fromLOS A (no congestion or delay) to LOS F (gridlock conditions with significant congestion and delay). LOS D is generally considered acceptable for urban conditions. Traffic count data collected in 2011 and 2030 projections for the PM peak hour are provided in Figure 5. Several microsimulation traffic models were developed using the VISSIM traffic simulation software to analyze existing traffic, and to predict future operations. Traffic models were created for the 2008 Pre-DDI, 2011 Post-DDI, and projected 2030 Post-DDI traffic conditions. The results of the analysis are provided in Table 1. Traffic operations at the interchange are anticipated to remain at acceptable levels through 2030 and beyond. The two-phase signals in the DDI provide for a very efficient traffic pattern. Conclusions The DDI presents several advantages over other freeway inter- change types. Significant cost savings can be realized when used in a retrofit application, or when a smaller structure is required. The ability improve signal operations with a retrofit of an existing interchange makes the DDI an attractive alternative for freeway designers. However, local conditions such as close signal spacing or lack of available right-of-way for the crossover signalsmay reduce the attractiveness of using a DDI. Since its conceptual introduction to the United States in 2003, the DDI has been gaining more sup- port as an effective design alternative to manage traffic demands. We expect that the DDI will continue to be a powerful tool in the designer’s toolbox to reduce traffic delays, even as demand increases on our streets and highways.  Endnotes 1 Chlewicki, G. New Interchange and Intersection Designs: They Synchronized Split-Phasing Intersection and the Diverging Diamond Interchange. 2nd Urban Street Symposium, Anaheim, California, 2003. http://www.urbanstreet.info/2nd_sym_proceedings/Volume%202/Chlewicki.pdf. Accessed December 1, 2011. 2 Siromaskul, S. Speth, S. A comparative Analysis of Diverging Diamond Interchanges, 2008 ITE Annual Conference Anaheim, California, 2008. 3 Seegmiller, Luke W. Design Evaluation Diverging Diamond Interchange Guidelines No. UDOT-DE- BO, 2008. 4 Bared, J. G., P. K. Edara, and R. Jagannathan. Design and Operational Performance of Double Crossover Intersection and Diverging Diamond interchange. In Transportation Research Record: Journal of the Transportation Research Board, no. 1912, Transportation Research Board of the National Academies, Washington, D.C., 2005, pp. 31-38. 5 Boal, J. Crews to convert innovative I-15 interchange in one weekend, August 9, 2011, KSL News. http://www.ksl.com/?nid=148&sid=16738986 Accessed December 1, 2011 6 “Drivers’ Evaluation of the Diverging Diamond Interchange”, http://www.fhwa.dot.gov/publications/ research/safety/07048/, Accessed December 1, 2011 Table 1: VISSIM Model Analysis Comparison Interchange Intersections 2008 Pre-DDI 2011 Post-DDI 2030 AJR Post-DDI Volume Delay (sec/veh) LOS Volume Delay (sec/veh) LOS Volume Delay (sec/veh) LOS SB Ramps 2,077 21.8 C 3,197 16.3 B 4,544 24.3 C NB Ramps 2,832 15.8 B 3,443 20.7 C 4,582 31.1 C Figure 5: 2011 PM Peak (2030 PM Peak) Hour Volumes