2012 Issue

43 the ramp traffic. 2 The DDI design may become inferior to other interchange designs when ramp movement volumes approach through movement volumes. The DDI design may not be able to co- ordinate all movements effectively if they are all equally heavy. 1 Options for DDI Signal Operations Two basic signal phasing strategies have been developed, with one favoring cross-street movements and the other favoring off-ramp movements, as shown in Figure 2 and Figure 3, respectively. The goal of the phasing strategy should be to minimize the queuing between the crossover intersections, ensuring that left-turning vehicles onto the on-ramps are not blocked. 3 DDI Benefits Two paramount benefits of DDIs are the ability to retrofit an existing interchange and the efficient signal operations. A conventional interchange can be modi- fied to handle more traffic without the additional cost of widening the bridge structure. 4 The Bangerter Highway DDI and Timpanogos Highway DDI used this approach. For example, the cost to retrofit the Timpanogos Highway interchange to a DDI was $7.5 million rather than the estimated $25 million cost to construct a new interchange. 5 In addition to construction cost savings, the efficient signal operations result in user savings as well. In every traffic signal, there is a certain amount of time taken away from efficient operation. This lost time includes the transition time between movements, the yellow and all-red times when no ve- hicles are moving, and the time it takes to get vehicles moving again after stopping. Each signal within theDDI is timed as a two- phase signal, compared with three or four phases needed in other interchange types. This two-phase operation allows for shorter cycle lengths and less lost time. Recover- ing this lost time results in fewer delays for drivers using the DDI. DDI Challenges While the simpler two-phase operation provides for more efficient operation, some challenges exist when coordination with nearby traffic signals is required. The crossover intersection operation cannot serve two opposing directions at the same time, which reduces the ability to provide for coordinated traffic flows. For example, the northbound through can- not be served at the same time as the southbound through, as shown in Figure 4. While nearby intersections would serve both these corresponding movements simultaneously, this progression would be broken by the DDI crossover. A wider spacing between crossover and adjacent intersections is required to allow for more options in coordinating signal operations. Traffic modeling software (such as VIS- SIM) should be used to determine if the proposed signal spacing and phasing will provide adequate operation. Another challenge inherent in any new concept is in managing the driver expecta- tions. It takes time for motorists to become familiar with new traffic concepts such as the DDI. A combination of media campaigns, daily experience, and specific design ele- ments including signing and striping can be utilized to overcome this challenge. Research done by the Federal Highway Administration 6 determined that errors were no more likely with the DDI than with a conventional diamond interchange. Properly designed, the DDI provides an increased level of safety to drivers when compared to other diamond interchanges. Case Study: I-15/Pioneer Crossing in American Fork Selection of the DDI The original proposed design for the Pioneer Crossing interchange was to use a single-point urban interchange, similar to those found throughout the Wasatch Front. As part of the design-build selection process, an alternate concept for using a DDI was approved for use. The contractor was able to provide cost savings, better maintenance of traffic during construction, and, ultimately, decreased driver delay in using the new interchange design. INTERCHANGE | continued on page 44 Figure 1: Example layout of a DDI. 3