This article describes the design and benefits of the double crossover diamond (DCD) interchange, which is a variation of the traditional diamond interchange. The geometric design of the DCD channelizes vehicular traffic on a grade-separated crossroad from the right side of the road to the left side and then back again at the ramp terminals. What distinguishes the DCD from the conventional diamond interchange is that it combines left-turning traffic with through traffic. This is accomplished by having both left-turn and through vehicles cross over to the opposite sides of the roadway at the ramp terminals. The result is that northbound traffic travelling over the bridge travels on the roadway on the left (between the two ramp terminals), and the southbound traffic also travels on the roadway to the left. The DCD design offers advantages in operational capacity, safety, environment protection, and cost compared with the conventional diamond interchange. Although the DCD design has been used in France for 30 years, until recently few U.S. engineers were aware of its existence. The Federal Highway Administration began to explore the geometric and operational aspects of the DCD design in 2004. To accomplish this, they developed two types of simulations: microsimulations that facilitated modelling of design and operational characteristics, and a full-scale drivable model that allowed direct observation of driver performance. The Missouri Department of Transportation opened the first DCD in the United States on June 21, 2009. The Missouri project was significantly cheaper and faster to build than a single-point interchange. During the first 6 months of operation, the DCD reduced crashes by 50% between the ramp terminals and by 25% between the first intersections north and south of the interchange, compared to the same period in 2008. (Author/publisher)
Samenvatting