Performance Evaluation of the Diverging Diamond Interchange In Comparison With the Conventional Diamond Interchange

The decision making process in transportation is often about making a choice between alternative solutions to a problem. To select the best alternative, the decision should be based on results from a careful study of the implications of the alternatives. This paper presents the results of a study comparing two alternative interchange designsDiverging Diamond and Conventional Diamond Interchange. The two alternatives are studied for a range of volume scenarios using traffic micro simulation and the better performance in each scenario for Diverging Diamond Interchange in terms of delay, travel time, maximum queue length etc are reported. The comparison also consists of cost-effectiveness analysis of the two alternatives. The results from the analysis help in providing guidelines to the decision makers for selecting the best alternative in terms of performance and cost. Introduction The increase in surface transportation and number of vehicle miles traveled has led to new challenges for the transportation planners and engineers of ascertaining safe and uninterrupted flow of people and goods. These challenges can be met by employing some innovative and costeffective methodologies for solving transportation problems. One of these methodologies is the Diverging Diamond Interchange (DDI). The main goal of the DDI design is to better accommodate left-turn movements and hence eliminate a phase in the signal cycle. There is no change in the freeway portion but the offramp and on-ramp movements change for left-turns. In a DDI, through and left-turn traffic on the crossroad maneuver differently from a conventional diamond interchange as the traffic crosses to the opposite side in between the ramp terminals. The DDI design was first suggested by Chlewicki (2) and a case study was performed using Synchro. Edara et al. (1) had earlier performed an analysis of the DDI using VISSIM for different volume scenarios and lane configurations. The results from both of these studies suggested that the DDI design outperforms the conventional interchange design for a balanced interchange traffic volume scenario. Federal Highway Administration (FHWA) has recently performed a driver’s evaluation of the DDI and reported that the design has safety benefits combined with the predicted operational benefits and reduced right-of-way requirement. In this paper we further analyze the DDI design for unbalanced interchange traffic volume scenarios. Also, this research includes a cost-effectiveness analysis of the DDI as compared with a conventional interchange design. The aim of this research is to determine the additional benefits of using a DDI design in place of a conventional interchange design in terms of user and construction costs. The results are reported and some recommendations are made on the circumstances best suitable for selecting DDI as an alternative. The first section of the paper describes the designs of the DDI and the conventional diamond interchange under analysis. The second section describes the methodology used, model development, traffic scenarios analyzed and the type of data collected. The third section presents the results and findings of the research and the last section gives conclusions and recommendations and the need for further research. Siddharth Sharma, Indrajit Chatterjee 3 Design of Investigation Conventional Diamond Interchange (CDI) The interchange connects the freeway to the crossroad with the help of two on-ramps and two off-ramps. Figure 1 shows that the CDI under analysis consisted of two through lanes and a separate left-turn lane in each direction between the ramp terminals. The approaches to the signalized intersections consisted of two through lanes and a separate right-turn lane on the arterials. The off-ramps from the freeway consisted of two left-turn lanes and a right turn lane. Both the off-ramps were single lanes. The distance between the ramps was 500 ft. Radii of crossover movements ranged from 150 ft to 200 ft and that of the left-turn movement was 100 ft. Diverging Diamond Interchange (DDI) Figure 2 shows that the lane configuration of DDI was similar to the CDI, but there was a change in the number of lanes between the terminals in both directions. The DDI consisted of two lanes in each direction, one through and a left-turn from a shared through and left-turn lane between the two terminals. Clearly this reduced the right-of-way requirement for the DDI as compared to a CDI between the terminals. The off-ramps consisted of two left-turn lanes and one right-turn lane. One left-turn lane and one right-turn lane led to the on-ramp. Distance between the two terminals was fixed at 500 ft. The arterial consisted of one through lane, one through + left-turn lane, and one dedicated right-turn lane. Radii of the curves were same as the radii for CDI. The design and lane configuration can be better understood with the help of the Figure. FIGURE 1 Conventional Diamond Interchange. Siddharth Sharma, Indrajit Chatterjee 4 FIGURE 2 Diverging Diamond Interchange. Model Development and Volume Scenarios The analysis methodology followed for the performance comparison of the two interchanges is microscopic simulation. The tool used for this purpose was VISSIM 4.3. This tool was chosen due to its ability to perform a detailed analysis at the microscopic level. The base model for the interchanges was coded in VISSIM using Figure 1 and Figure 2 as the background image. The traffic signal timings were obtained using SYNCHRO for signal optimization. As shown in Figure 3, for the CDI, a three phase signal was used and optimized signal timings were obtained. Figure 4 describes that for the DDI, a two phase signal was used at both the intersections and signal timings were obtained. It can be seen that DDI used one less signal phase than a CDI for each intersection. The total number of interchange phases for a CDI was therefore eight whereas the DDI reduced the total number of phases to six. Also, the left-turn movement leading to the ramp in a DDI transforms into a free movement thereby increasing the left turning capacity and decreasing delay. The signal timing used was optimized for the highest flow within each scenario. Siddharth Sharma, Indrajit Chatterjee 5