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Transportation planners and traffic engineers are facing the challenge of inventing ways to mitigate congestion during the peak hours. Alleviating delays and improving safety for passengers and pedestrians is the primary motive. One way of achieving this objective is to search for alternative intersection and interchange designs. This paper presents the results of a study on two new alternate designs-Double Crossover Intersection and Diverging Diamond Interchange. These designs are studied for different traffic scenarios using traffic simulation and the results showed better performance during peak hours when compared to similar corresponding conventional designs. Better performance includes better level of service, lesser delays, smaller queues, and higher throughput. BACKGROUND Transportation planners and traffic engineers are facing the challenge of mitigating congestion during the peak hours and at lower costs. Alleviating delays and improving safety for motor vehicles and pedestrians are primary motives. In urban areas the land available for constructing roads is less and hence should be used more judiciously by designing roads, intersections, and interchanges that occupy less right of way. One way of achieving all these objectives is to search for alternative intersection designs. Researchers have developed several innovative intersection designs in the past to address these problems. These designs include the quadrant roadway intersection, median U-turn, superstreet median, jughandle, split intersection, and the continuous flow intersection (CFI). The most influential factor in the intersection performance for heavy flows is achieved by reducing the number of phases in the signal cycle. The CFI especially is finding increasing acceptance in the United States lately (1). Chlewicki (2) suggested two new designs for intersection and interchange designs-the Synchronized Split-Phasing (SSP) Intersection and the Diverging Diamond Interchange (DDI). As in the CFI, SSP design also disperses the flow of traffic before reaching the main intersection. The synchronized split phasing design allows both the through and the left movements to cross over prior to the intersection. (see Figure 1(a)) The main goal of the DDI design is to better accommodate left-turn movements and hence eliminate a phase in the signal cycle. Figure 1 (b) shows the layout of the diverging diamond interchange. The freeway portion does not change but the movements off the ramps 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. Chlewicki (2) discusses the simulation tests performed for a case study intersection and interchange using Synchro as the simulation tool. Results showed that the SSP and DDI designs outperform similar corresponding conventional designs. In his conclusion, Chlewicki (2) discusses the future scope of research including analysis of different volume ratios and tu...

Evaluating the traffic impacts of work zones is vital for any transportation agency to plan and schedule work activity. Traffic impacts can be estimated by using microscopic simulation models. One challenge in using these software models is obtaining the desired work zone capacity values, which tend to vary from state to state. Thus, the default parameter values in the model that are suitable for normal traffic conditions are unsuitable for work zone conditions, let alone for conditions specific to particular states. Although a few studies have been conducted on parameter selection to obtain desired capacity values, none of them have provided a convenient look-up table (or chart) for the parameter values that will replicate field-observed capacities. Without such provision it has not been possible for state agencies to use any of the research recommendations. This study provides the practitioner a simple method for choosing appropriate values of driving behavior parameters in the VISSIM microsimulation model to match the desired field capacity for work zones operating in a typical early-merge system. The two most significant car-following parameters and one lane-changing parameter were selected and varied to obtain different work zone capacity values. CC1 is the desired time headway, CC2 is the longitudinal following threshold during a following process, and the safety distance reduction factor is representative of lane-changing aggressiveness. It has been verified that the recommended parameter values not only produce the desired capacities but also create traffic conditions consistent with traffic flow theory.

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