Common use of SIMULATION EXPERIMENTAL DESIGN Clause in Contracts

SIMULATION EXPERIMENTAL DESIGN. To fully investigate TSP system performance under various traffic conditions, the Phase Two test corridor, i.e., the section of Washington SR 99 between 238th Street SW and 164th Street SW in Lynnwood, was selected as the simulation test site because of its large scale, complex traffic conditions, and diverse control strategies. There were 13 signalized intersections along this test corridor. Semi-actuated control strategies had been executed to coordinate signal control at these intersections. In our study, three typical signal plan groups were used to investigate TSP system operations. Each signal plan group consisted of 13 individual timing plans for the corresponding intersections. These signal plans included phase structures and timing parameters exported from the corresponding controllers. An individual VISSIM model was configured for each plan, including various field-observed traffic volumes, traffic control parameters, and so on. Because of consistent cycle lengths for all the intersections under the coordinated control mode, we were able to distinguish these signal groups by using their unique cycle length as follows: the 120-second signal plan group, the 130-second signal plan group, and 150- second signal plan group. In TSP systems there are two important pre-specified control parameters, early green time and green extension time. In the SS-RTSP system, the early green time and green extension time were pre-set to 15 seconds. It is widely recognized that these two parameters have significant impacts on system performance because they indicate the extent of priority treatment for transit vehicles. Therefore, optimizing these two parameter settings is important for improving TSP system operation efficiency. Simulation experiments were conducted with different early green time and green extension time settings under different time plans. Various ▇▇▇▇ were applied to quantify their impacts on system performance, including delays, stops, and throughputs for both transit and general vehicles. Six simulation scenarios were established as follows to fully examine TSP system operations and explore the optimal control strategy settings: Scenario 1: Fixed early green and green extension times of 15 seconds under 120- second-cycle signal plan In VISSIM, traffic generation is manipulated by a random seed number; by employing different random seeds, simulation results can be changed correspondingly, but within a certain range. To minimize the randomness of simulation results and enhance the credibility of simulation models, multiple simulation runs should be conducted. In this study, 20 simulation runs were conducted for each test scenario; among them were ten iterations with TSP functions and ten without TSP functions, each with a different random seed arbitrarily selected. The integrated results from these simulation runs were considered statistically reliable and unbiased. On the basis of actual traffic conditions and control plans, the VISSIM model was configured and calibrated. Slight corrections were made to strengthen the model’s appropriateness to the corresponding applications. Details of simulation model configuration and calibration are described in Chapter 7. The test period was specified as three hours. The outputs from the calibrated simulation models, including traffic volumes and speeds, reasonably matched our field observations. We believe that the simulation models were reasonably calibrated and did represent the real-world TSP system operations. Simulation results and discussions are provided in the next section.