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PDF (DX094490.pdf) - White Rose Etheses Online

PDF (DX094490.pdf) - White Rose Etheses Online

154 real time would take

154 real time would take 10 to 40 sec of CPU time depending on the volume of the circulating and demand flow and on the turning proportions. 5.6 Description of SPHT A simplified flow chart of the simulation program is shown in Fig. 5.2. SPHT consists of a MAIN segment and sub- routine RANDOM. The MAIN segment can be divided in the following parts according to their function: (a) Generation of circulating vehicles. The individual and cumulative headways of the circulating vehicles are calculated. (b) Generation of entry vehicles and turning movements. The turning movement of the vehicle is established using the same random generating algorithm as for generating the circul- ating and entering vehicles. The individual and cumulative entering headways are calculated. (c) Assignment of position and departure time. The turning movement of the entering vehicle determines the position of the vehicle in the queue. The position associated with the earliest departure time is the one preferred by the entering vehicle. (d) Entry check. The departure times are compared with the arrival time of the next circulating vehicle and the number, if any, of possible entries is noted. Ce) Updating of departure times. Any queueing vehicle which refuses an offered gap has its departure time updated to account for the incurred delay. (f) Moving-up sequence. When the junction operates at or above capacity, most gaps that are accepted by some queueing

155 vehicles are not of such length to allow the complete discharge of the queue. In cases when vehicles remain in the queue this sequence moves them forward and re-assigns them in new positions depending on their turning movement. (g) Calculations. These are performed in two positions. After each vehicle enters into the circulating flow, its individual delay and other relevant characteristics are computed. The overall figures are calculated after the simulation period has ended. Subroutine RANDOM generates the pseudorandom fractions required to generate the circulating and entering vehicles as well as the turning rLlovement of the entering vehicles. 'Each sequence is initiated by a different initial value, therefore each one is different and can be varied independently by the other two. 5.7 Input and Output of SPHT The input consists of the following parameters: Ql: the circulating flow (veh/hr); Q2: the demand flow (veh/hr); TAU: the minimum circulating headway (sec); ALPHA: the critical gap (sec); BETA: the move-up time (sec); 19, J9, K9: initial numbers for the pseudorandom fraction sequences; NS: duration of simulation (sec); AN: the number of position of the first row; AP: the number of positions per row before the flare; AL: the number of rows over which the flare is developed; OTL: a parameter, in seconds, which determines whether

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