CALIBRATION OF TRANSYT PLATOON DISPERSION ALGORITHM FOR CONDITIONS ALONG MAJOR ARTERIALS IN THE CITIES OF DAMMAM AND AL-KHOBAR, SAUDI-ARABIA

The reliability and effectiveness of the TRANSYT model depend primaril y on the ability of its platoon dispersion algorithm to accurately pre dict the flow pattern from one signal to another. The algorithm is bas ed on the theory that a platoon of vehicles starting from an upstream intersection will continuously disperse as it travels downstream along the link. The amount of dispersion in the traffic flow pattern, as pr edicted by this algorithm, depends on the proper value of an empirical constant in the algorithm referred to as the ''Platoon Dispersion Fac tor'' (PDF). The objective of this study was to determine the value of PDF which best simulates the traffic conditions in the study area. Th e study was conducted along two major arterials in areas of mixed resi dential and commercial activities, Each arterial consisted of four sig nalized intersections with four approaches to each of them. The signal s at these intersections were pre-timed with four protected phases. It was concluded that the average best fit (calibrated) PDF values in th e study area were 28 and 40 for low and moderate friction links, respe ctively. On the other hand, the TRANSYT-7F manual suggests a value of 25 for low friction links and 35 for moderate friction links. Neverthe less, the change in performance measures resulting from using the cali brated PDF value over the suggested ones was small and seems to be wit hin the accuracy of the TRANSYT-7F model.