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PDF (DX094490.pdf) - White Rose Etheses Online

PDF (DX094490.pdf) - White Rose Etheses Online

166 500, 1000, 2000,

166 500, 1000, 2000, 3000 veh/hr Q 2 : 500, 1000, 1500, 2000 veh/hr. Thus the effect of the turning proportions was examined using two sets of gap-acceptance parameters which represented the higher and lower possibilities of observed values. The above analysis was carried out for flared and straight entries. Figures 6.6 to 6.9 show the results of the above analysis for four selected sets of the possible coirthinations for both the flared and straight entries. The results of the remaining sets were similar, hence this selection was con- sidered adequate. The first and second sets represent operation of below saturation flow levels (figures 6.6 and 6.7), while the third and fourth sets represent operation with the entry saturated, (figures 6.8 and 6.9). The conditions of figures 6.6 and 6.7 allowed virtually the whole of the demand flow generated to enter. In such below capacity cases it can be seen that variation in turning proportion is not affecting the entering flow for both flared and straight entries. At-capacity operation differs between the two types of layout. Turning proportion does not affect capacity of straight entries, while for flared the capacity is directly related to the left-turning proportion, (figure 6.8a and figure 6.9a). The explanation to this lies on the use of the lanes. At flared layouts it was assumed, and supported by observations, that the extra lanes near the stop line are used by left-turners, (see Chapters 3 and 5). At 0% left turning proportion, the nearside lane which is exclusively used by

167 left-turning vehicles, is not used at all. Therefore the entry becomes equivalent to a three-lane entry. The gradual decrease in capacity with a reduction in the left-turning proportion is also associated with the difficulty such vehicles have in reaching the flared area, since they share the nearside approach lane with straight-through vehicles. The entering flows for the flared approach, of figure 6.6, have a difference of the extreme values of 9 veh/hr (2.0%), whilst for the straight entry, it is 5 veh/hr, (1.1%). Similarly for figure 6.7 the flared approach difference was 7 veh/hr (0.7%) and for straight 20 veh/hr (2.1%). As can be seen the variation is small, further there is no discernable pattern in. the variation over the lef t- turning percentage. The flows of the straight approach for figures 6.8b and 6.9b present similar values, respectively 20 veh/hr (3.9%) and 6 veh/hr (3.7%). However, the flared approach represents a much wider range. The 'difference between the extreme values is 196 and 79 veh/hr respectively (24.4% and 30.7%). The effective numbers of lanes of the maximum and minimum values respectively are 3.65 and 3.05 for figure 6.8, while for figure 6.9 they are 3.91 and 3.11. 6.3.3 Delay and Turning Proportion The simulation model estimates values for the average delays incurred by vehicles of the entering flow while queueing to join the circulating flow. The simulation runs over which delay was estimated lasted one hour. Over this period the values of the circulating and entry demand flows

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