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PDF (DX094490.pdf) - White Rose Etheses Online

PDF (DX094490.pdf) - White Rose Etheses Online

164 on the above range

164 on the above range is shown on figures 6.4 and 6.5. Both figures relate to the effective lanes obtained for = 2000 veh/hr. Figure 6.4 is a plot of Ne vs a while figure 6.5 of Ne vs . It can be seen that N is sensitive to the value of a, also, to the values for a = 2.00 sec but not for a ^ 2.50 sec. As carl be seen from figure 6.4 Ne is directly related to a, implying that the maximum effective lane number is achieved when drivers have high critical gap values. The conclusions reached from the above is that flared entries are more beneficial and, therefore, justified for con- itions of medium to high circulating flows with driver populations exhibiting slow gap-acceptance behaviour. Before- and-after studies at roundabouts being converted to flared layouts have so far reported only on the change in capacity. It would be of interest to examine whether such changes are accompanied, also, by changes in gap-acceptance parameters. It is conceivable that at the new layouts, due to the- capacity increase and hence due to the reduction of pressure on individual drivers, their gap-acceptance parameters would increase in value. This would result in asra1ler increase in capacity than would have been expected. .6.3.2 Entry Flow and Turning Proportions The simulation was used to establish the effect of turning proportions of the entering vehicles on the entry flow. The term entry flow is used instead of capacity to indicate that this section included the study of roundabout operation under such conditions that the entry was not saturated. The turning proportion was assumed to consist of at

165 least 50% straight ahead traffic, while the other two directions could each achieve a maximum of 50%. The set of proportions used was the following LT(%) ST(%) 0 100 10 90 5 90 0 90 25 75 20 75 15 75 10 75 0 75 40 60 30 60 20 60 10 60 0- 60 50 50 40 50 25 50 10 50 0 50 RT (%) 0 0 5 10 05 10 15 25 0 10 20 30 40 0 10 25 40 50 The above sets were considered that they covered satisfactorily the range of values that could be expected in practice. The above turning proportions were used with two sets of gap-acceptance parameters, which used the values of two of the entry data sets recorded at the Sheffield sites and analysed by the method described in Chapter 4. The values used were (1) cx = 3.75 sec, = 2.60 sec, lane 1 of Moore Street Roundabout, (2) cx 2.80 sec, = 1.68 sec, lane 1 of Castle Square Roundabout. The values of the circulating flow, Q1, and of the entry demand flow, Q2, were:

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PDF (267549_VOL2.pdf) - White Rose Etheses Online
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