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PDF (DX094490.pdf) - White Rose Etheses Online

PDF (DX094490.pdf) - White Rose Etheses Online

76 This is useful in

76 This is useful in providing a direct comparison with the results obtained by the analysis of the present study for both observed and simulated data. It should be noted, however, that the values of q5 as supplied in Armitage and McDonald (1977, Appendix 1) might be a source of errors. In that study it is not mentioned whether the values given have been divided by the number of lanes for each site. If they have consistently followed the practice of dividing the slope by the number of lanes, as indicated in Fig. 4.6, then the values provided can be used to calculate by equation 4.8. Otherwise serious errors can be introduced. Studying the results of Table 4.1 the values of calculated as above often appear very low, sometimes they are less than 1 second. This suggests that the values of given are for the whole entry and are not per lane. However, among the data provided for each site, the number of lanes is not included, therefore it is difficult to justify any other use of the q 5 value. The gap-acceptance parameters estimated in the above way have been grouped according to area and whether the site was a public road or a test track at TRRL. For each group the average values of a and were calculated. They are included in Table 4.2. The mean values over all the sites are the following a 2.86 sec 1.43 sec. The a value compares favourably with values proposed by other researchers. However, the value is lower than any

77 proposed by Bennett (1971), Horman and Turnbull (1974) and Armitage and McDonald (1974). The lowest suggested value by any of the above is 2.00 seconds. This discrepancy must arise because the q 5 values have not been divided by the number of lanes of each entry. Furthermore, there are no data relating to the actual use of the entries, some of the plans included in the 1977 report do not indicate the number of lanes each entry was designed to have, and finally the entry width, El, as defined (see Fig. 4.5) does not represent a satisfactory alternative to the number of lanes. From the above, it follows that if is under- estimated so will be the value for c, the critical gap, as the two are related. This can be seen in equation 4.7. Therefore, both averages given above should not be considered accurate, as both underestimate the true values. 4.4 Some Linear Models Suggested by Previous Research The analysis of data to abstract values for gap- acceptance characteristics is similar for both T-junctions and roundabouts. In both cases the entry/minor road vehicles give way to circulating/major road vehicles while they wait for a suitably long gap to enter or cross the priority flow. Therefore the concepts of "critical gap" and "move-up time" are relevant to both situations. Some previous research into gap acceptance at T-junctions has proposed models for estimating these parameters which are directly relevant to the current project. They are described in more detail in the following section.

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