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PDF (DX094490.pdf) - White Rose Etheses Online

PDF (DX094490.pdf) - White Rose Etheses Online

78 Some aspects of

78 Some aspects of T-junction operation are significantly different from roundabout operation. They have to be taken into account when the models for T-junctions are compared with models for roundabouts. The major points of difference are: (1) The major road flow can be in two directions while the circulating flow is always one directional; (2) The minor road vehicles can either merge with the stream coming from the right or cross that stream and merge with the stream from the left; (3) There might be right-turning major road vehicles whose queue can inhibit the right-turning minor road vehicles; (4) The major road vehicles usually have higher speeds than the circulating ones at roundabouts since they do not have to slow down as they approach the junction; (5) The design of a T-junction minor road and an entry road to a roundabout differ in such ways as to be easier for vehicles to enter from a roundabout entry than from a minor road at a T-jurction, for example flaring is almost exclus- ively used at roundabouts, there is better visibility at roundabouts especially for vehicles not at the give way line etc. From the above it is clear that methods developed for T-junctions are not directly relevant for roundabout operation. However, the analysis of the acceptance behaviour by minor road vehicles can distinguish left- and right- turning streams. In such cases the relationships for the left-turning minor road stream have similarities with roundabout operation. Even in such cases, however, only the form of the relationship is relevant and not the reported values for the gap acceptance parameters which tend to be

larger than the respective ones for roundabouts. 79 4.4.1 Description of the Models The four models described here are linear relation- ships between the number of vehicles entering, N, and the gap-length in seconds, T. Pearson and Ferreri (1961) examined queue acceptance in terms of the percentage of gaps of a given size accepted by streams of vehicles entering a freeway. From their gap acceptance distributions, they derived a linear relationship between N and T: N = 0.28 T - 1.07 (eq. 4.9) They claim a high correlation coefficient for this relation- ship but the method of derivation is not clear. In 1974, Watson proposed a capacity model for roundabouts which related the gap-acceptance parameters to the geometry of the site. He reported that N and T have a linear relationship. The two gap-acceptance parameters used were m and c, where m was the slope of the straight line and c the intercept with the y-axis. In the regression no rejected gaps are included, data from the whole entry are included, and N is assumed to be the independent variable. In his analysis Watson does not relate the gap-acceptance parameters of his method to the critical gap and the move-up time, but the relationships are as follows 1 m 1 1

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