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PDF (DX094490.pdf) - White Rose Etheses Online

PDF (DX094490.pdf) - White Rose Etheses Online

4.1 Introduction 62 The

4.1 Introduction 62 The computer program, developed to simulate an entry into a roundabout, is designed to generate individual entry vehicles which progress through the queue of traffic until they reach the stop 1ine where they reject or accept the gaps in the circulating flow as they are presented to them; (for further description of the model see Chapter 5). This entails the assignment to the entering vehicles of a set of parameters related to their gap-acceptance behaviour. The parameters involved are the critical gap, c, and the move-up time, 3. The critical gap is a measure of the minimum length in time, between circulating vehicles, for the first vehicle in the queue to join the circulating flow; the move-up time is a measure of the additional length required for any sub- sequent queueing vehicles to accept the same gap. Another parameter involved is the minimum headway, t, of the circul- ating flow. The values of these parameters are significant as they describe the performance of the queueing vehicles in the simulation and the size of gaps offered to them. For the model to give realistic predictions these parameters must have values that correspond to observed data. This Chapter describes various methods to obtain these values from observations proposed by previous research. It suggests some modifications to these methods, and finally describes the analysis of the collected data to obtain the values used to validate the simulation. Notation: There has been no uniform notation which has been universally adopted by previous researcheis in

63 this field. The conventions used here are: a the critical gap (sec) the move-up time (sec) T the minimum circulating headway (sec). These notations will be applied throughout this Chapter. When previous research, which has used different notation, is described the present notation will be used instead. However, it will be made clear that a change has taken place from what was the original notation. 4.2 Gap Acceptance Studies The study of parameters associated with the accept- ance of gaps was initially related to priority junctions and pedestrians crossing roads at non-signalized positions. Gap acceptance became relevant to studies of roundabouts only after the introduction of priority-to-the-right rule in 1966. The operation of a roundabout was likened to that of a. series of T-junctions, and Tanner's formula of capacity prediction for priority junctions was applied to roundabouts (Tanner 1962, Tanner 1967). Tanner's formula uses two parameters relevant to the minor stream, the critical gap, a, and the minimum headway, 2 The latter is defined as the time between successive vehicles accepting the same gap, therefore 2 is analogous to the move-up time, , used in the present study. Before the introduction of the priority rule at roundabouts, a lot of research was carried out relating to the estimation of the critical gap parameter for T-junctions. After 1966 this research has become relevant to roundabouts. Simultan- eously, other models of theoretical gap-acceptance behaviour

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PDF - White Rose Etheses Online
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