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PDF (DX094490.pdf) - White Rose Etheses Online

PDF (DX094490.pdf) - White Rose Etheses Online

6.1 Introduction 160 The

6.1 Introduction 160 The simulation model SPHT was used to establish the effect of turning proportions on roundabout capacity and delay. It was used to establish the relationship between delay and the flow and gap-acceptance parameters. The per- formance of flared and straight entries was compared throughout. The chapter also covers the validation of the model using the observed capacity figures and the relationship between input and simulated circulating flows. 6.2 Validation of the Model The capacity estimates produced by the computer model developed were compared with observed values of capacity at three entries, to establish how realistically the model behaved. The values of circulating and entering flows abstracted from the videotaped data were used as the observed values. As was explained in Chapters 3 and 4 the observed sites were operating in a way that did not allow the abstraction of a figure of capacity for the whole of the entry. Instead, it was possible to estimate only the capacity of specific lanes. Therefore, the comparison would be valid only if the model simulated the operation of individual lanes. The model has the ability of both simulating a single-lane entry and, also, of providing estimates of the use of each lane. The second case involves the simulation of a flared entry at such conditions that the entry demand flow is greater than the capacity of the entry as a whole. This ensures that the capacity of each lane not directly affected by the flare is

161 equivalent to the capacity of a single-lane entry. The data collected at the three public road sites in Sheffield provided values for the circulating and entry flows, the gap-acceptance parameters and the minimum circ- ulating headway for each site, (see Chapters 3 and 4). They were used as input for the simulation except f or the entering flow. A sufficiently large demand flow was input to ensure Continuous queueing and capacity operation of the junction. Therefore, the simulated conditions resembled the observed as close as the model allowed. Table 6.1 and figure 6.1 demonstrate the agreeement between the observed and simulated values. The largest percentage difference is 15.2% while the average percentage difference (ignoring signs) is 7.4% and the standard deviation of the percentage difference is 5. 1%. A further point of interest is the relationship between the input value for the circulating flow and the ones actually simulated. The agreement is demonstrated in Table 6.2 and figure 6.2. 6.3 Roundabout Performance The simulation . model SP liT was used to model an entry which had two lanes at the approach section and four at the stop line. At times it was modified to allow the simulation of a straight entry having two lanes throughout. The modifications involved changes in the input values and in a DATA statement in the program itself. The simulation provided estimates of average delays to queueing vehicles and entry flows for a wide range of flow

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