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# PDF (DX094490.pdf) - White Rose Etheses Online

PDF (DX094490.pdf) - White Rose Etheses Online

## 96 between the

96 between the predictions for each lane. Consistently the value of the critical gap of the offside lane is higher than for any of the other lanes. This is more pronounced in the cases of the Castle Square and the Park Square Roundabouts. The explanation could be that vehicles using the offside lane tend to be right-turning. Their manoeuvre usually involves circulating near the island, therefore they have to take into account the flow pattern of all streams circulating. The manoeuvre can be described as involving merging and weaving. Also, often the angle they approach the give-way line is sharper than at the other lanes, especially the flared ones. Their manoeuvre has some similarities to "crossingt' at a priority T-junction. Vehicles turning left or going straight ahead have only to merge with the nearside circulating flow stream. The difference in the predicted values of the move-up time does not demonstrate any consistent pattern. The higher values observed at Castle Square Roundabout r;ay be associated with the poor visibility ot vehicles in the queue at Arundel Gate. 4.8 The Minimum Headway of the Circulating Flow 4.8.1 Introduction The simulation model developed in the previous research (Natsinas, 1979) assumed that the headway distribution of the circulating flow follows a shifted negative exponential distribution. The input to the program included a variable, TAtJ, that described the minimum allowable headway of the distribution. During that research the value of TAU was assumed to be constant and equal to 1 sec.

97 When the model was validated by comparing its predictiorto observed data, it became important to examine critically the behaviour of TAU in the simulation. One of the first tests was to examine the sensitivity of the capacity predictions to variations of the minimum headway. It soon became obvious that the predictions were very sensitive to the TAtJ value. For example, one of the simulation runs assuming 2596 veh/hr circulating flow, predicted a capacity p = 774 veh/hr for i = 0.50 sec and p = 129 veh/hr for T = 1.00 sec It was considered necessary to examine in more detail the suitable models for the headway distribution, as well as, the suitable value of TAU if a shifted negative exponential distribution was used.. This section looks at some theroetical models proposed for the headway distribution, presents the results of the analysis of the observed and simulated data, and concludes by proposing the use of shifted negative exponential distribution with the minimum headway equal to 0.20 sec. Note on notation: throughout the section the notation followed is the following t: the minimum headway of the circulating flow. 4.8.2 Distributions of Traffic Headways The description of the traffic distribution along a road has attracted considerable attention from traffic engineers and statisticians. Statistical distributions are useful in describing a wide variety of phenomena where there is a high element of randomness. Such distributions can be divided into counting and interval distributions. Counting distributions describe the occurrence of events that can be

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