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

PDF (DX094490.pdf) - White Rose Etheses Online

## 162 and gap-acceptance

162 and gap-acceptance parameters. The results were used to study the effect on capacity, entry flow and delay of the gap- acceptance parameters, the turning proportions and the circ- ulating flow. Further, the performance of flared and straight entries were compared. The following sections describe the above in detail. 6.3.1 The Effective Number of Lanes A measure of the increase in capacity due to flaring that has been proposed (Ashworth & Laurence, 1977; Laurence & Ashworth, 1979) is the effective number of lanes, N. If a flared entry has N lanes at the stop line, Ne is defined as the number of non-flared lanes that could have the same capacity as the flared layout. They tentatively suggested that there is a linear relationship between Ne and N: N = 0.33N + 1.3 e The simulation model was used to predict the capacity.of (eq 6.1) flared and straight entries which, subsequently, were compared to establish the effective increase in capacity. The compar- ison was performed over the following ranges of values: circulating flow (Q 1 ) 500 veh/hr steps, 0.0 - 4000 veh/hr in critical gap (a) = 2.00 - 3.50 sec in 0.50 sec steps, move-up time () = 1.50 - 3.00 sec in 0.50 sec steps. Throughout it was assumed that a ^ 3. According to equation 6.1, a flared entry with N = 4 has an Ne = 2.62. The formula does not account for any other

163 parameters. It was found that as capacity is a function of the circulating flow so is the effective number of lanes. Values of Ne were calculated for all combinations of the above range. Figure 6.3 is a plot of all the points obtained together with an envelope within which all such points lie. The common elements of behaviour are that: (1) For all combinations of the gap-acceptance para- meters, and Q 1 = 0.0 veh/hr the value of Ne is equal to 2, i.e. the flare is not contributing any extra capacity than a two-lane straight entry. (2) As increases N also increases but at differing rates for the various gap-acceptance parameter combinations. The value of N = 3.00 (i.e. 50% increase in e capacity) was achieved by all such combinations for = 2300 veh/hr approximately, while at = 4000 veh/hr only the combinations a = 3.50 sec, = 2.50 sec and a = 3.50 sec, = 3.00 sec had achieved N = 3.99 (i.e. almost 100% increase in capacity). The value Ne = 2.62 (suggested by Ashworth & Laurence) was achieved by all combinations at = 1525 veh/hr approximately. See figure 6.3a for com- parison with observed values of Ne reported by previous research. (3) At each Q1 value, the range of Ne values over all the gap-acceptance parameter combinations differed, the largest range being at = 2000 veh/hr. The maximum Ne at that value, was 3.52 while the minimum was 2.83, i.e. a difference of 0.69 lanes. At = 0.0 veh/hr there was no difference, while at Q 1 = 4000.0 veh/hr the range was 0.31 lanes. (4) The effect of the gap-acceptance parameters

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