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PDF (DX094490.pdf) - White Rose Etheses Online

PDF (DX094490.pdf) - White Rose Etheses Online

72 on roundabout

72 on roundabout performance during the 1970's. The investigation included the prediction of gap acceptance parameters from roundabout geometry and they proposed a method for obtaining these parameters from observed data using a least squares best-fit curve. Armitage and McDonald (1977, 1978) assumed that roundabouts operate as a series of linked T-junctions. They were interested in developing a formula that would predict the capacity and not the delay of the entering vehicles. This allowed them to use assumptions that gave simpler formulae. Thus they developed two concepts incorporated in their capacity formula. They were the concepts of lost time and saturation flow. Lost time is assumed to be a period associated with the passage of each vehicle of the circul- ating flow. During this time no entry vehicle can join the circulating flow, while at all other times they join at a constant rate which is the saturation flow. the following: The formula they proposed as the most useful is -q1(L-T = q 5 (l - Tq) e (eq. 4.2) where q 2 : entering flow (veh/s) q 1 : circulating flow (veh/s) q 5 : saturation flow (veh/s) L : lost time (s) T : minimum headway of circulating flow Cs) For further description of their capacity formula see Chapter 2 section 5. Originally they used the notation for the minimum headway.

73 They related the gap-acceptance parameters, q 5 , L and 1, to the geometric characteristics of the layout. In order to achieve this they collected data on all of the above parameters at a large number of public road sites and also in a series of test trac k experiments conducted by the Transport and Road Research Laboratory. Each of the sites was described by the geometric factors shown in Fig. 4.5. They tested all three gap-acceptance parameters against all these character- istics. The formulae they proposed are the following: = 0.12 EQ + 0.04(E1 + EO) for non-flared entries (eq.4.3) q 5 = 0.12(EO + F1(E1 + EO)/(F1 + 69) ) for flared entries (eq. 4.4.) L = 2.3 + O.006K1 - 0.04 W2 (eq.4.5) 1(j) = 1/(0.12 EO () + O.04(E1 () - EO () ) ) (eq.4.6) where all the geometric notations are as defined in Fig. 4.5. The subscripts (1) and (j) in eq. 4.6 signify the following: (i): parameters relating to the study entry (j): parameters relating to the immediately previous entry. Five different methods were used to estimate the minimum circulating headway. Briefly, these methods were: (i) the theoretical headway distribution was fitted to the observed headway data by the method of moments; (ii) the theoretical headway distribtuion was fitted to the observed headway data by minimizing (iii) the minimum circulating headway was related to the mean rejected headway;

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