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PDF (DX094490.pdf) - White Rose Etheses Online

PDF (DX094490.pdf) - White Rose Etheses Online

where = Q e cx e -1 17

where = Q e cx e -1 17 the maximum minor flow (veh/hr) Q: the major priority flow (veh/hr) Qt2 a 3600 t-t - 3600 1 2 the minimum gap acceptable by drivers (sec) (eq. 2.12) t 2 : the minimum time interval required for one vehicle to follow another from the minor stream-termed the "following-gap". (sec). 2.6 Empirical Capacity Models Most of the work under this heading tries to relate the capacity of roundabouts to geometric characteristics of the junctions. The majority of this work has been developed at the Transport and Road Research Laboratory. The first attempt to describe the performance of the new layouts was carried out at the TRRL and reported by Blackmore (1970). The formula suggested deals with the whole of the junction and gives a single value of capacity. Q = K( w (eq. 2. 13) where Q: the capacity (pcu/hr) K: an efficiency coefficient w: the sum of the basic road widths in metres used by traffic in both directions to and from the junction a: the area of widening, i.e. the area within the intersection including islands, if any, lying outside 2 the area of the basic crossroads (m ) (see Fig. 2.9).

18 Blackmore reported that the highest capacities obtained by different junction types were approximately equal for the same values of parameter a. In 1969 Grant investigated some roundabouts in Aberdeen with dimensions outside the limits of the Wardrop equation. He treated each approach separately as a priority junction and developed a graphical relationship between the capacity of each entry and the dimensions of the entry. He observed that smaller gaps than usual were accepted at the small roundabouts, resulting in high capacities. Murgatroyd (1973), while examining the validity of Wardrop's formula, proposed an alternative one. It is similar to Wardrop's with p = 1.00 and with a subtractive constant: - 90w(1 + e/w) - 1 + w/ 1100 (pcu/hr) (eq. 2.14) The above symbols have the same significance as for Wardrop's equation (eq. 2.1), and again all dimensions are in feet. In 1974 Maycock proposed a model from which the capacity is determined by the conflict of entering traffic with traffic already using the circulation. He proposed a linear model approximating Tanner's relationship: q = q(1 c/cm) (eq. 2.15) where q: the maximum entry flow (pcu/hr), C: the corresponding circulating flow, and cm: constants specific to the roundabout. would be equal to the entering flow when there is no circulating flow, while cm is the circulating flow at which no entering flow would be possible.

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