In 1997, N. Wu proposed a Universal Gap Acceptance Approach to calculate roundabout capacities as an expo ential equation relating entry capacity to circulating flow, the number of circulatory lanes, the number of entry lanes, the critical gap, move up time and minimum headway in circular stream(53).
Kreisel is the software program developed by W. Brilon and his research team at the Ruhr University in Bochum, Germany to calculate roundabout capacities and delays. Version 4.1 of Kreisel was published in November 1996. The unique aspect of this program is that it calculates entry capacities for the German methods, as well as for the British method by Kimber, for the French methods (by Louah, CETUR, and Girabase), for the Swiss methods (Emch + Berger and ETH Lausanne) and for the Troutbeck method
(1989). For the German method, Kreisel also takes
account the effect of the pedestrian crossings on entry capacity.
The Swiss Roundabout Guide, prepared by the Institute of Transportation of the Federal Polytechnic School of Lausanne under the direction of Professor Bovy and under conractwith the Swiss Fund for Roadway Safety, proposes a linear empirical formula, similar to the CETUR formula, but with a d i f f e r e n t s l o p e ( 1 0 ) . I t a l s o e x p r e s s e s t h e e n t r y c a p a c i t y C e a s function of the impeding flow Qg: a
Ce = 1'500 8/9 * Qg (pcph)
Q g = b * Q c + a * Q s ( p c p h )
FIGURE 19 Capacity factors from Swiss roundabout guide(54).
Q c = c i r c u l a t i n g f l o w , Q s = e x i t i n g f l o w , a pcph = passenger car equivalents per hour. n d
The coefficient a takes into account the impedance of the entry due to the exiting flow. It has been determined by the simulation model to be a function of the distance between the conflict points of exit and entry (see Figure 19). The value of a is to be taken from the diagram in Figure 19.
Coefficient takes into account the number of circulatory
lanes as follows:
one circulatory lane: two circulatory lanes: three circulatory lanes:
= 0.9 1.0
= 0.6 0.8
= 0 5 0.6.
To determine the capacity in the case of several entry lanes, a saturation coefficientTCU is determined at the entry point e and at the point of conflict on the circulatory lanec:
T C U e =
γ * Qe Ce
= percentage of saturation
γ * Qe 8 . 9 *Qg 1'500
= percentage of saturation at conflict point
The variable takes into account the number of entry lanes and its value is:
one entry lane: two entry lanes: three entry lanes:
= 0.6 0.7