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T.P.D.M.V.2.4
4.5.9 Capacity of Roundabouts
4*5.9.1 What were previously referred to as "conventional roundabouts w
consisted of large central islands with long parallel sided
weaving sections. The capacity of the roundabout was measured
in terms of the capacity of its individual weaving sections
using Wardrop f s Formula. With the introduction of small and
mini roundabouts the emphasis was placed on shorter wider
weaving sections (in fact little weaving takes place) and flared
approaches. The capacity of this type of roundabout was
calculated for the junction as a whole based on the basic road
widths and areas of widening at the junction. These formulae
have now been superseded by a predictive equation, giving the
capacity of each entry to the roundabout, which is applicable to
all types of roundabout.
4.5*9*2 Since the introduction of the offside priority rule traffic
waiting to enter a roundabout on one am has to give priority to
traffic already on the circulatory carriageway crossing the
entry. Consequently, the entry capacity decreases as the
circulating flow increases, since there are then fewer
opportunities for waiting drivers to enter the circulation.
When entry opportunities do present themselves the number of
vehicles which are able to avail themselves of the opportunity
depends on the entry width, the circulatory width, the entry
angle and other geometric characteristics of the roundabout
layout. The predictive equations are thus stated in terms of
the circulating flow and the geometric parameters, and are based
on multiple regression analyses from observations at a large
number of sites in the United Kingdom.
4.5.9*3 In evaluating a proposed layout, the design flow for each entry,
derived as described in para* 4.2.4, should be compared with the
calculated capacity to produce a design flow/capacity ratio
(DFC). The entry capacity is defined as the maximum in-flow
from an entry when the demand flow is sufficient to cause steady
queueing in the approach. A DFC ratio of 100$ would therefore
indicate continual queueing and could not be considered
acceptable. A DFC ratio of 8535, indicating that queueing would
theoretically be avoided in 85% of cases, can be considered
reasonable. A DFC of 70% indicates that queueing will be
avoided in 95! of cases. The acceptable value of DFC will vary
in relation to individual circumstances. For example in a
situation where future improvement to the junction would be
impossible a lower DFC than otherwise considered reasonable may
need to be achieved.