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T.P.D.M.V.2.4
4.5.10 General Layout Requirements
4.5.10.1 The principal objective of roundabout desigc is to secure the
safe interchange of traffic between crossing and weaving traffic
streams with minimum delay. This is achieved by a combination
of the geometric layout features described in the following
paragraphs. As relationships between aspects of design are not
always mutually compatible, design becomes a trade off between
operational efficiency, minimising delays and achieving safety.
4.5.10.2 Where geometric design parameters are speed related, at existing
junctions or new junctions on existing roads, the measured 85
percentile speed should be used, rounded up to the next highest
design speed step* For completely new junctions the design
speed of the roads should be used.
4.5.10.3 The geometric parameters described in subsequent paragraphs are
depicted in Figures 1-5 of Appendix 2 with accompanying detailed
description*