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T.P.D.M.V.2.4
4.5.6 Siting of Roundabouts
4*5*6.1 Roundabouts should be sited on level ground or In sags, where
drivers 1 advanced visibility of the junction is good, rather
than on crests of hills where, while approaching, they will have
difficulty in appreciating the layout,
4.5.6*2 Roundabouts are appropriate in urban areas but are generally not
compatible with Area Traffic Control systems* Roundabouts, in
these situations, interfere with the platoon movement to the
extent that inflows to downstream traffic signals cannot be
reliably predicted and optimisation of signal setting cannot be
achieved*
4*5.6*3 As already stated the viability of roundabout design depends on
the need to reduce all vehicle speeds through the junction*
This property of the roundabout may be used to good advantage in
the following circumstances :
(a) Where there is a significant change In road standard,
say from dual to single carriageways or from grade
separated to at-grade sections of road;
(b) To emphasise the transition from rural to urban
environment.
In such circumstances the roundabout serves as a very useful
punctuation mark in the road network.
4.5*6,4 On single carriageways where overtaking opportunities are
limited, it may be advantageous to replace a large radius curve
at a junction with two straight sections leading into a small
radius curve at a roundabout, as shown In Diagram No. 4.5*6*1.
By so doing, overtaking sections are created and the revised
alignment may also have advantages in reduced land take.
4.5.6.5 Roundabouts may also be sited to good effect to provide "0" turn
facilities for example to service frontage properties along a
dual carriageway district or local distributor road.