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T.P.D.M.V.2.4
4.5.17 Superelevation and Cross fa 11
4.5.17-1 Superelevation is not required on the circulatory carriageways
of roundabouts whereas crossfall is required, to drain surface
water, but on the approaches and exits superelevation can assist
drivers to negotiate the associated curves.
4.5*17*2 On entry the degree of superelevation should be appropriate to
the speed of vehicles, as they approach the roundabout, but
should not exceed 5%. It should be reduced to the crossfall
required merely for drainage in the vicinity of the "Give Way tt
line, since with adequate advance signing and entry deflection,
speeds on the approaches should be sufficiently reduced.
4.5.17.3 On exits, superelevation may be provided to assist vehicles in
accelerating away from the roundabout* However, as with
entries, crossfalls adjacent to the roundabout should be kept to
the minimum required for drainage.
4*5.17*4 Normal crossfall for drainage on roundabouts should be 2.QJ5 and
not exceed 3.0J. To avoid ponding, longitudinal edge profiles
should be graded at not less than 0.67JJ desirable, and 0.5Jf
absolute minimum.
4.5.17*5 On the circulatory carriageway the values of crossfall should be
governed by drainage requirements. However the direction of
crossfall, particularly at normal roundabouts, should be
arranged to assist circulating vehicles. Hence the carriageway
adjacent to the central island should slope toward the central
island while the carriageway adjacent to the outside perimeter
will slope toward it. The crown line thus formed may be a
straight line joining the entry and exit deflection islands or a
line dividing the circulatory carriageway in the proportion 2:1
internal to external as shown in Diagram No. 4.5.17.1.
Alternatively, a subsidiary crown line, as shown on Diagram No.
4.5*17.2, may be used to avoid excessive changes in crossfall at
the single crownline. Maximum recommended algebraic difference
in crossfall across the crown line is 5$, and lesser values are
desirable.
4.5*17*6 Adverse crossfall should be eliminated from the paths of the
main traffic movements at normal roundabouts* Mini roundabouts
and smaller normal roundabouts in urban areas are often
superimposed upon existing pavement profiles and in these cases,
the cross section of the existing roads will influence
crossfalls at the roundabout. T-junctions require particular
attention. Some adverse crossfall can be accepted in order to
fit the existing levels provided approach speeds are low.