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T.P.D.M. V.2.3
(Al/2001)
3.11.5 SightliHes
3.11.5.1 Where the road is single track a motorist should be able to see between one passing bay and
the next. A desirable minimum stopping sight distance is 50m and an absolute minimum
30m, although at hairpin bends or similar hazards where speeds will be very low, a lower
sightline distance will need to be accepted. Care is required where only the minimum sight
distances or even smaller sight distances are provided that vehicle speeds can be adequately
controlled and that hazards to vehicles travelling in opposite directions are avoided in
particular.
3.11.6 Vertical Alignment
3.11.6.1 Where the road is to be used by large or heavy vehicles the maximum desirable gradient is
10%. For smaller, lighter vehicles the maximum desirable gradient is 16%. If the terrain
necessitates a steeper road the design engineer should consider the following before
exceeding the 16% desirable maximum :-
(i) length of gradient which is to exceed 16%.
(ii)
(iii)
(iv)
Visibility.
Type of vehicles to use the road - Emergency vehicles would not normally operate at
gradients in excess of 16%, and public service vehicle operators may refuse to use
those roads.
Topography - alternative but longer routes.
3.11.6.2 On any steep section of road particular attention will need to be paid to the texture of the
finished road surface to ensure that adequate frictional resistance can be provided.
3.11.6.3 On single track road the inner edge of the carriageway on a horizontal curve should be used
as the vertical control line. Vertical curves should be at least 10m long. At summit curves
special consideration will need to be given to maintaining adequate stopping sight distances
and in these locations a desirable minimum K value is 3 with an absolute minimum K value
ofl.