ble in order to ensure adequate surface water drainage. (This can also be dealt
with through the provision of a drainage system running parallel to the
highway.)
6.6.2 K values
The required minimum length of a vertical curve is given by the equation:
L=K(p -q) (6.33)
K is a constant related to design speed. K values are given in Table 6.12.
Geometric Alignment and Design 179
Design speed (km/hr)
120 100 85 70 60 50
Desirable minimum K value – Crest curves 182 100 55 30 17 10
(not recommended for single carriageways)
Absolute minimum K value – Crest curves 100 55 30 17 10 6.5
Absolute minimum K value – Sag curves 37 26 20 20 13 9
Full overtaking sight distance (FOSD) K — 400 285 200 142 100
value – Crest curve
Table 6.12K values for vertical curvature
Example 6.6
Calculate the desired and absolute minimum crest curve lengths for a dual
carriageway highway with a design speed of 100 km/hr where the algebraic
change in gradient is 7% (from +3% (uphill) to -4% (downhill)).
Solution
From Table 6.12, the appropriate K values are 100 and 55.
(1) Desirable minimum curve length = 100 ¥ 7 =700 m
(2) Absolute minimum curve length = 55 ¥ 7 =385 m
6.6.3 Visibility and comfort criteria
Desirable minimum curve lengths in this instance are based on visibility con-
cerns rather than comfort as, above a design speed of 50 km/hr, the crest in the
road will restrict forward visibility to the desirable minimum stopping sight dis-
tance before minimum comfort criteria are applied (TD 9/93). With sag curves,
as visibility is, in most cases, unobstructed, comfort criteria will apply. Sag curves
