The vertical alignment of a road consists of gradients(straight lines in a vertical plane) and vertical curves. The vertical alignment is usually drawn as a profile, which is a graph with elevation as vertical axis and the horizontal distance along the centre line of the road as the the horizontal axis. Just as a circular curve is used to connect horizontal straight stretches of road, vertical curves connect two gradients. When these two curves meet, they form either convex or concave. The former is called a summit curve, while the latter is called a valley curve. This section covers a discussion on gradient and summit curves.

Summit Curve: Summit curves are vertical curves with gradient upwards.

when a positive gradient meets another positive gradient [figure 1a]. when positive gradient meets a flat gradient [figure 1b]. when an ascending gradient meets a descending gradient [figure 1c]. when a descending gradient meets another descending gradient [figure 1d]. . Requirements

sight distance requirements for the safety The stopping sight distance or absolute minimum sight distance should be provided on these curves and where ove Also if the curve is provided with adequate sight distance, the length would be sufficient to ease the shock due to ch Parabolic curves is easy for computation and also it had been found out that it provides good riding comfort to the dr

Length of summit curve Shape Considerations Sight distance y = a = Deviation angle N Length of curve L L>sight distance

Parabolic

a N/2L

x^2

Case a

L=

h1 h2

1.2 m 1.2 m

h1 h2

1.2 m 0.15 m

Overtaking

stopping sight distance

For ssd L

NS^2/4.4

L

470 2S-4.4/N

For osd

Stopping distance at slopes Lag distance t v Table 4.7 Khanna & justo Stopping sight distance N

46.02273 m

ok

4

vt

27.77778 2.5 s 40 kmph

11.11111 m/s

90 m 1 in

40

OSD Design speed L if L>S L

470 m 80 kmph 575.2604 ok

if L<S

556 not ok

OSD Design speed L if L>S

340 m 65 kmph 301.0417 not ok

L

if L<S

296 ok

ISD Design speed L if L>S L if L<S There is no problem of sight restriction at the curve

180 m 80 kmph 84.375 not ok -24 ok

OSD Design speed L if L>S

340 m 65 kmph #DIV/0! not ok

L

#DIV/0! ok

if L<S

tical curves. The and the horizontal used to connect urves meet, they form ey curve. This section

ed on these curves and where overtaking is not prohibited, overtaking sight distance or intermediate sight distance should be provided as far ficient to ease the shock due to change in gradient. vides good riding comfort to the drivers. It is also easy for field implementation. Due to all these reasons, a simple parabolic curve is preferre

distance should be provided as far as possible.

a simple parabolic curve is preferred as summit curve.