Highway Engineering

6.6.7 Vertical sag curve design and sight distance requirements

In general, the two main criteria used as a basis for designing vertical sag curves

are driver comfort and clearance from structures.

Driver comfort

Although it is conceivable that both crest and sag curves can be designed on the

basis of comfort rather than safety, it can be generally assumed that, for crest

curves, the safety criterion will prevail and sight distance requirements will

remain of paramount importance. However, because of the greater ease of vis-

ibility associated with sag curves, comfort is more likely to be the primary design

criterion for them.

Where comfort is taken as the main criterion, the following formula is utilised

in order to calculate the required curve length:

(6.66)

where

Lis the required vertical sag curve length (m)

Vis the speed of the vehicle (km/hr)

A is the algebraic difference in the straight-line gradients

The vertical radial acceleration of the vehicle is assumed to be 0.3 m/s^2 within

Equation 6.66.

Clearance from structures

In certain situations where structures such as bridges are situated on sag curves,

the primary design criterion for designing the curve itself may be the provision

of necessary clearance in order to maintain the driver’s line of sight.

Commercial vehicles, with assumed driver eye heights of approximately 2 m,

are generally taken for line of sight purposes, with object heights again taken as

0.26 m.

Again, as with crest curves, two forms of the necessary formula exist, depend-

ing on whether the sight distance is or is not contained within the curve length.

Case (1) S £L

(6.67)

where

Cl is the clearance height on the relevant structure located on the sag curve,

generally taken in ideal circumstances at 5.7 m for bridge structures.

Lm S

2

12

A

8C1 H H /2

=

[]-+()

L=V

(^2) A
3.9
Geometric Alignment and Design 189