# Geometric Design II

1. Gradient is the rate of rise or fall along the length of the road with respect to the horizontal. While aligning a highway, the gradient is decided for designing the vertical curve. The positive gradient or the ascending gradient is denoted as +n and the negative gradient as −n.

2. The deviation angle N is: when two grades meet, the angle which measures the change of direction and is given by the algebraic difference between the two grades (n1 − (−n2)) = n1 + n2 = α1 + α2.

1. The ruling gradient or the design gradient is the maximum gradient with which the designer attempts to design the vertical profile of the road.

2. This depends on the terrain, length of the grade, speed, pulling power of the vehicle and the presence of the horizontal curve. In flatter terrain, it may be possible to provide flat gradients, but in hilly terrain it is not economical and sometimes not possible also.

3. The ruling gradient is adopted by the designer by considering a particular speed as the design speed and for a design vehicle with standard dimensions.

4. But our country has heterogeneous traffic and hence it is not possible to lay down precise standards for the country as a whole. Hence IRC has recommended some values for ruling gradients for different types of terrain.

1. This gradient is adopted when the ruling gradient results in enormous increase in cost of construction. On rolling terrain and hilly terrain it may be frequently necessary to adopt a limiting gradient.

2. But the length of the limiting gradient stretches should be limited and must be sandwiched by either straight roads or easier grades.