what is limiting gradient and exceptional gradient in transportation engineering

Limiting gradient 

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 limiting gradient stretches should be limited and must be sandwiched by either straight roads or easier grades.

Exceptional Gradient

Exceptional Gradient are very steeper given at unavoidable situations. They should be limited for short stretches not exceeding about 100 metres at a stretch. In mountainous and steep terrain, successive exceptional gradients must be separate by a minimum 100 metres length gentler gradient. At hairpin bends, the gradients is restricted to 2.5%.