What is the significance of gradation with respect to Road Construction?

The gradation, of an aggregate is one of the most influential aggregate characteristics in determining how it will perform as a pavement material. gradation helps determine almost every important property including stiffness, stability, durability, permeability, workability, fatigue resistance, frictional resistance and moisture susceptibility .

material being used for road construction excution should be pass all required test before proceeding 

Thank you for the invitation

I will speak succinctly because this is a very important topic especially for road engineers

Grading in civil engineering and landscape architectural construction is the work of ensuring a level base, or one with a specified slope,[1] for a construction work such as a foundation, the base course for a road or a railway, or landscape and garden improvements, or surface drainage. The earthworks created for such a purpose are often called the sub-grade or finished contouring (see diagram).

The particle size distribution, or gradation, of an aggregate is one of the most influential aggregate characteristics in determining how it will perform as a pavement material. In HMA, gradation helps determine almost every important property including stiffness, stability, durability, permeability, workability, fatigue resistance, frictional resistance and moisture susceptibility. In PCC, gradation helps determine durability, porosity, workability, cement and water requirements, strength, and shrinkage. Because of this, gradation is a primary concern in HMA and PCC mix design and thus most agencies specify allowable aggregate gradations for both.

Depends  on the  gradation , material Qaulity , methodology of road  construction , such as Base course, top layer/ pavement / asphalt compacted very well using sophisticated machines for this purpose and run a test before use the road .Also I want to add that the slope , curves turns on fly zone should be consider carefully to avoid cars overlap in mechanical view.

Gradation is very significant in road construction.  Ungraded or poorly graded material used in the sub-base or base course will have adverse effects in the long run even though it may show good compaction readings during construction.  Too much fines will cause loosening and settlement under traffic while excessive larger sizes may create voids in the underlayers that will subsequently cause the upper layer to cave-in especially if subjected to frequent rains.  AASHTO or other local requirements provide details of the gradation and other specifications for material and compaction details and procedures of filling and road base material very clearly and should be complied.

suitability of the material

and not suitability

it would define the material size 

gradation is defining the material size and type of aggrigates

Tie-in levels to surrounding topography/ building & gate levels

helps in removing the top black soil to get a clear foundation and strong base for the road