Process for the production of an additive composition intended to be mixed into bituminous conglomerate, bitumen and bituminous products for road paving comprising the following steps: a) providing a mixed waste material containing a mixture of plastic materials, wherein said mixture of plastic materials comprises at least one plastic material based on a polyolefin thermoplastic polymer; b) grinding said mixed waste material until reaching a particle size between 40 mm and 80 mm, preferably a particle size equal to about 60 mm; c) washing the thus ground mixed waste material and separating in a controlled manner a portion of plastic material having a characteristic average density from said mixed waste material, wherein said portion of plastic material comprises said at least one plastic material based on a polyolefin thermoplastic polymer; d) grinding said portion of plastic material having a characteristic average density until reaching a particle size between 10 mm and 20 mm, preferably a particle size between 12 and 15 mm; and, e) mixing at room temperature said portion of plastic material having a characteristic average density so ground with a material based on polyvinyl butyral and further grinding the mixture so obtained, in order to produce a ready to use granular additive composition having a particle size between 4 mm and 6 mm and having a melt flow index higher than or equal to 1 g/10 min, preferably between 1.5 g/ 10 min and 2.5 g/10 min, as determined according to the ISO 1133 procedure at a temperature of 190°C with a load of 2.16 kg; wherein said step c) of washing and separation is carried out by means of a technique of density separation, thereby selecting a predetermined limit density value and separating from said mixed waste material said portion of plastic material having a characteristic average density which is lower or equal to said predetermined limit value, said characteristic average density being lower or equal to 1.0 kg/m 3, as determined according to the DIN 55990 procedure. a) providing a mixed waste material containing a mixture of plastic materials, wherein said mixture of plastic materials comprises at least one plastic material based on a polyolefin thermoplastic polymer; b) grinding said mixed waste material until reaching a particle size between 40 mm and 80 mm, preferably a particle size equal to about 60 mm; c) washing the thus ground mixed waste material and separating in a controlled manner a portion of plastic material having a characteristic average density from said mixed waste material, wherein said portion of plastic material comprises said at least one plastic material based on a polyolefin thermoplastic polymer; d) grinding said portion of plastic material having a characteristic average density until reaching a particle size between 10 mm and 20 mm, preferably a particle size between 12 and 15 mm; and, e) mixing at room temperature said portion of plastic material having a characteristic average density so ground with a material based on polyvinyl butyral and further grinding the mixture so obtained, in order to produce a ready to use granular additive composition having a particle size between 4 mm and 6 mm and having a melt flow index higher than or equal to 1 g/10 min, preferably between 1.5 g/ 10 min and 2.5 g/10 min, as determined according to the ISO 1133 procedure at a temperature of 190°C with a load of 2.16 kg;