I wrote an extensive thesis on the use of recycled materials in asphaltic pavement. Aside from RAP, shingles are used as they are composed of the same basic materials as pavement (bitumen and aggregate). Rubberized surface courses containing rubber from recycled tires has desirable properties as well: reduced permeability, resistance to cracking, a darker appearance to allow for more contrast with pavement markings and a quieter ride. Experimentation has begun using plastic fibers as reinforcement to add to a mix's tensile and shear strength as well.