whilst this perpixel shaders are one way to do a road, you should use KSmultilayer_fresnel_nm shader for an Assetto Corsa visual quality road.
to get started. create a blank white version or light grey texture for your "new" diffuse. and a "new" mask texture to use (all same dimensions), colour this Mask pure Red (255,0,0) and use the texture you have the tarmac grain in on the Red channel Tab. with a figure of 1.0 for the scaling. and place the normal map as usuall with the same scaling figure as your "old" diffuse".
effects. when you zoom far from road it blends to the "new" diffuse that you created. when you zoom close it blends into the channel masked texture with grain on it. then you can dirty up the New diffuse to get some dark or light variations, these variations also affect the look of the close grain. wonder full shader !