With the P and R maps (texture with suffix _p and _r). More you do them white and more you make the vanilla paved and asphalt textures visible (included Snow effect). They are Additional textures that overlaps your road texture, so more you do maps white less you will see yours
The strip of snow, I suppose is the vanilla road with the flowerbed. So that area takes the textures from the ground. You can control it with the Alpha map, suffix _a. Basically where you paint this texture black you will hole your texture showing the ground below but on the maps without snow you will see the grass. This is valid only for the ground segment, while for bridges, where paint black becomes transparent (For example you can draw a railing on a flat polygon and make the empty spaces between the rods invisible).