Depending on how efficient your routes are, you will unlock this as you play through. When I finished all maps however, I was at around 95/100 km. You can boost this quickly on the DLC map "The Ridge" if you have it. You start on pavment and you can drive the main road to grind out the 100km. If you don't have this map, you can do it quickly on "Island". This map starts you off close to the main road.