You have to drive on EVERY road marked on the map. You will see that roads you have not visited yet will be grey on the map and roads you passed are white or orange. You can easily check on your map by taking away all markers (the LB menu) so you can see the roads more clearly. You have to cover all roads. Some tipps for this:
1. Broader roads like highways you will have to cover TWICE - once on each direction.
2. Be thorough: Especially small "roads" like a little road connecting two other roads or roads that connect to the highway can be easily forgotten. Or little roads in the towns.
3. There are roads that connect to named "open world" areas like the abbey or the airport. Double check on them so that you are sure they are registered.
Last but not least:
4. Once I was done with all roads on the map I was still missing 4 roads. There were none left to cover. What I did: I drove to every Horizon Festival checkpoint. You will see that there are tiny "road" leading to them. They were all marked white on the map but once I went to the checkpoints they were finally registered.
If you are still missing roads, take the map, zoom in and check road for road... Good luck!