If cross, or MTB is available, then take gravel roads. More scenic, safer (In my opinion).
I would question whether gravel roads are safer than paved roads. Cars/trucks on gravel may be going slower, or not. Where I live people drive fairly high speeds on gravel roads. Gravel roads also require you to ride on one of the two packed strips of the road. These two strips are where all the cars drive. They are roughly in the middle of the entire roadway where the cars drive. The two feet between the strips and the ditch and the four feet between the strips are loose gravel. You cannot ride a bike on this loose gravel. You can only ride a gravel road on these two packed strips. So you are right in line with any car coming up behind you. And in line with any car you meet too. Gravel roads are really one way roads. That go one way in both directions at the same time. You can hope a car will go to the side and into the loose gravel to pass you. Or you can go into the loose gravel and crash yourself in front of the car and then maybe the car will run over you. In contrast, on paved roads you are usually riding a few feet from the edge of the road. So you have a few feet to move over if need be. And the car can easily move side to side and be on nice fast paved roadway to pass you.