I dont have the map in front of me, and I know the most obvious route would take you through Moca, and onto San Fransisco de Macoris, but I suggest that you take the rte 1 route through laVega and then take a left on the nice flat straight road to San Faransisco deMacoris. i went this way and believe it is easier on the mind and actually faster, even though the milage meter may say its LONGER.
Rte 1 is the main road between Santiago and Santo Domingo....and where route one runs around LaVega, just after you pass that town there is an exit or turn to go to San Fransisco de Macoris( north). that should shave 15 minutes off the trip.
then just follow the main road from S F de Macoris to Nagua, get on route 5 and head towards Las Terranas....now I went down when I hit the pennisula and went onto Samana itself..las Terranas is on the coast and you will have to stay on the coast.....
ok after I read it, the only good tip will be the santiago to S F de Macoris tip....but when i get home, I can give you better details if no one else posts
good luck
the route through Moca and onto S F de Macoris is rough and the road sucks bad... and you will get stuck behine traffic and busses alot
bob