Note on the new highway...
Several posts reference the new highway between Las Americas and Hwy. # 5, enrought to Samana and Las Galeras. It is true that the road is VASTLY superior to many other roads in the Dominican and is the quickest route to Las Galeras, whether driving or by public conveyance.
However, you should be aware that this road is not engineered to the standards you will find in the US or in Europe, even on very old routes. Curves are not banked as you might anticipate, and other drivers frequently and blindly disregard the no-passing zones. So be forewarned and pay attention.
Also, the alternate route via the Duarte Hwy. and passing through San Francisco de Macoris and Nagua is only a couple of hours longer and provides a more direct experience of "not-so-touristy" town in the DR, so don't dismiss that possibility out-of-hand.
Finally, a hearty "amen" to the suggestion that you avoid driving at night. Besides dogs, cows, horses, children and people sleeping on the pavement you will most certainly encounter other vehicles, many of which will have no lights and several of which will have the most powerful high beam, fog, offroad, and spot lights you will have ever seen. They have the right of way.
Enjoy.
Bill