Both routes pass round the Étang de Saint-Quentin before merging at Plaisir; the old road course is often numbered RD912.
The road now bypasses the small town of Houdan and crosses the river Eure into the city of Dreux, one of many that circle Paris.
The road leaves Dreux and heads broadly southwest through rolling countryside of southern Normandy.
The countryside is more wooded now as the road crosses the Les Bois Francs before coming to the Forêt du Perche.
In the Forêt de Multonne Mont Souprat rises to 385 m. At Pré-en-Pail the RD176 heads northwest to the coast and Avranches.
The road heads southwest below hills such as Mont du Saule and Bois de Buleu both 327 m high.
Due to an old royal act of parliament, roads cannot be tolled within Brittany, and so the RN12 was upgraded to autoroute standard but not numbered as such.
The road then enters the town and port of Saint-Brieuc on the southwest corner of the Baie de Saint Brieuc on the English Channel.
The countryside becomes more and more undulating with steeper river valleys the Monts d'Arrée lie to the south (394 m).
The roads both enter the large Atlantic coast port of Brest from the east.