When you travel from Dalaman the big road that you pick up which takes you to Fethiye and beyond to Antalya etc. is the D400. It's number is on the sign post a lot of the time.
It seems to start at Datca and Mugla then takes you a long way to the east side of Turkey following the coast a lot of the way,although from Fethiye to Antalya it does go across the mountains more.The big roads in Turkey have numbers like the main roads in Britain.