D400 is a major highway in Turkey, more or less following the southern edge of the country. In Mediterranean Region, it closely follows the shoreline, connecting all major coastal towns. From Mersin eastwards, it retreats inland, also forming the main backbone of traffic in Southeastern Turkey.
The western terminus of D400 is at the coastal town of Datça, on the geographical border between Aegean Sea and Mediterranean proper, and the eastern terminus is at the remote border post of Esendere east of Hakkari, on the Turkish-Iranian border (where it connects to Iranian Road 16, which leads to Gilan on the Caspian coast via Tabriz), with 2,057 km between its two termini. For part of its route, European route E90 either lies parallel a few km north of D400 (between Adana and Nizip), or is coterminous with it (between Nizip and Cizre). Other important routes connecting to D400 include E87, which branches off northwards from Antalya.
The volume of traffic and level of hitchability varies along the length of the route (harder in western and central parts, pretty easy as you approach east), so refer to individual articles on towns for more details.WikiPedia:State road D400 (Turkey)