Douz is about 450 km south of Tunis. There is no direct train service; the nearest railhead is Gafsa, about 130 km north. Buses (SNTRI and private companies) run from Tunis via Kairouan, Sfax, and Kebili. The journey from Tunis takes around five to six hours.
By car, take the P14 south from Sfax through Skhira and Gabes, then Route 16 west through Kebili to Douz. The roads are good. Having your own vehicle is a significant advantage for exploring the desert and surrounding area.