Take Bus 5515 at the Luisenplatz in the direction of Erzhausen, until the "Am Rotböll" stop. From there you walk a bit back to the highway, go right to the asphalted street parallel to the highway to the service staiton "Gräfenhausen". Walk 2 km through the woods or try to hitch a ride from there to the service station. The station is the best in the region, for going any direction.
If you want to go into a direction south of Darmstadt(Mannheim, Stuttgart, Freiburg) take every bus from Luisenplatz which is goning to Rhein-Neckar-Strasse und then walk 200 meters into the north. There is an Aral Gasolin Station where you can easily find persons which are going to the south
If you want to go to the north-east take the bus 5515 and leave at Riedbahn-Robert-Bosch Strasse and walk 20 Meters towards the highway A5. Be aware that it is not allowed hitch a car directly there, but it works.