A3 South-East towards Frankfurt am Main
You can get the Bus 101 Horhausen (Kardinal-Höffner-Platz) from the city (make sure is goes to Horhausen and not only to Straßenhaus) to the station "B256" and walk from there just a few meters to the roundabout. The roundabout has a exit to the A3 in direction of Frankfurt (south-east) and you can stand there very good, as cars will not be too fast and have enough space to hold on the right side.
Else (if raining) you can also try the petrol stations, but most people might go nearby and not on the motorway. There is one petrol station you already passed at the Bus stop and another one behind a second roundabout.