Changes

Take the metro/tramway to La Joliette or Euroméditerranée Gantès and walk down the main street (Boulevard de Dunkerke) for a few blocks. There is the highway North-West (A55) that starts there (it's a bridge going up so it's better to stand at the traffic light just before it). A lot of the cars leaving from that point can take you out of Marseille's metropolis and leave you at Fos, where the road is only one-lane and it's easy to get another lift.  In the early morning (6-7 a.m.) a lot of people that live in Marseille go to work to Fos, so it's definetely the best time to be there. Try putting your destination directly on the sign you have (especially if it's big cities like Montpellier or Toulouse everyone will know where it is), your driver might actually give you further advice on how to go there.