Bilbao is a city in Spain.
Toward San Sebastian
Heading toward San Sebastian there is a roundabout near the airport on the strada (street not the motorway) you can wait at. That was much quicker for Markus than waiting near the toll where the road goes in 2 directions.
Near the toll there are traffic lights, but better is a little further on where the road forks, though its not a good place to wait as its in the middle of the road it is possible. The 2 directions are Santander and San Sebastian.
Another spot to hitch to San Sebastian: take the metro to Etxebarri and walk to this spot: (enter the coordinates 43.235063,-2.868845 in Google maps). I got a ride there although it took 45 minutes and the cars were whizzing by.
The A8-E70 starts in downtown Bilbao. We tried there without success. Not much space for cars to stop.
So instead we took the subway to the last station on the red line: Santurtzi. When you get to the Santurtzi station go to the exit that leads another small train that just takes 5 minutes, in order to take the passangers stright up to Parque Santurtzi. From there you just go down towards the water and then follow the road and it will lead to the highway entrance. We waited 10 minutes and then got a ride to santander.