East towards Toulouse
Take bus C in the direction St. Pierre d'Irube and get off at the stop Dasbendia. The bus stop is just before a roundabout. Following the signs to the motorway (walk left), it will take less than ten minutes. You will pass a college on your left and soon come to the next roundabout which leads to the A36 on the right. Drivers can see you and there is enough space to stop.
West towards San Sebastian
Most traffic coming out of San Sebastian will be only going to local towns along the coast so write you sign/ ask for lifts to Saint Jean de Luz or Hendaye.
Go to the entrace to the motorway on Avenada de Grand Basque.There is a round about at it is pretty busy but in the direction of the river there are plenty space here for cars to pull over before they turn right. The only problem with this spot is that because it is in the north of Bayonne alot of the traffic might be going north. Operation Mayhem was here for an hour and a half before he got a lift to an exit toll station and then got another ride to Hendaye metro station very quickly.
About 15 kms south on the motorway there is a service station
This is a better option as there is more likey a chance that there will be more cars heading south-west towards Spain but its a few kms from the city centre. Follow Bd d'Aritxague south to the entrace and up the ramp you will see a toll station. This could be a could spot to get a ride, at least on to the motorway where you can then change at the first service station. Be careful not to stand too close to the ramp, where cars are driving to fast to stop.
Option 3 - Public Transport
If you get can get a ride at least to Hendaye, the border town with Spain, you can just jump on the metro and it will take you all the way into the centre of San Sebastian-Donastia. It takes about 45 minutes and costs less than 2€.
There is an excellent and cheap bus system covering the whole area of Bayonne, Biarritz and Bidart. A single ticket costs just 1€ no matter how far you go and is valid for one hour. A 24-hour ticket costs 2€, so well worth it.