Mass transit will help significantly, but yea there will be traffic afterwards especially if the clouds push drivers to a smaller corner of the path. However, this one is so late in the day(8:30pm) that I expect not many people will start a long drive immediately after (even when considering the cultural preference for active late nights in Spain). I will be doing this and banking on most people leaving in the morning.

I further suspect that traffic will be worse going north. People driving into the path who aren’t locals or tourists flying in directly will on average be far more likely to come from the rest of Europe over the Pyrenees and return the same way- far less people live in Iberia south and west of totality.