Walking to the autoroute is a very long, but very nice morning activity. Just follow the signs for Barcelona out of the city. You'll walk for hours probably, through many roundabouts and eventually you'll get to a long, lonely but beautiful road with viniards on either side. Keep going and eventually you'll get to the autoroute onramp situated on a bridge over the A9. You can stop here if you want to try your luck, or you can go to the Péage which is visible from the bridge. Just follow the road down there and cut into the woods to get around and come out on the south side of the Péage. From there you can thumb a ride and there's even a parking area big enough for a lorry to stop for you. Note: getting to the other side of the Péage involves crossing a creek and jumping one or more fences! Attempt at your own peril (or pleasure!).