Changes

=== North towards Extremadura & Castilla y León ===

Go to the main bus station and take the line bus 41 (if I'm not mistaken) headed towards Camas and Santiponce, the ticket costs around 1'45€.

Get down at Santiponce, this is a tiny industrial outskirts town.

Walk back towards the little rest area on the right of the picture shown on the link below. It's not fantastic, but I believe this is definitely the best spot you can find to get on to the E-803 legally and safely. Tip: I got super lucky because during the oranges season a man sets a stand right there to sell oranges, and some extra cars stop there because they know about it :-)

If there isn't much traffic going by, you can always "dangerously" walk up to the highway and try to find a car there. I didn't find any other possible spot going out north from Sevilla.