A general problem is that lots of people take the National Road next to the motorway because it's free. There it is not that good to hitchhike because there aren't as many petrol stations.
The bus #28 is not the best choice to hitchhike out of Zaragoza. You will get to a kind of good looking point with a petrol station and a red-light before the motorway but you will soon figure out that not even 10% of the cars go on the motorway. Most of them just cross it over on the bridge. Now Fabzgy would rather try his luck with the bus 29 and get of as close to the cross of the city motorway.
Wladi tried the bus28-thing: It really doesn't work! Better to follow the "Ronda Hispanidad Z-30" (Wladi hasn't tried yet - finally he took the alsa.es-bus to Barcelona) to get to the motorway access.
By the way - the second petrol station is bigger but not really on the motorway. So try the first one after Zaragoza if your driver doesn't go far.
Barcelona --- Succesful hitchhike after 3 hours ---
I first went to this round-about like intersection and stayed exactly after the first road sign saying BARCELONA - on the first lane. I stayed there for 3 hours and the closest success was some guy shouting out the window 'Where do you want to go' in Spanish because I didn't have the sign posted and I was just signing by hand. This was completely useless because he just went with 80km/h so I didn't have time to respond. In my opinion Spanish people do not hitchhike and tend to make fun of people who do. Anyway, the roundabout intersection is here. Then I got pissed off and considering the previous reviews about bus 28/29 I said I'm going to try something 'new' so I just walked randomly to this place
You can get on Av Alcalde Francisco Caballero with bus 44 until the intersection with Ronda Hispanidad and then walk all the way up on the bridge towards the Nord Este Autovia junction. That whole area is an industrial site so there are a lot of trucks passing by and the chances are bigger with them than with normal cars. In the same time the problem with that junction is that trucks don't have place to stop but the guy who picked me up just stopped in the middle of the road so apparently that is not a big deal for some of them at least. As I found out from the driver, the best hours are around 10:00 and 18:00-19:00 because there are generally 2 shifts of transporting merchandise between Zaragoza and Barcelona. I got picked up at 18:30.
He also told me almost all merchandise trucks are leaving from Mercazaragoza which is a little bit to the north from the junction I've hitchhiked from, and that it's way better to go there and ask the drivers. Mercazaragoza is here
He said any city in Spain should have a Merca<name-of-the-city> where the trucks get loaded with stuff and that from now on I should go directly there. Hope this information is useful to anyone in the future
I think the best option is the petrol station/service area in "La Puebla de Alfinden" - I just don't know how to get there by public transport. Any suggestions of locals? Alternatively you could try to hitch with a sign saying something like: bring me to the highway, I'm a traveller from...etc. Had really good experience with such big, amusing signs in Japan and the US.
Southwest towards Madrid
To hitch Southwest towards Madrid you are best to catch a bus towards the airport (501, 1,85€ as of 2018), which stops at a large shopping plaza. You can get out here and try to hitch from an on ramp to the A2, but you are better to walk along the side of the A2 for about 2kms until you find a large 24hr Gasolina-Restaurant. From here it is easy to approach drivers for a lift to Madrid.
Catch either bus 27 or 52 to Centro Augusta. From here follow the N-232 for 5 minutes till you reach a petrol station. There is a lot of local traffic, but the service workers are very nice they arranged me a good lift to Logroño.
After Centro de Agusta, there are three petrol stations. Most drivers will be staying within Zaragoza or going to one of the outlying pueblos such as Utebo. Therefore, go to the third gasoline station. From there, try to get a ride to the truck station which is about 10 km away. If there are no vehicles, or it is night, you can walk there. The name of the truck station is La Joyosa and can be accessed by walking - illegaly - on the AP-68 towards Logroño. I arrived at night and found a ride the next morning.
The official charity project from FC St. Pauli football club ran a charity hitch-race from Hamburg to Zaragoza from the 23rd of August on to the EXPO 2008 with the topic Water and Sustainable Development. See also the viva con agua website.