They tell drivers well in advance of what parking garages have spaces free, and how many. I should point out that I took this picture on a Sunday - on Saturdays the numbers are closer to 0.
I forgot to mention in my Barcelona post that we avoided 2 pickpocketing attempts in our first 15 minutes in the city. It can happen anywhere, but late at night in a subway station does seem to be about the worst.
Everything is closed here today for Labor Day, held on May 1 in Germany (and most other countries, I think).