A bit of history about that one:
It started installing billboard in the parisian subway by 2009. There was quite violent reactions at that time. Some people painted the billboards with messages like "fed up with Ads", "Fuck big Brother"... Some MPs asked them to be removed. And the CNIL investigated them. However, they never were condemned because they do not *store* nor *transfered* any data, and the CNIL (for historical reasons) nearly only cares for data storage and data transfert.
Now, those billboards do not raise much questions anymore. People are just used to them. But honestly, I never saw anyone actually watching the Ads while rushing to get their subway train. And Quividi recently combined with Amscreen in the USA to display those kind of billboards in BP and Shell gaz-stations.
Sylvain