On Belgian motorways I'm usually asking for rides directly. As I speak both Dutch and French that works well for me. As a whole my statistics tell that my avg wait when asking is about the same as when using a cardboard sign or only thumb. In reality though, asking scores much, much, much better, since I use that technique always in more difficult situations (little traffic, uncommon direction, darkness, rain).

Of my 21 rides longer than 200 km I got 13 on petrol stations. Of those 11 by mainly asking around and 2 by using a cardboard sign. But situations with cardboard sign were in Luxembourg, where almost all Dutch leaving would pass by my Maastricht on the way (and there are many, because of the lower petrol prices in Luxembourg). Further 7 times more with cardboard on places where asking would be inefficient, impossible or hard: three times on a national road, twice on an official hh spot, twice just after a non-Schengen border). On these spots way more drivers pass by than at the end of a motorway service area.

Once I got a >200 km ride on a petrol station after I took my cardboard sign from my bag and fixing it on the backside of my bag, so people could see my destination. In a minute a driver told me that he had seen me putting on the Berlin sign and offered me to go along to Berlin.

Just remember it took me a year to understand many basic techniques of hitchhiking and to overcome the "fears" of asking for rides and all the time I develop the techniques to (literally) open doors.