Simple answer: YES. Not necessarily in that there are signs, but certainly there are many hitching-friendly countries. Some people on this site would claim you can hitch in any country.
Hitchwiki is a great site for checking out the 'hitchability' of a country. It has a list of all countries and their ratings for your quick reference guide.
So for example, they would consider Moldova to be better for Hitching than the Netherlands! I'll leave it to you to read why.
Now of course, you have to know the reasons why some are considered good. For example, in Uzbekistan and other ex-Soviet states, it's VERY easy to get someone to stop and pick you up. The problem then is that that car is transformed into a taxi. Allegedly it's because of socialist times helping each other out, and you usually pay a small amount to your new 'taxi driver', who will take you if you're headed in a similar direction.
But because they aren't official taxis (although these do exist) often you then begin a fun dance of 'so...how much? I dunno, how much do you think? Is ... this enough? Maybe?' between driver and passenger. As such, however, it's harder to hitch for free because it's often just assumed that you'll be paying - even a small amount - for your ride.
So after the clarification, you can still use Hitchwiki to check out which countries do have good support for it. For example, Cuba has the Amarillo police, who are there specifically to drive around and enforce the LAW that drivers are REQUIRED to pick up hitchers! How awesome is that!