So, the most recent (29/01/2014) newsletter mentions the effort we have made in México to improve Waze routing through highways. I'll share just a bit more.
I don't know about other country's experience but our highways where just useless on Waze… I once had a 14 hour route suggested for a trip that should take about 9 hours, so you can see how big the problem was.
In response to this and to the enthusiasm to have big editing projects we decided to have a push for better highway mapping.
So far the main focus has been on the 14 main highway links as identified by the national government based on vehicle flows.
Based on personal testing (admittedly nothing scientifically rigorous) we now route correctly in between 70% and 80% of those corridors so I would say that we have had a big improvement so far.
Some of the other local champs are in the process of creating a wiki article with a list of the segments covered and the ones still in need of work so that we can have better tracking of the process.
One of the happy results of the project has been that people have now liked working with highways and are starting to give attention to their local and state highways. Giving promotion to that kind of work will probably be stage two for the project and we hope that it will be an incentive for even more editors to join in.