I've seen billboards on random interstates across America. I wonder what the ROI on those are.
Also in France.
Add Germany.
Given the low yield of many Interstate billboard adverts ("Your message here" being amongst the more popular), I'd suspect minimal cost and some return.
DDG is making progress but it's still far away from being good especially for local, non-English results. I'm worried whether advertising at this stage is a good idea as it could make people try it, be disappointed and leave a bad impression.
I saw ads for DuckDuckGo at the airport the other day too