United Coalition of Reason. Note the light bulb, which stands for "Brights," a name some atheists have given to themselves, indicating they think they are "brighter" (= smarter) than theists. (As I write this I cannot help but think of one of our local atheist leaders. When I met with him I found he was far from "bright," being mostly logically incoherent. Yet he thought himself to be a "Bright.")
I see nothing wrong in atheists doing this. If they want to spend their money that way, they have the right to. I can see their ads fueling discussion, which for me is always a good thing.
I will say this, and it may just be me. Were I a strong atheist (viz., one who believed they could make a rational case against the existence of God, as opposed to a "weak atheist," who just believes there is no God but cannot speak reasonably about their unbelief), I would not waste my time promoting my atheism. This is because I would be inclined to embrace an existentialist view of atheism, and find life to be fundamentally absurd. Hence, to spend money on proclaiming the ultimate absurdity of life would strike me as but another absurdity. If "life's a tale full of sound and fury, signifying nothing," surely atheistic bus-signs signify the same.