Only knowledge can combat ignorance and stoke the fire that would change public opinion assuming it's presented in a way that's easily palatable. Placards on public transportation are great places to plant seeds to grow curiosity.

You’ve got to take the bitter with the better for this. In between all the smiting and stoning and stuff, the Pentateuch includes the following innovations:

1. Infectious disease / pathogen control
2. Improved agriculture by application of mixed manure at harvest time.
3. Separation of malfeasances into crimes, torts, and breaches.
4. Building codes
5. Fair trade codes
6. Polycrop agriculture and national security food system

This doesn’t mean there’s an invisible old man with a long grey invisible beard who lives in the sky and knows if we’ve been naughty or nice. But it is an interesting observation when compared to books of similar vintage.

How about this simple test that will easily fit on the side of a bus:

"Did a talking snake trick a rib-woman into eating a magic apple?"

Yes: National Alliance on Mental Illness 1-800-950-NAMI (6264)
No: www.agnostic.com

A religious person posted a sign by the road which I travel going to town with the following words, "Don't think - PRAY!"

I think this is a great way for the atheists/agnostics/skeptics to get a laugh.

I think it's too much reading for the average person to process as a bus drives by and even when parked most people won't read a complete passage. You would need a fast message like "BIBLE STUDY" Read: A Rebellious Son - Deuteronomy 21:18-21 -LOGIC- or something like that. My fear is Christian parents might start stoning their kids instead of understanding how immoral it is.

I'm not sure I agree with the idea in general. I'm of the opinion that individuals should come to their beliefs by actually seeking it out for themselves. The kind of ad campaign you're proposing seems to me just another form of indoctrination. I would certainly be annoyed by religious ads (and have been). I would think that theists would be equally or moreso annoyed by anti-theist ads. In short, I think it would essentially backfire.

Belief is a very personal matter, and as such I think the only thing we should actively do is to address this on a one-to-one basis, as the subject arises. The good thing is that someone actively seeking enlightenment might view the discussions such as on this site, and get something from it. Better this than having it shoved down their throats or shoved in their faces.