I just bought two hundred posters of the earth from space and will be putting them up on west coast freeways for the next few months. Remember all those flags that went up after September 11th? This is like that, except instead of putting signs up after the tragedy, I'll be putting them up before.
These are the three things that worry me most:
1) The amount of carbon we're putting in the atmosphere.
2) The amount of carbon dioxide and methane seeping out of a now melting permafrost, and
3) The fact that an ice-free Arctic will start absorbing all the heat from the sunlight it used to reflect.
It's that last one that really gets me: the thought that the top of our planet is turning blue. While I don't know precisely the atmospheric effects of CO2 emissions from industrial society, or the methane release of ancient plantlife, I do know this: in direct sunlight, a thing that's blue gets a hell of a lot hotter than a thing that's white.
Posting signs is simply a matter of driving around and looking for those places that are easy to see but not so easy to reach. With hammer, nails, spring clamps, bungee cords and duct tape, you can put a sign up almost anywhere.
How long they stay up depends on how difficult they are to reach and how motivated someone is to reach them. The sign above is in a (not entirely) sealed-off overpass and has been up for six days now. Generally speaking, signs placed over the freeway will only last a couple of hours. Those placed next to the freeway however will usually last a couple of days.
As for the legality, I go back to the days after September 11th, when people across the country put American flags all over the freeways. Nobody questioned their reasons or their right to do it, and that's why you should feel free to do it too: either with signs, pictures of the planet, or both. The benchmark case on this was Brown vs. California Dept. of Transportation when CalTrans tried taking down anti-war banners but leaving up the flags. The courts eventually ruled against them, this being, after all, the land of free speech.
If you do it in front of the cops though, they'll probably ask you to take it down.
Signs posted since January 1st: 1012