The best way to find good posting spots is to simply drive around and see where your eyes are naturally drawn. The above bit of fencing is directly next to I-5 in Seattle by the Ravenna Ave. offramp.
Other spots are good by dint of the sheer numbers of traffic lanes that overlook them. While not readily apparent by the photo, the fencing above, bordering Kobe Terrace Park in Seattle's City Center, is highly visible to some sixteen lanes of heavy, heavy traffic, none of which have a prayer of getting to it. It took three days for someone to take down the giant "The War is a Lie" I'd put up before this "Chimpeach", which was still up 24 hours later. If you live in Seattle and have something to say, there's no better place than Kobe Terrace Park to say it.
Article in The Stranger
(note: I was a political correspondent for Penthouse in the late 80's/early 90's - not the 70's)
I-90 West just after the causeway from Redmond. Sign reads "Impeach Bush for Blowing the Job."
Unless, of course, you know how.
FB - 966