Scarlet P. the anti-war activist has put up more than 2,000 cardboard signs on his own in California. " When you put a sign on the freeway people will read it until someone takes it down. Depending on its size, content and placement it can be seen by hundreds of thousands of people." The freeways are more and more equiped with digital bilboards but they are only open for commercial used. Since 9/11 and the beginning of the War in Iraq, freeway blogging has exploded in the US as reclaiming a space for free speech where you can also reach a big audience. (
see the interview with him)
Online he has put guidelines and a picture blog...

Here my own image from recently driving to LA:

$2.99
Thanks Pieter Boeder for the hint!