What they mean is
Here's how it works:
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.
So you write your message on a large piece of cardboard, and put it up on the road.
This is, obviously, not blogging as we know it. If they had then put in the requirement: "take a picture of the sign and post it on this blog" we could talk about freeway blogging. But the fact that they call it blogging without any kind of online publishing except for the initial initiative is interesting, as this indicates that the word "blogging" is about to change meaning dramatically.
From "posting online in a public journal," it changes into: "Writing a statement and make it visible and available to a large group of strangers." I can see how this drift can happen. Over the last year the focus on blogging in US media has highlighted the free expression, the political statement and the random viewers. The discussion has been less focused on the meaning of blogging to the online culture, and more on the effect of blogging as a political tool and an open, free channel.
And so putting up a poster on a freeway may become blogging. Not the way I would have used this word, but definitely an interesting twist.