Here's how it works: When you put a sign on the freeway people will read it until someone takes it down.
Placement: Anything you can see while driving is a place you can put a sign. The more difficult it is to reach, the longer it'll stay up. Tens, even hundreds of thousands of people can drive by a sign before one of them takes so much as five minutes to go take it down. Apart from actual prisoners, you won't find a more captive audience than people in their cars.
Safety First: Like anything else, free speech is fun until someone gets hurt. Signs should be placed on the INSIDE of fencing, NOT suspended directly over traffic. So long as your letters are at least six inches tall, they'll be perfectly legible through the fencing.