I wouldn't put anything on the back indicating I'm going slow because I'm trying to make good gas mileage.
From what I've seen, it's an invitation to Road Rage.
What I have done is install a rear-facing camera (and a front-facing one too). If a tailgater spots it, shehe will know they're on Candid Camera and that the video could be replayed to a jury someday.
Maybe that's an idea: put a "Smile, Your're On Candid Camera" sign right below the camera.
On second thought, that's an invitation to Road Rage too.
You can't assume other people are sane, especially when they're driving a metal box at deadly speeds.
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