Impersonating a cop in order to not get a speeding ticket might sound like a good idea, but it turns out it isn’t. The Supreme Court decided that would be stretching the right of speech a little too far.
But shouldn’t the question be: Why should a law enforcement officer off duty get away with a speeding ticket in the first place?
Here’s more on that topic:
http://www.wired.com/threatlevel/2012/08/impersonating-cops-ruling/