This is not a 100% hoax as CreedFreed says.
Check out:
http://www.hoax-slayer.com/issue59.html
Then, scroll down the page to read the article.
It depends on where in the world you are at that moment in need...
Best bet, put you hazards on, call 911 to verify it is a cop behind you and keep driving to a more populated area.
You have a right to acknowledge the officer (if this is a real officer of the law), by putting on your hazards and continuing to drive to a more public area before pulling over.