I think at one time I had a similar idea for this very concept, except my version was a bit less confrontational:

http://www.roadragecards.com/

I think from a non-situational standpoint, these are quite funny.  However, in application to actual driving scenarios, I'm not sure it is a good idea to flip through a booklet to hold up an inflammatory comment to someone you know nothing about.  How would they react?  Are you putting yourself in danger (both by the act of disrupting your driving focus to flip through and hold up cards, and potentially inviting a fight with someone who may be more irrational than you bargained for)?

You can disagree, but I think there is only one type of card that it may be worth the risk to hold up for viewing.  It is the one for when you do something on the road that is blaringly misguided or you make a mistake that inconveniences or potentially endangers other drivers:

Oops, I'm sorry!