Ads That Make You Do a Double Take

In the past few weeks, every time I drive through the city, I've noticed a particular billboard that I couldn't quite understand.  The first time I saw it, I did a double take, trying to figure out the message.  I could read it just fine, but what was it an ad for?  What was it trying to tell me?  Even after seeing it multiple times I still couldn't figure it out.

Here's the billboard:
 

According to this site, the ad refers to John Wayne determination not to give up during his battle with cancer, and encourages others to do the same.  Not knowing John Wayne's history, I never would have guessed this.  So is the ad intriguing because its ambiguity makes you curious?  I don't think so.  I feel that most people wouldn't bother to look up the meaning and would just forget about the ad, therefore having no effect on them at all.  Its ambiguity could be dangerous, too - double takes while in cars can certainly be dangerous.  A billboard probably isn't the place for an unclear ad.