Adverbs: Public Service from Weird Al

Weird Al Yankovic has YouTube.com channel. In this video, he claims to do a public service by changing a traffic sign that said 'Caution Drive Slow.' The edited sign said:

Caution Drive Slowly

Sorry, Al, but I'm going to have to pull you over. The original sign was grammatically correct.

Where does Weird Al go wrong? He mistakenly assumes that all adverbs must end in '-ly.'

Quick rule:

• Some adverbs end in '-ly.' (Take a look at 'Lolly, Lolly, Lolly' from Grammar Rock.)
• Some adverbs don't end in '-ly.' These flat adverbs are not only perfectly acceptable but commonplace in our language. My mom braided my sister's hair too tight. He hit the ball hard. My name is spelled wrong on my nametag. My grandmother drives slow. The bus driver drives too fast.

Correct: 

• Caution Drive Slow (stet!)