Drive Slow

I've always been annoyed by "Drive Slow" signs since I learned in school as a youngster that adverbs should end in -ly. I thought "Drive Slowly" was the correct form because "slowly" was an adverb modifying "drive."  It appears this issue has been ongoing for centuries about adverbs.  So with the power of the internet, I can finally end the confusion in my own mind.  There is apparently something called a "flat adverb," for example: drive fast, shine bright, jump high, take it easy, sleep tight, and of course, drive slow. My conclusion: since all adverbs in English apparently don't have to end with -ly it's time to grow up and get over my annoyance.  Go ahead, see for yourself - Google it quick, er uh, maybe I mean Google it quickly?