hello, I'm really confused now.
I haven't thought that 'drive slow' is grammartically right.
But, I happened to hear that from the movie 'Rumor has it.'
It's kind of trauma when the things you've believed strongly turned out to be wrong.
Always good luck!
Slow can be used as an adverb says the dictionary.
slow - Definitions from Dictionary.com
By the way they give the same example that you used here.
Anyway, as I'm neither a native speaker nor a teacher I don't know if it's common to use it as an adverb.
Similar structures: speak slower
speak more slowly.
It reminds me of a double predicate: The moon rose round and yellow,
where 'round and yellow' characterize the moon, not the action 'rose'.
adverb :at a slow speed
I can't walk any slower.
a slow-burning candle
Amer.Eng.: He drives too slow!
I suppose it's not always been an adverb. It probably became one because of the road signs as it was: imperative, easier to read when driving fast and even saved space on the sign