slow/slowly

I love to travel. I especially like driving trips. Here's one that irks me every time. You're driving along, and sure as anything, you run into road construction where traffic is narrowed to one lane going in both directions. Obviously, to avoid collisions, traffic has to be stopped in one direction so traffic moving the other way can proceed.

Here's my issue. You know that guy holding the sign that reads "Stop" on one side and "Slow" on the other? It's grammatically incorrect (of course). The nice man flips the sign from "Stop" to "Slow" when he wants me to continue on my merry way. . .but I don't know if the "Slow" part refers to my driving or to me.

You see, the word "slow" is an adjective as in "That is a slow car," or "Timmy is really slow." It should read "Slowly." Its intended meaning is that I am to proceed with caution; therefore, it's referring to my driving. If it's related to my driving, then it's modifying an action and should be the adverb form.

Besides, I'm not slow, and who is he to be making judgments such as that? He doesn't even know me.