nick mentions "dumbing down." Exactly!! So many of our fellow drivers are not very smart, not paying attention, not thinking. Highway engineers have to do everything (within reason!) to alert the inevitable fools who will misconstrue "passenger cars only."
Do I think that I would misunderstand that sign, or that anyone reading this thread would do so? No, of course not. But as H.L. Mencken famously observed, "No one ever went broke underestimating the intelligence of the American public.” A corollary to that rule is "if some fool can misunderstand a warning sign, he will." (I just made that up -- not very catchy, but true.)
