Entry 009: Caution -- Adverb Usage Error (Rules Project)

My father lives on a private road in the woods, which is maintained by his homeowner's association.  It's not a very wide road -- when two cars wish to pass one another on the road, one usually has to pull partly off of the road.  There are spots of the road where flat, solid asphalt has given way to pebbles and large, disjointed slabs of asphalt.  There are turns with little visibility.  There are trees that grow very close to the side of the road.

This still does not keep people from speeding down the road.

To combat speeding neighbors and visitors, people who live along the road have put up their own makeshift signs, admonishing people, in different ways, for driving too fast ("slow down -- children at play!"; "watch speed!"), or calling for them to slow down.

Many signs say this (sign found via google image search; text is identical to signs on road):

Can you spot the error? 

This sign is a command to drivers to "drive slow."  However, in this sign, "slow" is an adjective.  Adjectives, as Jane Straus writes on her blog*, are "a word or set of words that modifies (i.e., describes) a noun or pronoun."

Slow, however, is being used to modify a verb.  As such, slow should be changed into an adverb, since, as Jane says on the same page, adverbs are "a word or set of words that modifies verbs, adjectives, or other adverbs."

The correct version of this sign would read, "Caution - Drive slowly."

I was not able to find any explanation for why this mistake is made on so many signs.  Internet users on various forums suggested that by leavig off the "ly" from slowly, the government was saving millions of dollars -- I shall let you decide if that explains the error!


*http://www.grammarbook.com/grammar/adjAdv.asp