I don't think "according to the applicable vehicle code" is quite accurate, or, at best, it is somewhat ambiguous.
I think that, in your example sentence, "as specified in the vehicle code" could almost mean two slightly different things.:
- A. It is written (specified) in "the applicable Vehicle Code" that "It
may be unlawful for any driver to operate a vehicle without wearing
the proper seat belt while it is being operated on a public highway
or road." (Maybe not that exact sentence but something with the same meaning.)
- B. Whether or not "It" is "unlawful for any driver to operate a vehicle without wearing the proper seat belt while it is being operated on a public highway or road" is specified in whichever "vehicle code" is "applicable" to you. (In the U.S.A., different states have different vehicle codes and they don't all have the same seatbelt laws.)
Your guess of the meaning seems to me to be A, but, based on evidence will presently explain, I think B is more likely to be the meaning of this sentence.
- The sentence says that "it may be unlawful", but then it uses the phrase "any driver." The only reasons I can think of to use the verb form "may be" rather than is, are if: a.) The truth of the sentence depends on who the reader/listener is, where, what, or when they are driving, etc.; or b.) The writer/speaker simply doesn't know the law for certain, which is unlikely given the formal style and legal subject matter.
- The sentence says "the applicable vehicle code." If only only one "vehicle code" were "applicable" to all readers/listeners, then there wouldn't be any need to include the word "applicable." It is still possible that every "vehicle code" which might be "applicable" specifies the same law, but this sentence seems like an oddly indirect way to convey such a meaning.
Now for some discussion that might help you understand this phrase and others like it.
Specify is related to specific (and also special.) The meanings "to state explicitly or in detail" and (possibly) "to include in a specification" (both from The American Heritage Dictionary, 4th ed.) both might apply here.
Among the several definitions of the word as given by the Fourth Edition of The American Heritage Dictionary, the following is the only one possibly appropriate here:
3. When taken into consideration in a specific relation or form: this definition as distinguished from the second one.
The dictionary also asserts that this meaning of the word is an "adverb." Unfortunately, traditional English grammar uses the word "adverb" to refer to a number of words that behave quite differently from each other. Personally, this use of the word seems similar to a complimentizer(such as that or which), (and reminds me of subordinating conjunctions.)
Note that, in both your example and the dictionary's, "as" is the head word of a phrase which is actually just a reduced sentence (Words in brackets, [ ], may be added without changing the meaning or grammaticality though they may cause confusion with other meanings of as.):
Main-Sentence + "as [it is] specified in the applicable vehicle code."
= Main-Sentence + as the applicable vehicle code specifies.
= The applicable vehicle code specifies that + Main-Sentence + , and that is true.
"this definition" + "as [it is] distinguished from the second one." REMINDS YOU to distinguish* "this definition"* "from the second one."
Note that the dictionary's example seems to be affecting either a noun ("definition") or a noun phrase ("this definition"), whereas yours seems to be affecting either the whole sentence or the verb "may be". This, along with the somewhat unclear connection between the two meanings, might be cause (a good reason) to argue that it is actually a slightly different use of the word.
Whatever the grammar behind this is, here are some example constructions.
Verb(+Objects)+as specified. by itself implies either that something was specified previously in the text, or that you should already know what or who is specifying whatever is "as specified."
Verb(+Objects)+as specified by [eg. the applicable vehicle code, the highway commission, Bob] tells that whatever follows by did the specifying.
Verb(+Objects)+as specified in [eg. the applicable vehicle code, Vienna, the The Tunisian Constitution, the previous session, the living room] tells you either where or when something was specified or where you may find the specifications written.
Verb(+Objects)+as specified for [eg. Texas drivers, the particular engine, most safety applications, LTE systems] tells you who or what a specification applies to or is for the benefit of; or in what situations it applies.
and so on with on, with, at and probably any other preposition you can think of, though most are unlikely to be used.