Wow, that is some fine slicing on detailing the legal ramifications of the wording. They describe (page 419-420) how the provision saying a tax must be used for roads says it applies when the tax is "on the ownership, operation, and use" of motor vehicles. They point out that in a different section about fuels, they talk about "the storage, withdrawal, use, sale, distribution, importation or receipt" of fuels.
They point out the long standing legal principle that if a word is included in one place but excluded in another similar place, it must be interpreted that it was left out on purpose. So they say since the tax on "sales" was left out of the section of tax that must go to road maintenance, then it does not apply.