As someone who is fairly unfamiliar with tax law, small picture government budgeting and things like this, perhaps I am slightly underprepared to answer this question but I'll give it a shot nonetheless.
Frankly, my first answer would be that it seems fairly intuitive not to bridge that gap. If there is less income from the gas tax, it seems to follow that there are fewer people purchasing gas, or people are purchasing gas less frequently. If this is the case, then it seems fairly intuitive that people are using their cars less and less, therefore there is less and less wear and tear on roads, and therefore the roads need less repair.
Now there is an argument about gas mileage here: that the gap does not demonstrate fewer people using roads, rather people using cars with better gas mileage and they, in turn, have to fill up less frequently. However, typically the cars that get better gas mileage are lighter and produce less wear and tear on the roads, so there is a bit of a canceling-out effect to that argument.
However if the gap must be bridged, it ought to come from some sort of auto-related tax area. There is, in my mind, a direct correlation between auto usage and wear and tear of roads so it seems like a natural place to bridge the gap. It should not be bridged at the expense of education, research, or human well-being. Good roads are a luxury and ought to be treated as such: a secondary concern.
-- MICHAEL VOWELS, Richland