Some fun stuff:
A recent Strong Towns post tells us that (1) sales taxes are increasingly being used as DOT slush funds (due to excessive restrictions on local tax collection),
thereby (3) heading into the teeth of a likely large-scale retail downsizing,
which would (4) extirpate sales tax revenues in most locations--shrinking the size footprint of the transportation slush fund.
This is consistent with "tail fattening" (think Nassim Taleb)--especially the kind that, to reduce or mitigate risk now, kicks it down the road, compounding it when the interest's due.