Yes, I think you are right. Here is an analysis that places the 30-year lifecycle costs of a road at $8.63 per square foot: http://www.strongtowns.org/journal/2014/7/10/fiscal-impacts-...

This research seems more thorough, but still concludes that low-density developments often require much more than 30 years to pay off a road's 30-year lifecycle.

Edit: Looks like OP's calculations are based on how much of the collected taxes actually goes to the road budget.