Estimating cash flows for project finance

Calculating NPV and IRR is simple and all, but how would one go about to estimate the cash flows for an infrastructure project? I mean, it’s usually in a new area, few comps, and relies on the local development pushed by the government (ie. let’s say you build a toll road in the rural area, whether it generates sufficient cash flow depends on if the municipality is pushing for growing traffic / population)

Is there a framework I can use for the estimations?