Why India pays highway builders even when roads stay empty

Intuitively, a toll road company should want traffic. More cars, more tolls, more revenue, it seems like the whole point of building the road. But a large share of India's national highways today are built under a model where the private company that constructs the road gets paid on a schedule that has nothing to do with how many vehicles ever use it, and the government, not the developer, absorbs the risk of an empty highway.