As PMBOK says that you must idenfity all stakeholders. And if required then you must interview them to gather their requirements. I want to know how do you do that in case of a large project where the stakeholder is a large group?

For e.g. let's say there is a project about building a 50KM road. Now in this case general public is also a stakeholder who will use this road so how do you get their requirements? Or you just talk to the government and since government is representative of people hence what ever they say is public requirement?