Right, yup it was a census, which is manned by civi/government staff, but a policeman is present.
The use of these is to briefly ask people about their journey, choice of route, importance of using the car etc.
Usually done when they are considering a big project.

At a census you are legally obliged to stop, and would be commiting an offence to refuse to do so.
David, its not a road block, its a selected number of vehicles that are stopped. Sometimes only specific vehicle types are stopped..