Classically, fundamental diagrams are estimated front aggregated data at a specific location. Such a measurement method may lead to inconsistency, which mainly explains the current controversy about their shape. This paper proposes a new estimation method based on passing rate measurements along moving observer paths. Under specific assumptions, it can be proved that in congestion, the passing rate is independent of the traffic flow states. This property allows 1) proof that a linear fundamental diagram is suitable to represent traffic flow behavior involved in the Next Generation Simulation (NGSim) data set and 2) fitting of its two parameters, i.e., the congested wave speed and the jam density.