Classically, fundamental diagrams are estimated from aggregated data at a specific location. Such a measurement method may lead to inconsistency which mainly explain 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 passing rate is independent of the traffic flow states. This property allows (i) to prove that a linear fundamental diagram is suitable to represent traffic flow behavior involved in the Next Generation Simulation Model (NGSim) dataset; (ii) to fit its two parameters: the congested wave speed and the jam density. Index Terms : Passing rate, macroscopic, NGSIM, fundamental diagram, equilibrium traffic state, traffic flow theory.