This paper proposes a measurement-based method to assess transmission line loadability, particularly when determined by stability constraints. A power system two-area equivalent model provides the basis for the proposed line loadability assessment method, with the angle across this equivalent model defining a metric of the closeness to the system stability limits. In this paper, using advanced synchrophasor measurements, Kalman filter-based algorithms are applied to estimate the parameters and variables of the aforementioned equivalent system model. A significant benefit of the proposed method is that it does not rely on the knowledge of the system topology and parameter values. The proposed method is illustrated through a two-bus system example and a two-area system case study.