Traffic safety especially freeway safety has become of increasing concern of transport agencies due to the increasing losses to society. The crash surrogate, which is defined as the measure of crash proximity, is widely accepted as an alternative to crash data by taking advantage of many merits. With the increased travel demand, freeways frequently operate at capacity or even in a state of synchronized flow with high speed and small headway. Under this kind of state, even a slight disturbance might result in a rear-end crash or even a chain collision (multi-vehicle collision), which could consequently lead to serious health, economic and environmental problems. However, the traditional surrogate measures could not perform very well in this state due to some limitations. Surrogate measure Aggregated Crash Index (ACI), a tree-structure based indicator, is designed based on a hypothetical disturbance imposed. With the hypothetic disturbance, ACI can be applied to any car following scenario without any restricted conditions. In this paper, we carry out a case study to illustrate how ACI is applied to the distinct car-following scenarios. The result shows that ACI has more comprehensive capability and applicability on representing crash risk.