  Moderate length of time window can get the best accurate result in detecting
the key incident time using extended spectral envelope. This paper presents a
method to calculate the moderate length of time window. Two factors are mainly
considered: (1) The significant vertical lines consist of negative elements of
eigenvectors; (2) the least amount of interruption. The elements of
eigenvectors are transformed into binary variable to eliminate the interruption
of positive elements. Sine transform is introduced to highlight the significant
vertical lines of negative elements. A novel Quality Index (QI) is proposed to
measure the effect of different lengths of time window. Empirical studies on
four real traffic incidents in Beijing verify the validity of this method.
