We have examined a possibility for improvement of the STOA (Shock Time Of Arrival) model for interplanetary shock propagation. In the STOA model, the shock propagating velocity is given by Vs ~ R-N with N = 0.5, where R is the heliocentric distance. Noting observational and numerical findings that the radial dependence of shock wave velocity depends on initial shock wave velocity, we suggest a simple modified STOA model (STOA-2) which has a linear relationship between initial coronal shock wave velocity (Vis) and its deceleration exponent(N), N = 0.05 + 4 ¿ 10-4Vis, where Vis is a numeric value expressed in units of km s-1. Our results show that the STOA-2 model not only removes a systematic dependence of the transit time difference predicted by the previous STOA model on initial shock velocity, but also reduces the number of events with large transit time differences.