In this paper, we present a fast coupling aware delay estimation algorithm based on a simplified interconnect circuit model. The interconnect model consist of a coupled PI segment which represents the coupled interconnects after mapping and reduction of general coupled lines. Closed-form analytical formulas are derived for both propagating and crosstalk waveforms. Best-case and worst-case delays are obtained by using superposition to create composite waveforms without expensive search process. The circuit model has been verified by comparing with SPICE simulations, and it provides an efficient and reasonably accurate way to estimate timing transition intervals in presence of cross-coupling.