Abstract: The deteriorative traffic condition in many cities significantly augments travel cost, and hence resulting in so-called fat tail effect.This paper aims at depicting such phenomenon by employing such concept as travel time reliability(TTR) to recapitulate the variational character.Firstly, a no vel metho d to predict travel time, i.e.grid method, is designedfor the sake of integrating driving time and queuing time.And then, Gram-Charlier as a powerful probability tool is applied to describe the extrusive character of TTR based on its four parameters.Two approaches are discussed to assist derive those parameters in closed form.Finally, a simulation result illustrates that the method works, and further research is indicated as well.

Key words: travel time prediction, travel time reliability(T T R), Gram-Charlier distribution, urban transportation