Travel time can be defined as the total time required for a vehicle to travel from one point to another over a specified route under prevailing conditions. Its calculation depends on vehicle speed, traffic flow and occupancy, which are highly sensitive to weather conditions and traffic incidents (Park et al., 1998). Nonetheless, daily, weekly and seasonal patterns can be still observed at large scale. For instance, daily patterns distinguish rush hour and late night traffic, weekly patterns distinguish weekday and weekend traffic, while seasonal patterns distinguish winter and summer traffic. It has been increasingly recognized (Smith and Demetsky, 1996; Park et al., 1998; Chien & Kuchipudi, 2003; Stathopoulos & Karlaftis 2003) that for many transportation applications, estimates of the mean and variance of travel times affect the accuracy of prediction significantly.