This study presents a mechanistic-based overlay design system and its implementation in a computer program named "FLEXOLAY" for the state of Idaho. The concept adopted is based on analyzing the pavement distresses in terms of rutting and fatigue and determining an overlay thickness using strain-based fatigue and rutting models. The developed design procedures consider the past and the expected future traffic to calculate the required overlay thickness. Evaluation of existing pavements is performed by nondestructive testing using Falling Weight Deflectometer. Effect of seasonal variation is considered by employing seasonal adjustment factors for pavement layers moduli to account for each season. Seasonal factors for different climate regions in Idaho have been established and used in the environmental data base of the computer program. The program allows for entering other seasonal factors for other locations. The user has the flexibility to enter different shift factors based on local experience.