This study presents a mechanistic-based overlay design systemand its implementation in a computer program named "FLEXOLAY"for the state of Idaho. The concept adopted is based on analyzingthe pavement distresses in terms of rutting and fatigue and determiningan overlay thickness using strain-based fatigue and rutting models.The developed design procedures consider the past and the expectedfuture traffic to calculate the required overlay thickness. Evaluationof existing pavements is performed by nondestructive testing usingFalling Weight Deflectometer. Effect of seasonal variation isconsidered by employing seasonal adjustment factors for pavementlayers moduli to account for each season. Seasonal factors fordifferent climate regions in Idaho have been established and usedin the environmental data base of the computer program. The programallows for entering other seasonal factors for other locations.The user has the flexibility to enter different shift factorsbased on local experience.