From The Transportation Research Board:

TRB's National Cooperative Highway Research Program (NCHRP) Report 523: Optimal Timing of Pavement Preventive Maintenance Treatment Applications describes a methodology for determining the optimal timing for the application of preventive maintenance treatments to flexible and rigid pavements. NCHRP Report 523 also presents the methodology in the form of a macro-driven Microsoft Excel Visual Basic Application--designated OPTime.

http://www.trb.org/Main/Public/Blurbs/155142.aspx