TRAFFIC LOAD FORECASTING FOR PAVEMENT DESIGN. INTERIM REPORT

When a pavement structure fails prematurely, the constructing agency must pay millions of dollars in traffic control and construction costs to rehabilitate or reconstruct the pavement sooner than had the pavement survived its design life. The money required to rehabilitate or reconstruct the pavement could have been put to alternative uses during the remaining years of the pavement's design life; but because the money must be spent when the pavement actually ceases to provide adequate service, the opportunity to apply the much needed capital elsewhere is lost. A primary determinant of a pavement's actual service life is the traffic loading applied to the pavement. Consequently, an important consideration in pavement structural design is a forecast of the traffic loading expected to be applied to the pavement structure during its design life. This research evaluated the Texas Department of Transportation's traffic load forecasting procedures. The research found that traffic load forecast accuracy could be improved by more than 30% from current levels by conducting 24-hour manual vehicle classification sessions at specific pavement project sites and by more than 85% by conducting week-long weigh-in-motion (WIM) sessions at specific pavement project sites. The research shows that if forecast accuracy was improved by the amounts indicated above, fewer pavements would typically fail prematurely; and, while some pavements would still fail prematurely despite improved forecasts, these pavements would have longer lives than under current practice. The research found that the cost to improve traffic load forecasts is justified by the benefits received in return for almost all pavement reconstruction projects and most major pavement rehabilitation projects.