Effect of Including the Empty-adjusted Truck Factors on Pavement Design: A Case Study on Riyadh Area Road Network

Truck traffic is one of the major inputs to any pavement design procedure. In almost all pavement design procedures, truck traffic is represented in terms of the accumulated Equivalent Axle Load (EAL) applications. The level of EAL applications is mainly dependent on the prevailing values of truck factors. Thus efforts have to be made to accurately estimate truck factor values based on local loading conditions. The current practice of the Ministry of Communications (MOC) involves recording axle weights of loaded trucks only without including empty trucks. This, in turn, leads to higher truck factor estimates and consequently over-designed pavement structures. The main objective of this study is to quantify the effect of excluding empty trucks when estimating truck factors, on highway pavement design. The truck factors associated with loaded trucks were determined from MOC data files. The empty truck factors were determined from a field sample consisting of 4000 empty trucks. The empty-adjusted truck factors were calculated as the weighted average of the two truck factor values. An analysis has been conducted to study the effects of using the empty-adjusted truck factors on pavement design instead of loaded truck factors, using the AASHTO design procedure. The analysis indicated that the average truck factor for empty trucks is about 1.15, the average truck factor for loaded trucks is about 9.87 and the average empty-adjusted truck factor is about 6.63. It also showed that excluding empty trucks would result in over-designs with an average of about SR 1.0 / m2 or about SR 3500 / km-lane extra construction cost.