Effectiveness of Traffic Forecasting on Pavement Designs for Sri Lankan Roads

Abstract

Pavement design plays an important role in any improvement or rehabilitation. It is a responsibility of the road design Engineer to ensure that he has come up with an effective design, so that it will last for the design life. This effectiveness or the optimization is very important as otherwise it could lead to financial implications. The method adopted for the design of flexible pavements is the TRL, Road Note 31. The two main parameters considered in the design of the pavements under Road Note 31 are Cumulative Number of Standard Axles (CNSA) (i.e. Traffic Class) and the sub-grade strength (i.e. California Bearing Ratio (CBR)% class). In this research study, flexible pavement designs of recently rehabilitated or improved set of roads were analyzed to check the effectiveness of the traffic forecasting on pavement design. As the subgrade strength of the pavements is a fixed parameter in all the cases, the only possible variable is the Traffic Class relevant to predicted CNSA. It was found in the study that the actual traffic growth rates of different modes of traffic which travels along the selected roads is different to the predicted rates at the time of design. It has also been shown and statistically proved that the Equivalent Standard Axles (ESA) values actually applied on these pavements by large trucks / heavy goods vehicles are significantly high, compared to the ESA values recorded at the design stage. The authors have proposed a methodology to evaluate the effectiveness of traffic forecasting on pavement designs, and improvements to the present practice of pavement designs carried out by the Road Development Authority (RDA) and its presentation.