The design of flexible pavements developed by the U.S. Army Corps of Engineers is basically based on the California Bearing Ratio (CBR) equation that was formulated in the 1950's, and extended in the 1970's, on the basis of the results of numerous full-scale accelerated traffic tests. These tests involved full-scale load carts operated on various test section-pavements, for both highway vehicle and aircraft landing gear, with wide configurations. The test sections consisted of flexible pavements with much different thickness built on subgrades that had a wide range of strengths.
The CBR equation is empirical approach, and the recent method of design is deterministic. A probabilistic approach providing more reliable design-data potentially lower cost can be developed from the recent design procedure if the reliability of the CBR curve is known. This research is done to determine the reliability of the current CBR-based flexible pavement design model by using existing data from accelerated traffic tests. The reliability of the design model was found to be about 60 percent, neglecting the effect of conservative estimates of the design parameters. For the further research, it is suggested that include the difference between the performance of the accelerated traffic tests sections and the long term-performance of actual in-service pavement, and the effect of conservative estimate-values for material strength, traffic load, and traffic intensity.