In this research project a number of theoretical volume-delay functions have been proposed with some gaining wide practical applications. The major practical volume-delay functions (VDFs) include Bureau of Public Roads (PBR) function, the Davidson function, the Conical function, and the Akcelik function. The predictive accuracy of these models is heavily dependent on accurately specifying the free flow speed and practical capacity of the highway under study. In addition, properly calibrated parameters of these models are of paramount importance in ensuring realistic results.