Free-flow speed is one of key factors in the traffic congestion functions, which reflect the relationship between the cost on a link and its characteristics. To examine how properly the free-flow speeds in KHCM (Korean Highway Capacity Manual) reflect the cost-flow relationship, this study re-estimates the free-flow speed by utilizing the AVC (Automatic Vehicle Classification) data obtained from loop detectors at 109 sites on Korean rural expressways in 2009. This study also analyzes whether or not the traffic assignment errors can be reduced if the re-estimated free-flow speeds are used in the congestion function. The estimation results show that the re-estimated free-flow speeds are much higher than the KHCM free-flow speeds: the free-flow speeds for two-lane and three-lane expressways increased by up to 8kph and 12kph respectively. In addition, the traffic assignment results show that the mean absolute percent error between the observed and assigned traffic volumes is reduced from 189% to 88% when employing the re-estimated free-flow speeds in the congestion function. Hence, it is highly recommended to use the re-estimated free-flow speeds to reflect the cost-flow relationship more appropriately.
