At freeway weaving sections, traffic congestion and accidents are frequent because of the disturbance generated by merging or diverging vehicles. To reduce the disturbance and accordingly improve the level of service (LOS), an appropriate length of weaving section should be established. According to the US-HCM, the weaving section density, which is a surrogate for LOS, is determined by weaving length, traffic volume, and weaving volume ratio. However, it is not yet clear whether the US-HCM accurately estimates the density for various traffic conditions. The purpose of this study is to evaluate the effects of weaving length under various traffic conditions through microscopic traffic simulation VISSIM. The GA optimization method is applied to build the car-following and lane-changing models in VISSIM. The results show that weaving length is a critical determinant of LOS and short weaving length significantly increases traffic congestion. Increasing the length of the weaving section is a good countermeasure for alleviating traffic congestion. This study also proposes the appropriate weaving section length, with which the upstream LOS can be maintained in the weaving section. These results would serve as valuable information in the planning or design phase of a weaving section given that the flow demands are specified.