Abstract: Fog leads to a decline of visibility on freeways,which decreases safety and efficiency of traffic.A variable speed limit(VSL)control scheme for freeways under foggy weather conditions is proposed,including system design, workflow,and VSL operation strategy,etc.The speed limit of target sections is determined based on data of real-time road visibility and traffic flow,factors including the maximum safe speed,traffic state and non-compliance behaviors of drivers,etc are considered.Data collected from the G4 freeway and simulations are analyzed by Vissim.The results show that compared with no control scheme,the proposed scheme can increase the lowest speed in advection(agglomerate)fog by 27.24%(28.54%)and decrease the maximum speed difference of adjacent link by 26.42%(41.91%).This VSL control scheme is more effective in agglomerate.Furthermore,it ensures traffic safety and improves efficiency of freeways in foggy weather.