The objective of this study is to ascertain the engendering causes and evolving patterns of dense fog weather processes on expressways and to reveal their mechanisms of influencing visibility during transportation. Observed fog data from the automatic weather monitoring system (AWMS) along the Shanghai-Nanjing Expressway in China and that from conventional meteorological stations covering areas near the same region were collected and analyzed for a typical complex fog event that occurred over the expressway on November 7, 2009. Based on analysis of actual weather conditions during the fog event, the Weather Research and Forecasting Model for Version 3.1 (WRF3.1), a non-hydrostatic mesoscale numerical forecasting model with high spatial and temporal resolutions, was used in conjunction with National Centers for Environmental Prediction (NCEP) 0.5°×0.5° meteorological reanalysis data to simulate a typical fog event. Numerical simulations of several physical variables were verified from the observed data of the fog event data of AWMS. The dynamic and thermodynamic conditions engendering this complex fog event in addition to moisture levels were analyzed. The following were the main conclusions: (1) The atmospheric circulation conditions in Jiangsu Province and its contiguous regions were stable before and during the heavy fog event. These regions were affected by the southeastern stream at the southwestern side of an anti-cyclone system over the Yellow Sea. Moreover, a weak surface wind present during the entire fog process provided favorable dynamic conditions for engendering heavy fog. (2) The simulated distribution areas of the fog masses depicted by the liquid water content (LWC) agreed with observation data. (3) The simulated visibilities were very close to the observed AWMS visibility data. (4) The mixed fog event occurred during 18:00 (BST) November 7 to 08:00 (BST) November 8, 2009. The embryo of this heavy fog event was a local patchy fog that was converted into radiation fog under the influence of radiation cooling. Then, a warm and humid air advection mass from the surface of the East Sea moved southeast and entered into the southern part of Jiangsu Province to create an advection fog mass. This mass combined with the earlier radiation fog to form a complex fog mass from the dual effects of radiation cooling and advection cooling, enabling large-scale expansion. (5) The warming of the surface caused by short-wave radiation after sunrise was the main reason for the rapid dissipation of the fog body.