This synthesis will be of interest to traffic and safety engineers; highway and bridge designers; police, fire, and emergency personnel; toll authorities; and other officials concerned with roadway safety operations. It describes the forecasting and detection of fog, as well as countermeasures that are used by state and local transportation agencies to dissipate fog and to warn motorists during reduced visibility conditions. The presence of fog has often resulted in serious collisions due to reduced visibility. This publication of the Transportation Research Board presents a generalized background on the causes of fog, fog forecasting and detection methods, and how fog affects the roadway and the driver. The various countermeasures practiced by transportation agencies that are described herein range from techniques to dissipate fog to a variety of warning systems to alert drivers to adverse conditions. The National Transportation Safety Board (NTSB) has held hearings and made recommendations for fog countermeasures, which are described in this synthesis.  The report for this topic can be purchased at https://www.trb.org/news/blurb_detail.asp?id=3329