National Weather Service (NWS) observations were compared to Florida Highway Patrol accident site visibility reports to produce a Low Visibility Occurrence Risk Index (LVORI). When LVORI is compared with NWS visibility observations, significant differences are found. These differences appear to be related to two types of fog: advection and radiation. The data suggest that localized radiation fogs pose greater hazards than widespread advection fogs. Apparently, drivers are able to adjust when fog is widespread, but are less successful when very low visibility is suddenly encountered.