Highway Fog-Visibility Measures and Guidance Systems

Highway fog problems were analyzed and a fog visibility index was developed to quantify fog levels as a measure of detrimental effects on driver performance and traffic operations. The fog visibility index, which predicts the distance at which a vehicle stopped in fog can be seen, uses instrument visibility and ambient light level as inputs. Although the fog index and the associated safe stopping speeds appear as unique functions of fog density and ambient background brightness, the actual sighting distance is a probability function. A survey of fog measuring instruments showed that scatter types appear to be the most suitable for general highway use, although less accurate than transmissometers. All are subject to gross errors in patchy fog banks with single instrument installations. Several potentially useful fog countermeasures techniques are available to highway designers and traffic engineers. Variable message signs appear effective when properly used. Lane marking pancake lights have potential as guidance aids to prevent driver disorientation, but lights along the center of the lane may be also necessary to indicate lane blockage. Some advanced guidance systems appear technically feasible and may be economically practical in combination with freeway surveilllance and control systems.