This field study examined driver responses to two types of potential highway skidding hazard: wet pavements subjected to high frictional driving demands, and bridges during periods of possible preferential icing. Study objectives were to examine motorists' general awareness of the hazards and to assess the relative effectiveness of various signing treatments which warn of the hazards. Measures of signing effectiveness were motorists' speeds at critical driving locations and questionnaire responses regarding motorists' observations and interpretations of the signs. Three curved highway sections were treated with five experimental signing conditions. Experimental signing conditions incorporating flashing lights were effective at reducing highest quartile mean speeds below the critical safe wet pavement speed based on roadway geometry and surface conditions. Two bridge approaches were signed using combinations of activated and nonactivated signs both at the bridge and in advance of the bridge during periods of possible preferential icing.