Road surface freezing is a serious problem for road traffic in the winter in cold regions. This is especially true in mountainous regions, where the winter season is longer and roads are frequently covered with snow and/or ice. We sought to develop a detector for road-freezing conditions, consisting of optical sensors and an infrared thermometer. The measurement system has a light source with an incident angle of 45° and two photodiodes set with receiving angles of 0° (at the zenith) and 45° to measure the diffuse reflection and the specular reflection (mirror reflection). Using observations from a portable measurement system, road conditions (dry, wet, 'sherbet,' compacted snow, glossy compacted snow, black ice) were distinguished well in relation to the specular reflectance R_S and the diffusive reflection R_D. A mobile measurement system was installed at the rear of a car, and continuous observations were made along Route 39 around Sekihoku Pass, Hokkaido. The observed reflection data showed reasonable results for wet and sherbet surfaces along the lower parts of the course and for compacted snow and glossy compacted snow around Sekihoku Pass. The ratio of specular reflectance to diffuse reflectance, γ_SD (=R_S/R_D), enhanced the indication of the existence of black ice on the road surface, because R_S was large and R_D was small.