Road surface condition is considered by many to be the most important information of the road weather station. Traditionally, the condition has been assessed by measuring the electrical conductivity of the road surface with electrodes brought to the surface level. When a lot or even excessive amounts of salt are used and the climate is humid, the method is somewhat effective. It is often possible to distinguish whether the road surface is wet, damp or dry, and no icy road should appear. Under northern conditions, the method does not work well, as the data of this comparison clearly show. When salt is used sparingly, changes in conductivity are small and difficult to measure. As a result, the icy road will easily be reported as dry or, if there is little conductivity, saline moist. The job is saved by a good rain sensor, based on which the road can be said to be snowy when it snows, look at Evaluation of a winter road maintenance quality indicator (Mikko Malmivuo, 2018, in Finnish).
In early 2000, a method based on optical spectroscopy was introduced. The method can very well distinguish an icy road from a molten and even partially molten (meltable) road. The RWS10 measures the state of the road surface by an optical method. The benchmark sensor, Vaisala’s DSC211, has proven to be the best in measuring the surface condition and to perform well in many comparisons. Since a fixed road weather station always measures only a (very) small part of the road surface, an entirely correct result may not represent the surface condition very far from that location. The RWS10 tends to provide equivalent information compared to the DSC211 at a much lower cost. The structure of the entire RWS10 station is remarkably simple and compact. Only the longest measurement distance is less than that of the DSC211. The signals of optical sensors decrease inverse to the square of the distance especially when the road surface is not dry. Fog also strongly attenuates signals. In any case, since the analysis of very small signals is more susceptible to calibration and other errors, it is advisable to make the measurement as close as possible. The other limitations are the contamination of the optics by traffic pollution and a possibly too small measuring angle across the lanes.
In connection with the optical sensor, a friction figure has been introduced to describe the condition of the road surface. Friction is basically a reproducible and physical measure of slippery surface condition in winter weather. Since most roads in northern conditions are practically completely or at least partially icy most of the winter, even a person as a friction measurer has to choose the place and line where to measure. Nearby, the result can be a lot different. Thus, the location seen by one optical sensor may be different from that seen by another sensor, although it would be nearby. A very accurate temporal comparison of the data is therefore not a very useful approach. From the images from the traffic camera, you may try to guess what may have caused the observed differences.
Sensors from different manufacturers often interpret the same situation differently. Opinions can also be expressed on the numerical values of the amount of ice or snow. The amount of water on the road surface varies so much in any case that even an accurate measurement usually does not represent a larger area, especially when there is a lot of water. The optical road weather station measures the amount of water well enough after precipitation. In many cases, one wonders whether the road should be considered as wet or dry.