We collect weather and road condition data from road weather stations that are located along the road. The system suggests a suitable speed limit on the basis of the station’s observations, and the officer on duty at the Traffic Management Centre can change it, if appropriate. Changing the speed limit is not always an obvious choice. Rain and snow showers and slipperiness can be very local phenomena. For example, driving conditions may be poor only for one kilometre, but the section in question might not have a road weather station. In this case, we have no way of knowing about the poor road condition.

Speed limit traffic signs are usually located near junctions, and the distance between two signs can be 5 kilometres. In these cases, it is impossible to set a speed limit that would be optimal for the whole distance. In the winter, even salted roads may have individual slippery sections in cold places, such as on bridges, between rocks and in low valleys. Road users should not expect the quality of the driving conditions to remain the same throughout the trip, and should instead adjust their driving speed to the current weather and road conditions.

Variable speed limits are also used for controlling the flow of traffic. Higher speed limits are used in quiet times, and they are lowered during peak times in order to improve the safety and smoothness of traffic. Variable speed limits are controlled either based on time or traffic information. These speed limits are not affected by the weather or road conditions.