This study was designed to investigate the effects of weather-controlled speed limits and signs for slippery road conditions on driver behavior on the Finnish E18 test site. Local weather and road conditions were monitored from two unmanned road weather stations. The speed limits were lowered automatically during adverse road conditions, and in some cases signs for slippery road conditions were displayed as well. Speed and headway data were obtained from loop detectors. The results showed that in winter the change of the posted speed limit from 100 km/h to 80 km/h decreased the mean speed of cars traveling in free-flow traffic by 3.4 km/h, in addition to the average mean speed reduction of 6.3 km/h caused by adverse weather and road surface conditions. When poor road conditions were difficult to detect (e.g., there was no rain or snowfall or the rain was insignificant), the effect was 1.9 km/h higher (i.e., the reduction was 5.3 km/h). When road conditions were such that signs for slippery road conditions were also displayed, the variable speed limit system reduced the mean speed by 1.8 km/h, whereas the reduction caused by the weather was 9.3 km/h. In addition to the effects on mean speed, lowering of the speed limit decreased the speed variance. There was no remarkable effect on headways. The signs decreased speeds also on the road section next to the equipped section. The main implication of the study is that the system improved traffic safety by decreasing mean speeds and speed deviation.