Summarized are the results of studies that examined the effects of speed limits that were changed on a seasonal basis on Finnish main roads. The first of these studies was an experimental study (1987-1989) in which speed limits were changed according to time of year. A follow-up study was then conducted on the resulting and greatly expanded use of lower speed limits during the winters of 1989 to 1996. Reasons for lowering wintertime speed limits involved primarily adverse road and driving conditions, including slippery roads, the use of studded tires, frequent heavy rain or snowfall, and darkness. In the 2-year study, a statistically significant accident reduction of 14% was detected, and the followup suggests an even greater accident reduction. This significant accident reduction was achieved by cutting mean speeds as little as 3.8 km/h. Interviews with drivers showed they accepted the lower speed limits surprisingly well, most likely because they understood the reasons for them. Although significant safety advantages have been gained in poor driving conditions, the gains have been even greater in good conditions on roads of reasonably high standard because such roads are where speed limits dropped the most. Lower wintertime speed limits have had a positive safety effect, even on roads with a fixed 100-km/h speed limit. As a result, most 100-km/h speed limit signs in Finland are now changed manually to 80 km/h at the end of October and back to 100 km/h in early March. These safety measures have proven to be both cost-effective and well accepted. Lower wintertime speed limits are now being planned in other northern European countries.