Seasonally changing speed limits

The effects of seasonally changing speed limits were studied in Finland during two winters and two summers between November 1987 and September 1989. Speed limits of 100 km/h were lowered to 80 km/h during winter on selected roads in two phases. In the summer experiment the former 80 km/h limits were set to 100 km/h similarly in two phases. The study included three parts, which dealt with: (1) the effects of the experiment on the speed of automobiles; (2) drivers' opinions about speed limits and changing them; and (3) the number of accidents on the experiment roads. More than 3000 drivers were interviewed to clarify their opinions. Three drivers out of every four supported the idea of changing the speed limits seasonally. Because of the experiment, the speeds of more than 200,000 vehicles were measured by radar. The lowering of the 100 km/h limit to 80 km/h reduced the average speed of all vehicles by 3.8 km/h. In addition to this, the weather and road conditions lowered average speed by 3 km/h during winter. The use of higher speed limits in the summertime (100 km/h instead of former 80 km/h) increased the average speed of all vehicles by 3.7 km/h. The lowering of the 100 km/h limits to 80 km/h during winter months decreased all accidents by 14%. The lower wintertime speed limits decreased accidents most when the road was not dry, but otherwise road and weather conditions were good. The higher 100 km/h speed limits used in the summertime instead of the previous 80 km/h increased the number of accidents at least on the most heavily trafficked roads and in good road conditions. The results of the study show that by lowering speed limits the number of accidents can be reduced where and when the limits actually lower the driving speeds. Because poor road conditions also decrease speeds, the lower speed limits do not automatically decrease used speeds everywhere.