The number of serious traffic accidents (accidents resulting in hospitalization or death) that occurred on Park Road from 1986 to 1990 was 35 percent lower than the number of serious accidents from 1981 to 1985  The speed limit on Park Road was lowered in 1986
Hence, the reduction of the speed limit led to the decrease in serious accidents
Until 1986 accidents were classified as serious only if they resulted in an extended hospital stay.