Abstract: The author clarified the relationship between volume to capacity ratio (V/C) and crash rate using three years crash and traffic flow data of a freeway. This relationship can be used as a basic reference to predict and prevent traffic accidents. The hourly crash rate model, having an independent variable of hourly V/C, was established by regression analysis, and the relationship between crash rate and V/C ratio represented by a U-shaped curve. The crash rate is the highest in the low hourly V/C range, decreases with an increasing V/C ratio, and then increases as the V/C ratio increases. When the V/C is 0.58, the crash rate is the lowest.