Abstract: With 3-year's traffic flow data and crash data of a freeway in plain terrain in China,the relationship between traffic flow and safety was analyzed.Besides the amount of traffic volume influences the crash occurrence absolutely,truck percent,the speed difference between truck and car,and the ratio of traffic volume to capacity are the most three prominent parameters.The interactive impact also existed among the traffic flow parameters inside,so how and to what extent the single parameter impacted the road safety was changeable,and some functions made sense only when other parameters were confined in the specific conditions.The authors developed several kinds of mathematical relationships that described the combined effects of traffic flow characteristics on crash occurrences,which can enable engineers to identify and implement effective countermeasures to reduce the probability of crashes.