Abstract: On-ramp merging area of freeway is general accident-prone section．In this area,vehicles are used to be involved in two kind of risks：car-following risk and lane-changing risk．Based on critical acceleration and its physiological and psychological characteristics on driver， this paper advances a model to calculate car-following risk index and lane-changing risk index． which a used to mesure the risk of a single traffic behavior．Then，the concept of average risk is presented to evaluate the safety level of on-ramp merging area

Key words: on-ramp, car-following risk, lane-changing risk, safety evaluation

摘要： 合流区域是高速公路常见的事故多发区域，车辆在该区域行驶时，易引起跟驰风险和变换车道风险．本文以 临界加速度作为依据，结合驾驶员对向后加速度的生理心理效应，提出跟驰风险指数和变换车道风险指数的计算 方法，用于定量描述单起交通行为的风险大小．最后提出平均行车风险值的概念，用于评价合流区域的安全水平．

关键词: 合流区域, 跟驰风险, 变换车道风险, 安全评价