The safety of autonomous vehicles is hard to ensure in adverse weather since the sensors will degrade drastically. Setting a variable speed limit based on real-time weather condition is the most efficient method to make the vehicle safe. But most current speed limit methods are based on human visibility rather than the sensor, which is not suitable for autonomous vehicles. Thus, it is necessary to explore the performance of sensors in different weathers and propose a speed limit method based on sensor performance. Safety decisions will be made based on the calculated speed limit to ensure safety. This paper describes how to make safety decisions based on sensor performance and road conditions in real-time. The experiment explores the degradation of different sensors, and variable speed limit methods are proposed for rainy and foggy days. MPC controller is used to generate safety decisions.