High automated vehicles rely on the computing system in the car to understand the environment and make driving decisions. Therefore, computing system design is essential for ensuring the driving safety. However, to our knowledge, no clear guideline exists so far regarding how to guide the safety-aware autonomous vehicle (AV) computing system design. To understand the safety requirement of AV computing system, we performed a field study by operating industrial Level-4 AV fleets in multiple locations for three months. The field study indicates that traditional computing system performance metrics, such as tail latency, average latency, maximum latency, and timeout, cannot fully satisfy the safety requirement for AV computing system design. To address this issue, we propose the “safety score” as a primary metric for measuring the level of safety in AV computing system design.