Modern cars can monitor rear vehicles and detect unsafe driving before the risk of rear-end collision becomes maximum. In this paper, we focus on this use case. We propose a Rear Vehicle Behavior Awareness (RVBA) system to prevent rear-end collisions. RVBA detects unsafe driving of rear vehicles and alerts the driver with guidance to reduce the risk of rearend collisions. We tested the RVBA in field experiments using a test vehicle. Experimental results show that RVBA can detect unsafe driving of rear vehicles in 4 seconds on average, with 70% accuracy, before the risk of rear-end collision becomes maximum. Index Terms–erratic movement patterns, rear-end collisions, rear vehicle behavior awareness, unsafe driving detection