Warning the rear approach of objects before opening the car door is essential for safety reasons. Particularly in Taiwan, the traffic accident caused by car door opening were about 20% in 2016 and resulted in a lot of motorcyclists and cyclists being suffered from severe injury. For this, we propose an anti-collision system by using single digital camera and a Field Programmable Gate Array (FPGA) chip. The proposed system first detects approaching objects by motion detection and then derives their speeds to raise an alarm. Here, the warning threshold is set at 16 km/hr to detect an object from 3.8 to 18.8 meters away. Our experimental results indicate that the proposed system is practical for smart car designs with embedded collision detection.