Autonomous driving cars have become a trend in the vehicle industry. Numerous driver assistance systems (DAS) have been introduced to support these automatic cars. Among these DAS methods, traffic light detection (TLD) plays a significant role. This paper proposes a method to detect traffic lights (TLs) using color density identification (CD). The system receives an RGB image as an input and produces the traffic light state (red, yellow, green or no Signal) of the scene. The algorithm has three stages: clustering, filtering, and state identification. Experiments were conducted on both highways and in urban areas in Korea. The results achieved around 95% accuracy on highways and 85% in urban areas. Furthermore, the proposed algorithm is able to run in real-time with 60FPS.