A novel cellular automaton model for traffic flow on highway is proposed considering Intelligent Transportation System (ITS). Based on the Nagel-Schreckenberg model (NS for short), it includes effective gap and brake light. Moreover, a novel concept about the variable security gap is introduced. The simulation shows that the road capacity of the modified ITS cellular automaton model is higher than the measured data on certain highway, further indicating the important role of ITS,enlarged traffic flow and suppressed traffic jam. The mixed traffic flow related to two different vehicles' velocity is studied. The simulation shows even few slow vehicles will lead to drastic decrease of traffic flow, which demonstrates the necessity to implement strictly special fast lane.