With the development of vehicle technology, more and more autonomous vehicles appear on street, which will greatly impact on road traffic. This paper improves the NaSch cellular automata model by taking into account the Gipps safe distance algorithm. The traffic flow mixed by manual and autonomous vehicles are studied using numerical simulation method, and several new conclusions are drawn. First, the highway capacity can be dramatically increased, up to twice of the original capacity value, by adjusting the reaction time of the autonomous driving vehicle. Second, the influence of the reaction time on the highway traffic capacity can be ignored, when the value of the reaction time is reduced to 0.5s. Third, the proportion of the autonomous vehicles in traffic has significant impact on the road capacity and traffic congestion. When the autonomous vehicles is 80%, the highway capacity will be twice of the capacity of the traffic flow consisting of only manual vehicles and the traffic congestion can be reduced up to 50%. Fourth, in the fully autonomous driving traffic flow, increasing the autonomous driving reaction time can reduce the traffic congestion. Especially, when the density is in the range of 30~60 veh/km, the congestion can be reduced 20%, which can be used as an important strategy of traffic congestion mitigation.
