Traffic congestions that occur at sags caused by slowdown at uphill are one of prominent problems to be solved. Though there have been extensive studies on reducing congestion due to slowdown, they basically address velocity control of vehicles upstream in the congestion vehicle group. This paper presents a scheme to largely improve the traffic flow to avoid congestion where each vehicle detects the initial state of congestion and alerts all the vehicles running in the whole target area. Upon receiving alert information, a vehicle at the head of the congestion vehicle group and those behind the group perform different velocity control to increase traffic flow. Performance evaluation by simulation using trace data of velocity collected at the highway shows that the proposed scheme improves average velocity by about 50% at maximum compared to the case with no velocity control is applied.