The purpose of this work is to study on construction of driver-assist system on highways. In recent years, road environments have become extensive and, include public roads and highways for efficient transportation. Some drivers, however, feel cognitive load by relationship with other vehicle when driving on highways. Human error by cognitive load has hazardous for drivers. Therefor, drivers need a support system while relieves cognitive load. Therefore, we constructed a driver-assist system that provides information to drivers about highway junctions, and focused on their decision-making processes at junctions and verified our system by simulation. And, we constructed driver-assist system, and have tested the effect of the driver by that assist-information provided from this system.