Collision warning should have human interface, which can inform hazard situation to drivers fast, correctly and in safe. When two or more warning systems are to be installed in the vehicle to prevent collision with surrounding obstacles in various directions such as forward, side, and rear, there is concern that driver misjudges the meaning of warning, delays operation, and does a wrong operation. For this requirement, hypotheses are proposed that beep sound which has high emergency parameters, imitation sounds which driver experiences in a hazard situation, and sound issued from the direction of hazard, are effective for drivers to recognize direction and kind of hazard fast, correctly and in safe, and they have been validated using driving simulators.