This paper presents three experiments which were conducted under various road conditions to obtain the optimum forward visibility. The following vehicles were used: a high-speed bus having the front window extending to the floor with a curtain of variable length arranged on the inner surface; a heavy-duty normal control truck having variable visibility; a passenger car also having variable visibility. An oscillograph served to measure speed, acceleration, and deceleration, and a polygraph was used to measure psychogalvanic response which indicates driver excitability and psychophysiological reactions. A feeling test was also conducted. The test results were analyzed in terms of eye angular velocity, and it was found that the optimum lower visibility closely connected with 2 rad/sec of the eye angular velocity. This investigation recommends the optimum range of driving visibility for the vehicles.