This study estimates and evaluates the social costs of highways in Japan. Five kinds of social costs of vehicular transport are considered: traffic accidents, air pollution, noise, global warming, and traffic congestion. Major findings of this study are as follows. First, social costs increase at an accelerated rate as traffic volume increases. Second, the largest component of vehicular transport’s social costs on highways is traffic congestion, accounting for about 64%. The second largest component is traffic accidents, accounting for 24%. Third, among vehicle types, the social costs of buses are largest, at 105 yen per vehicle-km, about 16 times higher than the social costs of regular cars. Last, total social costs for whole highway networks are about 678,212 million yen, or about 0.13% of Japan’s GDP. Compared with highway fare revenues and the operating costs of highways, social costs equal 62.6% and 98.6%, respectively.