Recent annual CO2 emission from Japan sums up to 1.2 billion ton, and about 17 percent of it is from automobile traffic. There are many approaches to reduce CO2 emission and to prevent the global warming, and one of the important approaches is the Intelligent Transport Systems. This article surveys the effects of the ITS on the energy saving and global warming prevention based on the data in Japan, focusing on the applications of vehicular communication technologies. The ITS could contribute to the energy saving on two aspects: one is to eliminate the congestion, which enables each automobile to drive at the fuel optimal speeds, and the other is to provide means for modal shift, which reduces the traffic. This article surveys the ITS technologies for energy saving and CO2 emission reduction, which include traffic signal control, ETC, route guidance, cruise control, and automated driving. The road-to-vehicle and vehicle-to-vehicle communications play an essential role in energy saving as well as in the safety of automobiles and road transportation.