Tokyo Gov. Shintaro Ishihara is proving to be a man of his word -- up to a point. It remains to be seen whether or not he can keep some of his promises. Not long after announcing plans to seriously tackle the capital region's notorious traffic congestion, Mr. Ishihara and the Tokyo Metropolitan Government are now taking aim at highly pollutant diesel-powered vehicles. The governor has even vowed to "oust" diesel vehicles from Tokyo's roads. The metropolitan government is reported to be already asking automobile dealers in the capital to refrain from buying or selling diesel-powered passenger vehicles, an easy enough request to meet since there is so little demand for them.

It is diesel-powered trucks and buses, not passenger cars, that are responsible for much of the capital's air pollution. The metropolitan government estimates that the distance traveled by diesel-powered vehicles accounts for 20 percent of the total travel of all vehicles, but that their engines are responsible for 70 percent of the nitrogen oxides emitted by vehicles. Very few of these vehicles are passenger cars. The governor seems to have grasped the distinction. At a meeting with 160 Tokyo residents, he bravely addressed the question of whether cars "are worth the convenience," but concentrated mainly on the issue of ridding the capital of diesel-engine trucks.The diesel engine -- the most widely used power source for military equipment on both sides during World War II -- is today the standard engine for most railroad locomotives, construction and farm machines and large numbers of truck and buses. It has never proved popular for passenger vehicles, however, because of its high vibration rate and the limits it imposes on speed.

Some 650,000 diesel-powered vehicles, 16 percent of the total for the entire nation, are registered in Tokyo. Only 10 percent of the some 3 million vehicles that are on the capital area's roads every day, however, are estimated to be diesel-powered. But that is enough for sufferers from asthma and other respiratory ailments to welcome the metropolitan government's intention to do something about the problem, beginning by replacing the 40 diesel-powered vehicles it owns with gasoline-engine ones.