Urban traffic is far below its usual level during this holiday-filled Golden Week period. On good days with fair skies, the public has the chance, as welcome as it is unexpected, for a foretaste of the cleaner air that is promised by tough new control measures for diesel engine-exhaust pollution soon to be introduced by the central government. If preliminary reports are correct, the government -- after years of denying or minimizing any health threat posed by emissions from diesel-powered vehicles -- has done a turnabout and plans to implement stricter regulations for this type of pollution starting in 2005, two years ahead of the original target date of 2007.

The government's ostensible change of heart is bound to raise as many questions as it answers. It was only a little more than three months ago, after all, that the government joined with the Hanshin Expressway Public Corp. to file an appeal with the Osaka High Court against a Kobe District Court ruling that they should compensate a group of residents of Amagasaki, Hyogo Prefecture, for physical ailments they suffer as a likely result of pollution caused by suspended particulate matter, or SPM, in exhaust fumes from diesel-powered vehicles. In announcing its reasons for appealing the decision, the Construction Ministry took the dubious stand that medical evidence proving a link between SPM and health problems is still lacking.

Yet, the Environment Agency -- which should be expected to have a more realistic viewpoint -- had already decided to shift its emphasis from regulating nitrogen oxides in diesel-exhaust gases, which it had previously considered the main health threat in such emissions, to tougher controls on SPM, which scientific evidence increasingly points to as a clear and immediate danger to human health. In fact, the agency technically had adopted the change as early as 1993, although it took few concrete steps to support it and has been under growing fire for its inaction during the intervening years.