Have vehicle registration restrictions improved urban air quality in Japan?

About 2.6 million non-compliant vehicles were removed from designated metropolitan
areas in Japan after the introduction of vehicle registration restrictions under the 1992
Automobile NOx Control Law. Based on a difference-in-differences framework and using
a monitor-level panel dataset for the period January 1981–December 2015, we find that
the intervention led to a 3–6% reduction in the monthly mean ambient concentration of
nitrogen dioxide (NO2) in the treated areas. Back-of-the-envelope calculations identify
benefits equal to about US$104 million as a result of reduced mortality from asthma.