The measure is aimed at reducing accidents in urban areas with heavy pedestrian traffic and narrow streets, marking a major shift in the country’s traffic rules as these roads account for around 70% of all ordinary roads nationwide.
[...] above [30 kph] the risk of pedestrians suffering fatal injuries in accidents is said to spike.
[...]
The aim of the speed limit change “is to prevent accidents by stopping dangerous driving, such as failing to slow down even on busy streets”
Sidewalks are quite rare outside major corridors in space-constrained Japan (you're often lucky to get so much as a gutter or a hint of a shoulder to walk/cycle on), so this is an overdue move indeed.
Pictured below is a pretty normal street width/layout in both rural and urban Japan, and the street I lived on was about 30-50% narrower than even this, and had high walls on either side (zero setbacks).
