What changes
Under the new classification, general roads used for everyday local life and lacking a center line or traffic lanes will generally have a statutory speed of 30 km/h. The change concerns the default speed where no speed is posted; before the effective date, follow the law and signs then in force.
Main roads that do not become 30 km/h
General roads with a center line or traffic lanes, roads whose directions are structurally separated, and motor-vehicle-only roads are not uniformly reduced to 30 km/h by this change. Do not decide from road width or nearby houses alone; check the markings and structure.
Posted speed limits take priority
If a sign or road marking specifies a maximum speed, that specified speed is the limit. A posted 40 applies even on a road whose default is 30, while a posted 30 applies on a road whose default is 60. Even below the limit, choose a safe speed for weather, visibility and pedestrians.
GAIMEN JAPAN
Sources
Procedures vary by prefecture. Always confirm the latest details with the relevant prefectural police authority.