Maybe in Japan it will be effective. Since their "zone 30" creation was apparently a success, it seems probable that people will generally comply. My experience in France differs.
I live in a French town where the default speed limit is 30 km/h instead of the national 50 km/h. In the street where I live, the drivers that willingly (sometimes they can't overtake bikes and slow cars) observe that limit are a tiny minority. Why would they, since there are no automated controls in the city, and police sets manual controls once in a blue moon.
My street was already at 30 km/h in 2016 when the global speed limit was introduced by the city council. I'm pretty sure the mean speed has increased since then, though I don't have hard facts to prove it[^1]. At first I thought that, since all road signs were removed except on the city borders, the drivers needed a constant reminder of a limit that they'd prefer to ignore. Now I just think that people soon saw that there was almost no risk in breaking the law.
[^1]: There was a study, but the methodology was so poor that it was inconclusive. They monitored the speed in a dozen streets before and after the change, but it was done for different lengths of time (one or two weeks), at different times of the year, and with explicitly no attention payed to potential popular events or roadworks. So the average speed was globally lower after the change, but in most streets it had increased!