In the US, there is the “85th percentile rule” that says the speed limit can’t be lower than the speed 85% of the traffic travels on that road. It’s not always followed, but is a factor I’ve heard cited in speed limit discussions several places I’ve lived, mostly as an argument against traffic calming measures for pedestrian safety. But to my understanding, the law implies that if enough people decide not to follow the speed limit, it could theoretically be deemed legally too low and would need to be increased.