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@CityOfIowaCity raised the speed limit on a section of road. Reasoning that in the year has been open there have been no fatal crashes with peds = safe. Except their analysis is widely underpowered to detect an increase in risk. Like <5% power. #badstats #SafetyOverSpeed
If crashes happened before at 12 times a year, on average, they'd need the rate to *double* in order to have 80% power to find a signal. At 24 times/year, they'd need crashes to happen about 1.7 times more often.
For fatal crashes when happen less often (say 1 time per decade) they would need the rate to be well over 20 times worse than before to have a detectable signal.
