Fascinating. I just looked up a bit of research on it, and it seems there are two contradicting phenomena at play. Flashing helps in seeing cyclists further away and helps with visibility generally -- but it also makes it harder to estimate speed and distance.

Apparently, the absolute safest solution is to have two rear lights side-by-side -- one that is always on and one that is always flashing.

It doesn't seem like there's clear data on which is safer if you have to pick only one. Different countries/states have chosen differently.