I was looking at this post on Streetsblog about the safety in numbers rule and I got to this part
Do more people on bikes cause cycling to become safer, or does safer infrastructure attract more people to bike? There’s no conclusive evidence either way, but the answer is probably a mix of both. Safer infrastructure entices more people to ride, and more people riding instill greater awareness on the part of motorists and increase the demand for safer infrastructure.
What would be useful for answering cause and effect would be data from some place where cycling numbers changed suddenly, faster than infrastructure let's say. Something like a long-term transit strike, or a fashion-induced bike boom, or even perhaps an oil crisis would be a way to change the number of people cycling without any other variable to point to (except maybe fewer cars in the last example). Then we could know if more cyclists cause safer roads OR if building safer roads leads to more safety and more cyclists (or both as streetsblog asserts). If that's a reasonable premise than this
The report noted that Korea’s cycling fatality rates were greater than what its biking rates would suggest. Researchers speculated that might be due to a rapid recent growth in cycling. Perhaps, they write, “neither cyclists nor other transport participants have had time to assimilate each other’s presence.”
Might also indicate that getting more people to bike doesn't do much to make roads safer. Which would mean that if you want to make roads safer, you have to actually go out and do that. And then you'll get more cyclists.