Our companion site pays short shrift to weather problems, as the studies (Hurt, Maids) didn't seem to find that weather is a factor in bike crashes. As soon as I got a mile or so from home on my post-publication ride, it started raining really heavily, and I found myself doing all the usual things - adjusting equipment so I could see better, considering my conspicuity in reduced visibility and accelerating and braking easier. When I stopped by Terry at Motorcycle Maddness, he reminded me how good rain was for practicing these skills.
I think I figured out why rain doesn't show up as a major cause of crashes in the study. In the US, bikers are mainly recreational, and often ride plans are canceled if there is rain. One local riding club has a "50-50" rule, for instance, with automatic cancellation of posted day rides if there is a 50 percent rain chance or temperature under 50. In my own experience, when caught on the road by bad weather, I often tweak the itinerary or schedule to avoid storms. All very sensible, but this probably causes much fewer miles to be ridden in the rain than in good weather. As the studies are driven by actual crashes, fewer rain miles will tend to under-represent rain-related crashes, even if there are more crashes per rainy mile ridden than for dry miles ridden.
In the case of Europe, where there are much more commuter miles ridden, I'd imagine that commuters use other means if they can, like carpooling or public transport, when it rains. Also, as a practical matter, if a rider doesn't use extra precautions during rain, it won't be long before Darwin reminds him with bike-dropping event. In the wet, riders tend to have slickers on, and the road surface has a lower coefficient of friction, so as long as you don't hit an immovable object, your chances of surviving the drop are maybe a bit better. So you might survive your initial rain drop, but you will be much more careful in the future. To some extent it is a self-correcting problem.
In this case, bikesafer.com will be going with Terry's biker gut, ignoring the studies, and adding more pages soon on riding strategies in adverse weather.
Maybe there is some way the new, long delayed US study can control for the rain factor and come up with reliable numbers on weather as a crash causation?