R’ Asher Weiss in an August lecture discusses what levels of danger is allowed. His conclusion, I think, is that a “1 in 1000” (figuratively) dangerous situation is allowed provided the person is in control of the situation and is following safety measures, i.e. ok to ride a boat (with a life jacket), ride a bike (with a helmet). At the same time, it is not always an issue of percentage, but of numbers. That is, if 1% of women are at risk during birthing, we need to be careful. Given the large numbers involved, being careful will save, say, 100 lives (don’t think “it is only 0.1%”). He applies same logic to COVID.
Applying this to riding bikes in the city – 1) you are not fully in control, as you are at mercy of drivers and helmet is not going to protect from them, 2) think of psychology of a person who is going to do that – can you guarantee that they’ll stay safe (R’ Weiss starts lamenting how careless religious community in Israel is, and how many accidents happen)
When our oldest were small, we had a neighbor, an avid and a very experienced biker. He was hit by a truck, had a major operation and a titanium implant in his leg. My wife stopped teaching kids biking. No, not after she heard about an accident. Only after we asked the neighbor how he is doing, and he said it will be a couple of months before he feels good enough to bike again.