It’s not that there is a lack of data. Instead, it is that the data are inadequate to answer the questions. No one has good statistics, for example, on crashes per mile ridden
Decent data is available only on bicycle versus vehicle collisions as many of these crashes are reported to police.
Somewhere I read an estimated 90% of bicycle crashes do not involve a vehicle and as such, are virtually never reported. Most bike crashes are bicyclist versus road hazard or cyclist colliding with another cyclist and even drunk bicyclists. No motor vehicles involved.
Media stories focus on car versus bike and rarely mention the far more common bike crashes.
As a consequence , there are no useful statistics on bicycle safety (except for car versus bicycle).
We know that about half a million bicycle related injuries are treated in ERs each year (CDC, 2015 data) but some estimates are even higher – there is no estimate on the number treated at doctor’s offices. Some think this could be low millions per year with estimates ranging (typically) from 1 to 2 million. And many are self treated (road rash, lacerations, soft tissue injuries).
Understanding bicycling injuries is critical for public policy on bicycling. Some cities have – for a long time – tried to encourage more people to ride bikes (bicycle commuting overall sticks pretty much in the 1-2% range with some exceptions). Most policies focus on issues related to bike versus cars – but do little to nothing for the larger group of non-vehicle related crashes. With potentially high injury rates, a dramatic increase in bicycle commuting could have serious consequences for health costs.
Up through my 20s, I experienced multiple bike crashes, only one of which involved a vehicle (side swiped by a bus). I suffered a fractured skull in one crash, and knock out blows in 2 other crashes that broke other bones. Two of my doctors have advised me not to ride bikes again to avoid additional head injuries – the effects of multiple traumatic brain injuries may be cumulative and I’ve also had brain injuries in non-bike situations too. None of my injuries were treated at an ER (the standard of care has changed since then!)