This post discusses problems related to measuring road injury risk, but substantively takes a look at cycling risk by London borough, via an attempt to estimate cycling volumes by borough via DfT counts and pct.bike.
So, I’ve been doing some analysis with a view to putting in some evidence to the London Assembly Bus Investigation.
In particular I’m interested in Question 12: Are there any problems caused by bus and cycling infrastructure sharing road space (particularly kerb side) and how could these be resolved?
I’m interpreting this as referring to shared bus and cycle lanes, and thought I’d look at an under-researched issue – the safety of cyclists in shared bus lanes. More on this in due course. However, looking into this issue brought up an old problem when using routinely collected data to look at risk: namely, we count road injury collisions in a manner that, while not counting all collisions, is at least comparable between areas, but getting that crucial denominator (out of how many cyclists?) is the really hard bit.
That denominator is especially a problem, when a particular type of infrastructure, whether it be a cycle track or a shared bus lane, is likely to affect the numbers of cyclists who might choose to use that road. For example: we wouldn’t want to conclude that a type of infrastructure was riskier than an alternative route type, if it actually kept people safer but also encouraged more cyclists to divert to it – leading to a higher number of collisions than on the alternative route type, but a lower number of collisions per cyclist.
But it’s often really difficult to get hold of the denominator, so studies end up looking at factors associated with crash sites, but without having a sure sense of the extent to which we’re talking about more risk per cyclist, or more cyclists in the first place.