BBC article on cycling accidents

Clustering does come into it. Even if we look at fatalities in general it's still a small sample. You mention people knowing nothing about probability theory but one of the main principles about extrapolating probability is that you can only predict future events from very large groups of numbers. It's the Monte Carlo fallacy.

Clustering around what, though? The PD needs some dimension, such as time or distance, for observations to cluster in. But in this case we're not interested in time, or distance, or even sequence. We're interested in how many people died, from these populations, this year. And, sadly, this year's numbers aren't much different from last year's numbers.

Statistical significance does improve with a larger sample size, but as I keep saying, we have a large sample size which is all male and female cyclists this year. Not the 8 that we observed to die.

Your point about extrapolation and clustering would apply if, for example, out of the 50 or so cyclists that ride down the side road next to my flat each year, three of them had been killed by an HGV. 50 people is a small sample. Out of all of the people who had been killed by an HGV in London, one might expect to find some geographical clusters. That being so, you wouldn't want to extrapolate from the three deaths per 50 to a similar proportion of deaths in the wider cyclist population. But that's emphatically not what is going on here. The sample size is large to begin with.