Most likely due to traffic going so slow.
Not so. The cycling growth has been in commuting which is often over routes extending far outside the slowest central region and the traffic speeds have changed little over time. There's always been a slow central core and many road stretches with 40 mph or more commonplace. Also the highest cyclist death rates were from before the congestion charge was introduced and central traffic heavier.
The largest reductions have been during this decade in the most recent years, due to the actions taken to tackle the deaths problem. They include a combination of extensive publicity, route changes, safety additions to the vehicle types most often in collision with cyclists, education and police action.
The main credit for the reduction goes to these:
Cyclists now avoiding some of the most dangerous practices like riding into the blind zones of trucks at left turn junctions for example.
Greater driver awareness of cyclists due to publicity, the much larger presence of cyclists now and some driver education programs.
There's more planned, additional separated routes for cyclists like the current east - west route and priority traffic lights for cyclists among them, so we can expect further cyclist accident reductions.
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