Also it seems backwards - the UK got close to increasing its speed limits on motorways recently. The tradeoff was between economic benefits from more efficient travel on motorways and increased risk due to the higher speeds. The discussion was supported with ample data, although I believe it didn't succeed for political reasons.
You're suggesting almost exactly the opposite.
Basically the more homogeneous in speed and behavior the traffic is the safer it is.
You should be able to Google up some civil engineering studies at will. This stuff was "new" decades ago.
Uneven traffic flow results in a lot of lane changing and passing and when drivers are preparing to do those moves they're spending a lot of effort figuring out where the cars are and that creates a prime opportunity for anything motorcycle and smaller to be missed.
So yeah, a uniform 30ish MPH traffic flow on your city street is probably better than a more bimodal one even if the average speed is slightly higher.