Almost every speed limit is set lower than it should be, presumably to maximize revenue instead of maximizing safety. Until this changes, I want enforcement of speed limits to be poor.
It's a meme at this point, but Not Just Bikes does a great job covering the terrible "stroad".
https://www.youtube.com/watch?v=ORzNZUeUHAM
Most places in the US are filled with wide, multi-lane roads posted for speed limits of 35-45 mph. But those roads will be packed with unsignaled driveways, where drivers entering the road needed to make unprotected and right left turns with and against traffic that's likely going 45-55mph, and drivers on the road need to predict the behavior of cars coming from almost any direction.
Meanwhile, a thin stripe of white paint gets added to the gutter and is somehow called a bike lane - like anyone wants to ride their bike at the same grade as 2+ ton trucks flying by within a foot of your handlebars. Those same lanes will frequently disappear or reappear within a shoulder, and at intersections will suddenly cross or dump you out into a right turn lane. Better hope that driver turning right noticed you before you get hooked.
It's even worse for pedestrians, where even in dense cities, a walk signal is by no means a safe time to walk. Unsignaled left and right-on-red means a car will likely fly through their path at any given moment. On your way to this intersection, you'll continuously cross driveways and slip lanes with curve radii to encourage speeding, sidewalks that dump you into parking lots (if they exist at all!).
Sweden has a policy that all roads shared with pedestrians and cyclists have a maximum speed of 20mph, and have spent a lot of money on infrastructure to separate roads so that cars aren't completely inconvenienced by the rules.
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.