Frequency of accidents is probably more closely tied to driver education and enforcement, given that fewer are killed in Germany (per 1B vehicle/km, trusting Wikipedia here), where there is strict prohibition of undertaking/cruising in the left lane, than in the US.
Also, for what it's worth without looking too deep into the specs, the big curve down at the 22X/88 St interchange has too small a radius for anything higher than 100 posted (it's ~600 m, needs 950 m for a higher speed), and it makes sense that they'd want the limit to be the same for the whole leg. The hills on either side going down onto the bridge over the Bow between Sun Valley and McKenzie would also fail for being too steep. From what I can tell, the segment between 130 Ave and Peigan at least passes the 120 design spec and could be posted at 110.
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