A good comparison point would be Germany. It has a very large network of roads too, some designed for very high speeds, and a strong driving culture (perhaps stronger than the continental US).
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In the US, on an interstate, the MPH spread around the speed limit is probably -20 to +20 (i.e. limit is 75, slowest cars are at 55, fastest at 95)
In Germany, on autobahns, you have speed ratios of up to 2x. You have to constantly be 110% aware of every vehicle within 1/4 mile of you, because you could either be closing in the much slower vehicle in front of you, or suddenly approached and passed by a much faster vehicle from behind.
Not such a terrible thing honestly...
Personally, I find the lack of predictability on US interstates is much riskier. I'm pretty sure the accident statistics back this up too.