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).
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.
The number of big trucks hanging out in the left lane in the US drives me mad...