Going 110 on a dry empty highway? Carry on.
Going 110 on I-90 right outside of Billings? Pull over young man, its time for the highway patrol to have a philosophical discussion with you on the side of the road.
On a US highway you can glance in the mirror and see a vehicle, and you can work on the assumption that it's not going (much) more than 65mph. Or see a gap and have a general idea of how long you know it will still be a gap. Having a massive variety in possible speeds harms that.
I think many people seem completely unaware about how fast a car at 110mph (for example) can appear. It's in the order of time it takes to check your blindspot and other mirror - there's a hard limit to how many directions you can be looking at one time after all. No amount of "Personal Driving Skills" or "Being More Aware" can help that if the timing is unlucky.