Speed, in of itself, is not dangerous. What is more dangerous on highways is differential speed. i.e. if you're driving 60 mph when the traffic around you is doing 80 mph, you're the problem. Probably exponentially so if you're driving slowly in the "fast" lane. While rarely enforced, most states in the US have laws that require you to pull to the right if you're impeding traffic (usually defined as holding up +2 vehicles behind you) - even if you're doing the speed limit. Yet, these laws are rarely enforced, just like "No Trucks Left 2 Lanes" is never enforced in & around Chicago, despite being posted every mile or so on I 294.
I'd wager, also, that speed itself is rarely the direct cause of an accident. High speed, though, will make an accident more severe. My bet is that distracted driving is the #1 cause of accidents. On my daily 2-hour round trip on Chicagoland highways, I usually see 4-6 accidents each involving >2 cars. The shear amount of people I see on their phones is staggering. Hard to tell if it's social media or texting, but you can always tell because their head is staring at their lap, or they're holding their phones at the top of the steering wheel for all to see. I also see people with their phones mounted on their dashes watching movies or TV shows. Playing with your phone while driving is far more dangerous than paying attention & speeding.