cars do tend to group together instead of staying apart.
Yes, I have noticed that also. I try (emphasis on "try") to position myself between those "wolf packs" and match speed. I can actually go for up to 1/2 hour before one of them catches up to me if traffic isn't too heavy. That does not apply in cities, of course.
The unspoken "agreement" with the state police in the USA is that you can safely go 5MPH over the limit without getting pulled over. Lately I notice that drivers are going 10-15MPH over the limit. There just are not enough police to cover all the territory, but now some states are posting cameras on the major interstates that will take pics over a certain speed. I noticed that the last time I drove through Ohio on I-90. I don't like cameras, but these drivers are going crazy so I don't feel like I can complain about it.
Now there is a new wrinkle in the problem: insane motorcyclists. I just got back from driving on I-80 to another city. Twice I was moving at 70MPH in heavy traffic, and a crotch-rocket blew past me like I was standing still; he had to be doing 85 at least. He swerved, dodged and squeezed into spaces between moving cars like a video game. In 10 seconds he was out of sight. I try to check my rearview mirror at least once every minute, but this guy came so fast I didn't see him coming up behind me and scared me half to death. He went past me so fast that the sound of his engine was like an explosion. The second incident occurred on the return trip, this time it was a Harley Davidson and the guy was wearing colors.
The western states with the wide-open highways and light traffic density are a lot nicer to drive.