To me, this herd mentality goes hand in hand with the idea that people don't want to lead the pack and be in front. They have no problem following someone as they break the speed limit, but won't do it alone or are more comfortable with a bunch of other drivers around. They have to stay with the pack. I say this because I have noted these tendancies in myself at times and have tried to make sure I don't do it over the years.
The only time I drove at 90+ for a sustained period of time (in a 65) was during a trip to college when a group of 3 cars (including myself) drove at 90 to 100 mph most of the way across the top of Indiana.
I have (and had) no problem with much lesser speeding on my own, but the only way I would have done that is when I have the interference of other cars, guessing that the police will have to pick one of us and let the other two continue on. (Not a given, of course, but a decent guess.)
Being older and wiser (and generally travelling with children) now, I would not do that again, but I made a valid risk/reward analysis at the time.