Over the road trucks often have speed governors, some companies limit their trucks to 60 mph because it saves a lot of fuel and leads to a much (50%) lower risk of collisions.
Apples to oranges. Stopping distance of a 16-wheeler is magnitudes larger than that of a typical sedan, so in their case it makes sense.
For specific numbers (after subtracting reaction distance being the same for both):
55mph: car 165ft, 16-wheeler 225ft.
65mph: car 245ft, 16-wheeler 454ft.
As you can see, the gap between a car's stopping distance and a 16-wheeler's stopping distance increases with speed increasing, and non-linearly at that. Not even mentioning the destructive potential of a car vs. a 16-wheeler.
I would agree with your point if majority of the roads were occupied by 16-wheelers, but it isn't the case (at least in the metro area that I commute to work in).
Note: I agree that it would be safer if everyone drove the exact speed limit, as opposed to everyone going 10mph above the speed limit. However, in a situation where everyone is driving 10mph above the speed limit, you are creating a more dangerous situation by driving 10mph slower instead of driving 10mph above like everyone else.
